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Mechanisms of liquid-phase sintering in Fe-Cu mixtures Magee, Brian Eric 1975

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MECHANISMS OF LIQUID-PHASE SINTERING IN FE-CU MIXTURES by BRIAN ERIC MAGEE . A . S c . ( H o n o u r s ) , Queens U n i v e r s i t y a t K i n g s t o n , 1973 A THES IS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in the Depa r tment o f M E T A L L U R G Y We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o t h e requ i r ed s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA J a n u a r y , 1975 In presenting th i s thesis in par t i a l fu l f i lment of the requirements for an advanced degree at the Univers i ty of B r i t i s h Columbia, I agree that the L ibrary shal l make it f ree ly ava i lab le for reference and study. I further agree that permission for extensive copying of th i s thesis fo r scho lar ly purposes may be granted by the Head of my Department or by his representat ives. It is understood that copying or pub l i ca t ion of this thes is fo r f inanc ia l gain shal l not be allowed without my writ ten permission. Department of ffigfe U'lJjtf) The Univers i ty of B r i t i s h Columbia Vancouver 8, Canada Date JfiA JS l C 1 l ^ ABSTRACT Continuous d i l a t o m e t r y and m e t a l l o g r a p h i c techniques have been used to study the..dimensional changes which occur when iron-copper compacts are s i n t e r e d above the m e l t i n g p o i n t of copper. Among the v a r i a b l e s i n v e s t i g a t e d were: a) the e f f e c t of s o l i d - s t a t e p r e s i n t e r i n g the compacts, b) copper content, c) p a r t i c l e size,'d) i n i t i a l compact d e n s i t y and e) the heating r a t e through the melting temperature. F i v e d i f f e r e n t processes causing dimensional change were i d e n t i f i e d . Three c o n t r a c t i o n processes operated con-s e c u t i v e l y : 1) rearrangement, 2) s o l u t i o n - p r e c i p i t a t i o n and 3) coalescence. Two expansion processes operated simultaneously w i t h processes (1) and (2), they were: 4) expansion due to d i f f u s i o n of copper i n t o i r o n and, 5) expansion r e s u l t i n g from the p e n e t r a t i o n of y - i r o n g r a i n boundaries by copper l i q u i d . In the e a r l y stages of s i n t e r i n g the i r o n was i n a 'dispersed' s t a t e , w h i l e i n the l a s t stage i t e x i s t e d as a s o l i d network. That change i s a t t r i b u t e d to an increase i n the values of the d i h e d r a l and wetting angles (<j> and 8) of the l i q u i d - s o l i d system during s i n t e r i n g . I t i s suggested t h a t i n i t i a l l y high concentrations of oxygen at the s o l i d - l i q u i d i n t e r f a c e s causes a low value of yCT and thus, the values of <j> and 6 are i n i t i a l l y low. As the oxygen c o n c e n t r a t i o n at those i n t e r f a c e s decreases w i t h time, <j> and 9 become p o s i t i v e , p e r m i t t i n g coalescence to occur. D i f f e r e n c e s among the r e s u l t s of previous i n v e s t i g a -t i o n s i n the Fe-Cu system have been explained by a t t r i b u t i n g them to d i f f e r e n c e s i n the thermal h i s t o r i e s of compacts. I t i s suggested t h a t some a d d i t i o n s to the Fe-Cu system, which reduce expansion during l i q u i d - p h a s e s i n t e r i n g , do so by a c t i n g as i n t e r n a l de-oxidants and preventing a zero d i h e d r a l angle from being a t t a i n e d . TABLE OF CONTENTS Page ABSTRACT i i LIST OF TABLES v i i i LIST OF FIGURES x ACKNOWLEDGEMENTS xv Chapter 1 INTRODUCTION . 1 1.1 P r a c t i c a l S i g n i f i c a n c e of L i q u i d -Phase S i n t e r i n g 1 1.2 Iron-Copper Phase E q u i l i b r i a 3 1.3 Role of Surface Energies i n L i q u i d -Phase S i n t e r i n g 6 . ..<: 1.3.1 Contact and Di h e d r a l Angles 6 1.3.2 Factors A f f e c t i n g the Contact and D i h e d r a l Angles 10 1.3.3 Wetting i n the Fe-Cu System 12 1.4 Mechanisms of Liquid-Phase S i n t e r i n g . . 14 1.4.1 Rearrangement. . . 14 1.4.2 S o l u t i o n - P r e c i p i t a t i o n Theories 15 1.4.3 Coalescence IS i v Chapter Page 1.5 Previous Studies of the Fe-Cu System 19 1.5.1 E a r l i e s t Observation 19 1.5.2 P r a c t i c a l I n v e s t i g a t i o n s . . . . . 19 1.5.3 Dilatometer Studies 2 2 1.5.4 Other Studies 27 1.5.5 Expansion E f f e c t s 27 1.5.6 Shortcomings of Previous Work . . 30 1.6 Objectives of This Work 32 2 MATERIALS, APPARATUS AND EXPERIMENTAL PROCEDURE 34 2.1 Metal Powders and Specimen Pr e p a r a t i o n . 34 2.2 The Dilatometer 42 2.2.1 D e s c r i p t i o n 42 2.2.2 Advantages of the Dilatometer Design 4 6 2.3 D i l a t o m e t r i c Technique 47 2.4 Other Experimental Procedures 53 3 OBSERVATIONS AND RESULTS 5 7 3.1 Observations: Blending, Compacting and Cleaning 5 7 3.1.1 Blending and Compacting . . . . . 57 3.1.2 Cleaning 60 3.2 Changes i n Weight and Dimensions During Dilatometry 66 3.3 D i l a t o m e t r i c Data f o r a Pure Iron 71 Compact v Chapter Page 3.4 S i n t e r i n g Behaviour of Fe-22 Cu-Mixtures 73 3.4.1 Dilatometer P l o t f o r PCM-2-20 73 3.4.2 S o l i d State Dimensional Changes 74 3.4.3 The Onset of M e l t i n g 79 3.4.4 Temperature Gradients i n the Dilatometer Specimens . . . . 87 3.4.5 M i c r o s t r u c t u r a l Changes Coincident w i t h M e l t i n g 88 3.4.6 S t r u c t u r a l Changes i n Stages I I I , IV and V . 90 3.4.7 Summary of M e t a l l o g r a p h i c Observations 107 3.5 Proposed Mechanisms of S i n t e r i n g i n Fe-22Cu Mixtures 109 3.6 E f f e c t . o f S i n t e r i n g V a r i a b l e s 120 3.6.1 General . . . . . 120 3.6.2 E f f e c t of P a r t i c l e S i z e and P a r t i c l e S i z e D i s t r i b u t i o n . . . . 122 3.6.3 E f f e c t of P r e s i n t e r i n g 128 3.6.4 E f f e c t of Heating Rate and Holding Temperature 137 3.6.5 E f f e c t of I n i t i a l Density . . . . 143 3.6.7 E f f e c t of a D i f f e r e n t Type of Iron Powder 149 3.6.8 Use of P r e a l l o y e d Powder. . . . . 152 v i Chapter Page 4 COMPARISON WITH PREVIOUS WORK 155 4.1 Comparison w i t h the Results of Kingery 155 4.2 Comparison w i t h B o c k s t i e g e l 1 s Work 162 4.3 Comparison w i t h Other Previous Work 163 4.4 Grain Boundary P e n e t r a t i o n and the D i h e d r a l Angle 164 5 CONCLUSION 168 5.1 Summary 168 5.2 Future Work 1 7 2 REFERENCES 1 7 3 v i i LIST OF TABLES Table Page I Results of Previous Surface Energy, Contact and D i h e d r a l Angle Measurements i n the Fe-Cu System 13 I I Metal Powders 35 I I I Composition of Metal Powders 36 IV Powder S i z e F r a c t i o n s 3 7 V Powder Mixtures Used i n Compacts 3 8 VI S i z e F r a c t i o n s of Powders i n Specimen PCM-UF-2 . . 40 VII D i s p o s i t i o n of Samples 54 V I I I Average Density of Fe-22Cu^ll Carbowax Compacts Pressed at 60 k . s . i . . . 59 IX Compacting and Cleaning 61 X Weight Loss During Cleaning. . 65 XI T o t a l Change i n Dimensions and Weight During Dilatometry 68 XII Runs which were Reproductions of the E a r l y Stages of Run PCM-2-20 75 v i i i Table Page X I I I Runs Showing E f f e c t s of V a r i a b l e s on Liquid-Phase S i n t e r i n g Mechanisms 121 XIV E f f e c t of P a r t i c l e S i z e and P a r t i c l e S i z e D i s t r i b u t i o n on the Dimensional Changes of Compacts During Liquid-Phase S i n t e r i n g 127 /XV E f f e c t of P r e s i n t e r i n g on the Dimensional Changes of Compacts During Liquid-Phase S i n t e r i n g . . . • 1 3 4 /XVI- E f f e c t of Heating Rate and Holding Temperature on Dimensional Changes During Liquid-Phase S i n t e r i n g . 1 4 0 . XVII, E f f e c t of *nijti:alcDensity on Dimensional Changes During Liquid-Phase S i n t e r i n g 145 XVIII E f f e c t of Copper Content on Dimensional Changes During Liquid-Phase S i n t e r i n g 148 XIX E f f e c t of Powder Type on Dimensional Changes During Liquid-Phase S i n t e r i n g 150 i x LIST OF FIGURES Figure Page 1 Fe-Cu phase diagram . . . . . . . 4 2 The contact angle, 6 7 3 The d i h e d r a l angle, <j> . . . 9 4 S p h e r i c a l p a r t i c l e s h eld together by r-.^-p c a p i l l a r y pressure of the l i q u i d phase 11 5 M i c r o s t r u c t u r e of a 75/25 Fe-Cu compact, s i n t e r e d at 1120°C f o r an u n s p e c i f i e d time from Chadwick et al. [21], 500x 21 6 Log f r a c t i o n a l d e n s i f i c a t i o n vs. l o g time, f o r Fe-Cu compacts c o n t a i n i n g 11.3, 22.0 and 43 weight per cent copper at 1150°C, from Kingery [14] . 23 7 The e f f e c t of copper content on the s i n t e r i n g behaviour of Fe-Cu compacts of <150u powders a. at 1150°C, from B o c k s t i e g e l [15]. . m . . . . . . 25 8 The e f f e c t of powder s i z e on the s i n t e r i n g behaviour of Fe-»7.u5eG,upeompaef sf I f rom per -. B o c k s t i e g e l [15] m 25 9 Heating c y c l e f o r cl e a n i n g 41 10 D r i l l e d compact 43 11 The di l a t o m e t e r 44 x Figure Page 12 'Normal' heating c y c l e 49 13 'Rapid' heating c y c l e 50 14 'Slow' heating c y c l e 51 15 ' P r e s i n t e r i n g ' heating c y c l e 52 16 Se c t i o n of compact.PCM-2£14 a f t e r c l e a n i n g . . . . 58 17 Dilatometer p l o t f o r PM-2; a pure i r o n specimen heated w i t h a normal c y c l e 72 18 Dilatometer curve f o r PCM-2-20, showing stages of dimensional changes . . . . 76 19 E a r l y stages of Run PCM-2-20, showing where samples i n Table XII were stopped 77 20 A s e c t i o n of sample PCM-2-12 at lOOx 80 21 Section of compacts used f o r metallography. . . . 80 22 A s e c t i o n of sample PCM-2-6 at 300x 81 23 A s e c t i o n i n sample PCM-2-21 at 300x, taken at l o c a t i o n 'B' 81 24 Macrophoto of PCM-2-5 at 5x 83 25 A s e c t i o n i n sample PCM-2-5 at 300x* taken at p o s i t i o n 'A' 83 26 A s e c t i o n i n sample PCM-2-5 at 300x, taken at p o s i t i o n 'B' 84 27 A s e c t i o n i n sample PCM-2-5 at 300x, taken at l o c a t i o n 'C' 84 x i Figure Page 28 A s e c t i o n i n specimen PCM-2-5 at 300x, taken at p o s i t i o n 'D' 85 29 A s e c t i o n i n specimen PCM-2-5 at lOOx, taken at l o c a t i o n ' C . 85 30 A s e c t i o n i n specimen PCM-2-5 at lOOx, taken at l o c a t i o n 'D' 86 31 A s e c t i o n i n specimen PCM-2-5 at lOOx, taken at l o c a t i o n 'B' . . . . . . . 86 32 A s e c t i o n i n specimen PCM-2-5 at 1200x 89 33 A s e c t i o n i n sample PCM-2-18, 1200x 91 34 A s e c t i o n i n PCM-2-18, 1200x 93 35 A s e c t i o n i n PCM-2-17, 1200x 93 36 A s e c t i o n i n PCM-2-17, 600x 94 37 A s e c t i o n i n PCM-2-17, 3 00x 9 4 38 A s e c t i o n i n PCM-2-22, 1200x 95 39 A s e c t i o n i n PCM-2-22, 300x 97 40 A s e c t i o n i n PCM-2-20, 300x 99 41 A s e c t i o n i n PCM-2-20, 600x 99 42 A s e c t i o n i n PCM-2-6, lOOx 100 43 A s e c t i o n i n PCM-2-18, lOOx 102 44 A s e c t i o n i n PCM-2-22 (Area #1) , lOOx 102 45 A s e c t i o n i n PCM-2-22 (Area #2) , lOOx 103 x i i Figure Page 46 A s e c t i o n i n PCM-2-20, 10 Ox 103 47 Surface of PCM-2-18 a t 2000x, viewed through a scanning e l e c t r o n microscope 105 48 Surface of PCM-2-20 at 1040x, as viewed w i t h a scanning e l e c t r o n microscope 105 49 Absorbed e l e c t r o n image of a s e c t i o n i n PCM-2-20; lOOOx . . 106 50 Copper X-ray image of a s e c t i o n i n PCM-2-20-; lOOOx 106 51 Dilatometer p l o t f o r Run PCM-2-20 110 52 Dilatometer p l o t f o r Run PCM-A. 124 53 Dilatometer p l o t f o r Run PCM-5-6 125 54 Dilatometer p l o t f o r Run PCM-6 126 55 Dilatometer p l o t f o r Run PCM-UF-2 129 56 Dilatometer p l o t f o r Run PCM-5-12 . 130 57 Dilatometer p l o t f o r Run PCM-5-13 . . . . . . . . 131 58 Dilatometer p l o t f o r Run PCM-2-10 (spurious data) 132 59 Dilatometer p l o t f o r Run PCM-2-19 133 60 Dilatometer p l o t f o r Run PCM-2-23 138 61 Dilatometer p l o t f o r Run PCM-2-24 139 62 Dilatometer p l o t f o r Run PCM-2-25 142 63 Dilatometer p l o t f o r Run PCM-2-30 144 x i i i F i g u r e Page 64 Dilatometer p l o t f o r Run PCM-2-100 147 65 Dilatometer p l o t f o r Run ATOCM-5 151 66 Dilatometer p l o t f o r Run PA-2-1 153 67 Log f r a c t i o n a l change i n length of compacts ( r e l a t i v e to L Q M J the length a t melting) versus ( t - t ^ ) , the time from m e l t i n g , during Stage I I I shrinkage 156 68 Dilatometer curve f o r Run PCM-5-6 . . . . . . . . 158 69 Kingery's data, f o r f r a c t i o n a l d e n s i f i c a t i o n vs. s i n t e r i n g time, and the same data c o r r e c t e d to e l i m i n a t e the e f f e c t of Stage IV expansion . . 160 70 Re s u l t s of d i h e d r a l angle measurements i n the Fe-Cu system, from Berner et al. [12] . . . . 165 71 x i v ACKNOWLEDGEMENTS The author wishes to thank h i s s u p e r v i s o r , Dr. J.A. Lund of the M e t a l l u r g y Department, f o r h i s c o n t i n u i n g encourage-ment and advice during a l l stages of the work. I t made the undertaking an enjoyable and rewarding experience. Thanks are a l s o due t o : B.N. Walker, J . Brezden and A.L. Redenbach f o r t h e i r help i n the l a b o r a t o r y , and Miss E.A. B e a t t i e f o r her encouragement. This research was undertaken w i t h the f i n a n c i a l support of the N a t i o n a l Research C o u n c i l of Canada and A.G. Magee to whom s p e c i a l thanks are accorded. xv Chapter 1 INTRODUCTION AND THEORY 1.1 P r a c t i c a l S i g n i f i c a n c e of Liquid-phase S i n t e r i n g Liquid-phase s i n t e r i n g i s the s i n t e r i n g of metal powders or ceramic m a t e r i a l s i n the presence of a l i q u i d phase. In metal powder systems i t i s most commonly accom-p l i s h e d by mixing the powders of two or more d i f f e r e n t metals (and/or a l l o y s ) , compacting the mixture and heating i t to a temperature above the m e l t i n g p o i n t of one of the powders but below t h a t of at l e a s t one of the other powders. G e n e r a l l y , l i q u i d - p h a s e s i n t e r i n g leads to r a p i d d e n s i f i c a -t i o n of the compact. Exceptions are cases i n which the l i q u i d does not 'wet' the s o l i d , when there i s a very s m a l l amount of l i q u i d formed, or when unusual expansion processes operate. The d r i v i n g f o r c e f o r the d e n s i f i c a t i o n of powder compacts i s the r e d u c t i o n of t h e i r t o t a l s urface energy. When a l i q u i d phase i s present, because of surface energy r e l a t i o n s h i p s , the l i q u i d spreads throughout the compact, f i l l i n g the voids between s o l i d p a r t i c l e s . The s t r u c t u r e s which r e s u l t range between the two extremes of: 1 2 i ) A c o n t i n u o u s n e t w o r k o f t h e s o l i d p a r t i c l e s w i t h t h e l i q u i d p h a s e d i s t r i b u t e d among t h e v o i d s i n t h e n e t w o r k and, i i ) A s t r u c t u r e c o n s i s t i n g o f s o l i d p a r t i c l e s s e p a r a t e d by t h e low m e l t i n g ' b i n d e r ' p h a s e ( o n l y t h e b i n d e r p h a s e i s c o n t i n u o u s ) . The high r a t e s of d e n s i f i c a t i o n , and the a b i l i t y to produce s t r u c t u r e s of high d e n s i t y , w i t h correspondingly good mechanical p r o p e r t i e s , makes li q u i d - p h a s e s i n t e r i n g an a t t r a c -t i v e process f o r the manufacture of s m a l l metal p a r t s . I f the volume f r a c t i o n of the compacts which i s l i q u i d i s not excessive ( i . e . i f there i s l e s s than 40% l i q u i d ) they g e n e r a l l y r e t a i n t h e i r shape by v i r t u e of the c a p i l l a r y a c t i o n of the l i q u i d . Thus the use of moulds i s not necessary. The a b i l i t y to c o n t r o l and p r e d i c t shrinkage during l i q u i d - p h a s e s i n t e r i n g i s important because cUose dimensional t o l e r a n c e s are of utmost concern i n the manufacture of small p a r t s . In order t o be able to p r e d i c t the shrinkage during s i n t e r i n g i t i s important to have a c l e a r understanding of the mechanisms and r a t e s of the processes causing shrinkage. I t i s a l s o d e s i r a b l e to be able to p r e d i c t the e f f e c t s of a l t e r i n g such parameters as powder p a r t i c l e s i z e and s i n t e r -i n g temperature. In f a c t , i n most l i q u i d - p h a s e s i n t e r i n g systems very l i t t l e i s understood of the processes causing d e n s i f i c a t i o n . 3 Iron-copper mixtures are widely used i n conventional powder metallurgy because the product of s i n t e r i n g has good mechanical p r o p e r t i e s . For t h a t reason, the l i q u i d phase s i n t e r i n g of i r o n copper mixtures has been i n v e s t i g a t e d e x t e n s i v e l y i n the past. 1.2 Iron-copper Phase E q u i l i b r i a From the iron-copper phase diagram i n F i g u r e 1 i t i s p o s s i b l e to analyze the sequence of events which w i l l occur when a mixture of i r o n and copper powders i s heated, from below the mel t i n g p o i n t of copper, t o above 1096°C. I t i s assumed t h a t the mixture i s heated s u f f i c i e n t l y f a s t t h a t l i t t l e or no i n t e r d i f f u s i o n occurs u n t i l the copper has melted. The sequence of events i s as f o l l o w s : i ) C o p p e r me I t s (I084°C) i i ) L i q u i d c o p p e r d i s s o l v e s some i r o n i i i ) Some e-phase ( s o l i d s o l u t i o n b a s e d on c o p p e r ) may f o r m between I084°C and I096°C i v ) C o p p e r b e g i n s t o d i f f u s e i n t o s o l i d i r o n t o f o r m Y~ s°l'd s o l u t i o n V) A t I096°C, any c o p p e r - r i c h s o l i d (e) decomposes t o f o r m c o p p e r - r i c h l i q u i d and c o p p e r s a t u r a t e d s o l i d y v i ) C o p p e r c o n t i n u e s t o d i f f u s e i n t o y With prolonged ho l d i n g above 1096°C, the y becomes saturated w i t h copper. I f the t o t a l amount of copper present 4 Cu-Fe Copper-Iron °c 1600 2800 F 1500 2700F (8-Fe)-r 10 2 0 Atomic Percentage Copper 3 0 4 0 5 0 6 0 7 0 1400 2500F 1300 2300 F 1200 2I00F 1100 I900F 1000 1800 F 9I2N 900 I600F (a-Fe) 800 700 80 90 1538° -1 I480°,I0.3 »/ 1 1 L —1 I 1 " " 1 /o 1394° ! - (X-Fe) I I -9.5 1096° C ) / 1 1 108' 9 6 - / K ^ ».5°-^\ 1 ( C u ) * -851° -99 \ -N2.I 1 1 111111111 CURIE TEN PERATURE 1 Fe 10 Robert E. Johnson 20 30 40 50 60 70 Weight Percentage Copper 80 90 Cu Figure 1. Fe-Cu phase diagram [1]. 5 i s l e s s than the s o l u b i l i t y l i m i t i n y the l i q u i d phase w i l l disappear. Otherwise, an e q u i l i b r i u m i s e s t a b l i s h e d between an i r o n - s a t u r a t e d l i q u i d s o l u t i o n and a copper-saturated y s o l i d s o l u t i o n . Because the l a t t i c e parameter of y i r o n i s l i t t l e a l t e r e d by copper i n s o l u t i o n ( i n amounts l e s s than the s o l u b i l i t y l i m i t ) , the expansion of i r o n p a r t i c l e s , when copper i s d i s s o l v e d i n them, i s approximately equal to the amount of copper d i s s o l v e d . A pure i r o n p a r t i c l e expands approximately 8 per cent i n volume when i t d i s s o l v e s 8 per cent copper by weight. The i n t e r f a c i a l i n t e r a c t i o n of the l i q u i d and i r o n -r i c h s o l i d p a r t i c l e s above the m e l t i n g p o i n t of copper i s of great importance i n determining the nature of the processes le a d i n g to d e n s i f i c a t i o n during l i q u i d - p h a s e s i n t e r i n g . The most s i g n i f i c a n t i n t e r a c t i o n s are those r e l a t i n g to the wetting of the s o l i d surfaces by l i q u i d . The w etting c h a r a c t e r i s t i c s of a l i q u i d - s o l i d system are commonly described by two parameters: the contact angle, 0, and the d i h e d r a l angle, <J>, as d e f i n e d i n the next s e c t i o n . The s i g n i f i c a n c e of these parameters must be c l e a r l y under-stood and t h e i r values known before i t i s p o s s i b l e to t h e o r i z e on the processes l e a d i n g to d e n s i f i c a t i o n during the l i q u i d -phase s i n t e r i n g of iron-copper compacts. 6 1.3 Role of Surface Energies i n Liquid-phase S i n t e r i n g 1.3.1 Contact and Dih e d r a l Angles The angle of contact which a l i q u i d makes w i t h a s o l i d i n the presence of a vapour i s c a l l e d the contact angle (9). I f Y l v i s t n e l i q u i d - v a p o u r i n t e r f a c i a l energy and Yg-y i s the solid-vapour i n t e r f a c i a l energy then: cos e = Y s v v ^ S L (1.1) YLV Where Y s l i s the s o l i d - l i q u i d i n t e r f a c i a l energy (see Figu r e 2). I f 9 i s l e s s than 90° the l i q u i d i s s a i d to wet the s o l i d , and i f 0 = 0° the wetting i s c h a r a c t e r i z e d as 'complete' A low contact angle i s favoured: A) F o r a g i v e n s o l i d by: i ) a low Yg|_> and i i ) a low Y|_y B) F o r a g i v e n l i q u i d by: i ) a h i g h Ygy i i ) a low Y S L I t should be noted t h a t 0 can never be zero as long as the surface t e n s i o n of the l i q u i d (YTT?) i-s higher than t h a t of the s o l i d ( Y s v ) • I f 0 i s l e s s than 90° and y O T i s constant then 0 increases when Y l v i s reduced. I f Y s l i s in c r e a s e d , 6 i s increased. In a completely wetting system the l i q u i d w i l l coat a l l of the surfaces of the s o l i d which are i n contact 7 \\\Ys^X\\\\  Figure 2. The contact angle', 0; determined by the r e l a t i v e values of a g L , a g v and d L V . (a = i n t e r f a c i a l t e n s i o n = y x area; where y = i n t e r f a c i a l energy) 8 w i t h vapour. I f 6 i s greater 90° the l i q u i d 'sweats' out of the compact. Grain boundary p e n e t r a t i o n i s determined by the d i h e d r a l angle, <J), which i s a f u n c t i o n of the r e l a t i v e magni tude of the g r a i n boundary and s o l i d - l i q u i d i n t e r f a c i a l energies (see Figure©3). The equation d e f i n i n g the d i h e d r a l angle i s : COS i l2j YSS  2^SL (1 where Y s s i s the g r a i n boundary i n t e r f a c i a l energy. In a powder compact complete l i q u i d p e n e t r a t i o n of a g r a i n boundary (or s o l i d p a r t i c l e - s o l i d p a r t i c l e c o n t a c t ) , occurs when y S L i s l e s s than i y s s ( i . e . cj) = 0). When y S L i s g r e ater than i Ygg' ^ -"-s ? r e a t e r . t h a n zero and l e s s than 180°. According to Smith [2], i f the d i h e d r a l angle i s greater than zero degrees, l i q u i d w i l l penetrate the g r a i n boundary only to the p o i n t where i t makes an angle i n the boundary equal to (f). The l i q u i d only advances f u r t h e r i n t o the boundary by d i s s o l v i n g the s o l i d where i t f o r c e s the l i q u i d to make an angle w i t h i t s e l f equal to the d i h e d r a l angle, ( i f the s o l i d i s s o l u b l e i n the l i q u i d ) . A f u r t h e r e f f e c t of surface energy i n l i q u i d - p h a s e s i n t e r i n g systems i s t h a t , through c a p i l l a r y p r essure, the 9 i / ° ~ S L Dy L i q u i d S o l i d / So l i d °SS Figure 3. The d i h e d r a l angle, <J); a f u n c t i o n of the r e l a t i v e magnitudes of Y q t a n d Yod-tor a and a c c ) . 10 l i q u i d places the s o l i d p a r t i c l e s under compressive s t r e s s . A l i q u i d w i t h 0 = 0 ° w i l l coat the s o l i d p a r t i c l e surfaces and pores w i l l form i n the l i q u i d . According to Kingery [3] the c a p i l l a r y pressure f o r c i n g adjacent p a r t i c l e s together due to the presence of a completely wetting l i q u i d between them i s : (1.3) Where P i s the pressure and r , the radius of l i q u i d surface (pore), (see Figure 4). The r e l a t i o n s h i p a p p l i e s whether or not the l i q u i d penetrates the p o i n t of contact between two adjacent p a r t i c l e s ( i . e . i t i s true f o r cf> > 0) . Gessinger, F i s h m e i s t e r and Lukas [4] have shown t h a t a compressive s t r e s s e x i s t s a l s o f o r p a r t i a l l y w e t t i n g l i q u i d s , but i t i s not as l a r g e as i n completely w e t t i n g systems. 1.3.2 Factors A f f e c t i n g the Contact and D i h e d r a l Angles According t o Taylor [5] the formation of a t r a n s i t i o n zone between the s o l i d and l i q u i d , i n the form of an i n t e r -m e t a l l i c compound or s o l i d s o l u t i o n , can a l t e r the value of YOT a n d thereby change the contact angle. Eremenko [6] has pointed out t h a t a c h a r a c t e r i s t i c f e a t u r e of the surface f r e e energy of metals i s the e x t r a o r d i n a r y dependence of Y l v on even very s m a l l amounts of i m p u r i t i e s . 11 Figure 4. S p h e r i c a l p a r t i c l e s h eld together by c a p i l l a r y pressure of the l i q u i d phase; cj> i s equal to zerOj i n the example shown, r i s the ra d i u s of the pore. 12 The d i h e d r a l angle i s p a r t l y dependent on the value of Y S S f which i s considered to vary w i t h the degree of mis-o r i e n t a t i o n across the g r a i n boundary [7]. Approximately 5% of randomly o r i e n t e d g r a i n boundaries are low-energy boundaries, f o r which the value of cf> i s higher than f o r the other 95% [7]. In liquid-soldldssysfeemswwifchi/intrins'ically low d i h e d r a l angles, r e l a t i v e l y s m a l l changes i n y s s cause l a r g e changes i n the d i h e d r a l angle. A l s o , the adsorption of i m p u r i t i e s on s o l i d surfaces can s i g n i f i c a n t l y a l t e r t h e i r surface energies and thus the values of 9 and cf> i n a given system [8]. The d i h e d r a l angle i s a l s o s e n s i t i v e to a d d i t i o n s to the l i q u i d phase [4]. Van Vlack [9] has shown t h a t , i n the FeS/FeO/Fe system, increased i r o n contents i n the l i q u i d cause a lower d i h e d r a l angle. The value of <f> a l s o v a r i e s w i t h temperature [9]. 1.3.3 Wetting i n the Fe-Cu System Several determinations of the value of <j> and 0 f o r the Fe-Cu system have been made. They are reported i n Table I . The values f o r 9 vary w i t h the atmosphere used. S p e c i f i c a l l y , complete w e t t i n g (0 = 0°) was observed only when a reducing atmosphere was employed. The observed values of the d i h e d r a l angle vary w i t h temperature. T h e o r e t i c a l l y , they should not vary w i t h 13 Table I Results of Previous Surface Energy and Contact and D i h e d r a l Angle Measurements i n the Fe-Cu System Temperature Degrees Centigrade Contact Angle, Degrees Di h e d r a l Angle, Degrees YSS gm/cm2 YSL gm/cm2 Atmosphere Reference — 0 27 H 2 10 1160 30 * 1.0 * 0.51 — 1220 30 * 0.51 — 7 1105 25 850. 430. J 1100 34 444. Ar 1130 ** 23 34 444. Ar 11 1100 387. Ar 1130 ** 13 301. Ar J 1150 9 1H 2/4N 2 I 1180 21 1H 2/4N 2 12 1110 32 1H 2/4N 2 J 1100 780. Vacuum 13 * r e l a t i v e values ** a f t e r 30 minutes at temperature 14 d i f f e r e n t atmospheres, because the equation f o r cj> does not in c l u d e y T „ . LiV Hough and R o l l s [1.1] found t h a t 6 decreased l i n e a r l y during periods of up to 30 minutes and then remained constant. They concluded t h a t a l l o y i n g of the i r o n and the l i q u i d phase was the cause of the decrease i n 0. 1.4 Mechanisms of Liquid-phase S i n t e r i n g 1.4.1 Rearrangement Kingery [3] proposed t h a t upon i n i t i a l formation of the l i q u i d phase, c a p i l l a r y pressure w i l l rearrange (repack) the s o l i d p a r t i c l e s i n such a manner t h a t maximum packing d e n s i t y and minimum pore surface area w i l l r e s u l t . The r e -packing i s accomplished by the s l i d i n g of s o l i d p a r t i c l e s past each other. He noted t h a t i f the s o l i d p a r t i c l e s remain s p h e r i c a l complete d e n s i f i c a t i o n i s only p o s s i b l e w i t h l a r g e amounts of l i q u i d . A minimum of 35% (by volume) of l i q u i d i s r e q u i r e d to f i l l the voids between uniform-sized s o l i d spheres packed f o r maximum d e n s i t y . Kingery i d e n t i f i e d v iscous flow as the r a t e -c o n t r o l l i n g process i n rearrangement, from which a l i n e a r dependence of d e n s i f i c a t i o n on time i s p r e d i c t e d . Kingery proposed the r e l a t i o n s h i p : AL/Lo = 1/3 (AV/V 0) = t ( 1 + y ) (1.4) 15 where L and V are the length and volume-of the compact, and y1 i s the c o r r e c t i o n f o r i n c r e a s i n g r e s i s t a n c e t o rearrangement when the repacking nears completion, and f o r an i n c r e a s i n g d r i v i n g f o r c e due t o ever decreasing pore s i z e . L 0 and V 0 r e f e r to o r i g i n a l dimensions ( i . e . at the s t a r t of the p r o c e s s ) . Rearrangement i s only p o s s i b l e f o r a short time and once maximum packing de n s i t y i s accomplished i t w i l l cease i n those systems i n which the s o l i d p a r t i c l e s do not d i s s o l v e i n the l i q u i d . 1.4.2 S o l u t i o n - p r e c i p i t a t i o n Theories Several mechanisms of d e n s i f i c a t i o n and g r a i n growth during l i q u i d - p h a s e s i n t e r i n g have been proposed which depend on the s o l u b i l i t y of the s o l i d i n the l i q u i d . Because i r o n has an appreciable s o l u b i l i t y i n l i q u i d copper these t h e o r i e s may w e l l apply to the Fe-Cu system. Kingery [3] proposed a mechanism of d e n s i f i c a t i o n which he c a l l e d " s o l u t i o n - p r e c i p i t a t i o n " but which, i n order to d i s t i n g u i s h i t from other mechanisms, w i l l be r e f e r r e d to i n t h i s work as ' s o l u t i o n due to pressure'. According to the model, which a p p l i e s only to systems w i t h a completely wetting l i q u i d (and presumably a zero d i h e d r a l a n g l e ) , the l i q u i d coats the surface of the s o l i d p a r t i c l e s and penetrates s o l i d - s o l i d contacts between p a r t i c l e s . The c a p i l l a r y pressure of the l i q u i d exerts compressive pressures on those areas of "^y i s a s m a l l f r a c t i o n . 16 the s o l i d p a r t i c l e s which are separated from other s o l i d p a r t i c l e s by only t h i n f i l m s of l i q u i d , and the a c t i v i t y of the s o l i d i s l o c a l l y increased. The s t r e s s at the contact areas i s p r o p o r t i o n a l to the amount of c a p i l l a r y pressure, and thus the s i z e of the pores i n the l i q u i d . The increased a c t i v i t y provides a d r i v i n g f o r c e f o r the s e l e c t i v e s o l u t i o n of s o l i d at 'contacts', which allows the approach of p a r t i c l e centres and, t h e r e f o r e , d e n s i f i c a t i o n . The s o l i d d i s s o l v e d at 'contacts' r e - p r e c i p i -t a t e s at other areas of the s o l i d which are not under com-p r e s s i v e s t r e s s ; thus promoting a l t e r a t i o n of the shape of the s o l i d p a r t i c l e s t o allow increased packing e f f i c i e n c y ( d e n s i t y ) . Kingery [3] p r e d i c t e d t h a t shrinkage r e s u l t i n g from s o l u t i o n due t o pressure, w i t h d i f f u s i o n r a t e - c o n t r o l l i n g , would be governed by the equation [14]: AL/Lo = C i r - " 7 3 t l / 3 (1.5) where ' r ' i s the p a r t i c l e r a d i u s , ' t ' the time from the s t a r t of the process and * C i ' a constant f o r a given composition and temperature. I f the phase-boundary r e a c t i o n i s r a t e -c o n t r o l l i n g the equation i s : AL/Lo = C 2 r - 1 t2 (1.6) 17 where C 2 i s a constant for a given composition and temperature. Bockstiegel [15] has suggested that a s k e l e t a l net-work of s o l i d p a r t i c l e s permeated by l i q u i d may shrink by a process of solution and re-rprecipitation of-the s o l i d i n the l i q u i d phase "with the s o l i d . . . skeleton becoming ever more completely enclosed by l i q u i d . . . and therefore shrink-ing as a whole" [15]. He did not elaborate, but i t i s infer r e d that the theory i s based on the idea that s o l i d at the surface of the compact dissolves and i s transported to inner areas of the compact where i t re-precipitates on the s o l i d network. In th i s manner the external dimensions of the s o l i d network are reduced and d e n s i f i c a t i o n r e s u l t s . The theory i s expected to apply to systems with a completely wetting l i q u i d but a f i n i t e dihedral angle. In many liquid-phase s i n t e r i n g systems, the s o l i d phase grains (or p a r t i c l e s ) are observed to increase i n average si z e with increasing time at temperature. This growth process was f i r s t discussed and analysed by Price, Smithells and Williams [1.6] i n reference to W-Cu-Ni a l l o y s , and was described l a t e r by Lenel [17] as the "Heavy A l l o y Mechanism." The mechanism involves the solut i o n of small s o l i d grains i n the l i q u i d and r e - p r e c i p i t a t i o n on the larger s o l i d grains. 18 The d r i v i n g f o r c e f o r the process i s the increased s o l u b i l i t y of s o l i d at a curved surface according to the r e l a t i o n s h i p : Ira (S/S-b) = 2 Y s l V 0/RkT (1.7) where S/So i s the r a t i o of the s o l u b i l i t y at a curved surface of radius R to t h a t at a f l a t s u r f a c e , V 0 i s the molecular volume, k, the Boltzmann constant and, T the absolute tempera-t u r e . Thus, i n a powder compact the s o l u b i l i t y of a p a r t i c l e increases w i t h decreasing r a d i u s . 1.4.3 Coalescence Coalescence i s a process of d e n s i f i c a t i o n analogous to s o l i d s t a t e s i n t e r i n g . Presumably, by means of volume d i f f u s i o n , or p l a s t i c flow mechanisms a c t i n g i n the s o l i d , p a r t i c l e s i n contact w i t h each other grow together w i t h enlargement of the contact area (neck) between them. The t o t a l s o l i d - l i q u i d or s o l i d vapour i n t e r f a c i a l area (and thus the t o t a l s urface energy of the system) i s thereby reduced. White [7] has shown t h a t the t h e o r e t i c a l surface energy of two spheres i n p o i n t contact i n a surrounding l i q u i d may be reduced by coalescence. Adjacent s o l i d spheres may minimize t h e i r surface energy by approaching each other through coalescence and the formation of a ( f l a t ) i n t e r f a c e between them. The energy of the two spheres i s a minimum when they 19 i n t e r s e c t at an angle equal to the d i h e d r a l angle of the s o l i d - l i q u i d system [7]. The theory i n c l u d e s conservation of the volume of the spheres. ' Further d i s c u s s i o n of coalescence may be found i n a paper by Warren [1.9] . 1.5 Previous Studies of the Fe-Cu System 1.5.1 E a r l i e s t Observation P r i c e , S m i t h e l l s and Wi l l i a m s [16] reported i n 1938 t h a t an 80% i r o n , 20% copper powder mixture s i n t e r e d f o r three hours at 1400°C reached 93% of t h e o r e t i c a l d e n s i t y , and e x h i b i t e d a s t r u c t u r e of rounded g r a i n s of " i r o n " i n a matrix of copper saturated w i t h i r o n . Since then, the l i q u i d - p h a s e s i n t e r i n g of Fe-Cu mixtures has been the subject of many i n v e s t i g a t i o n s [6,12,14,15,16,20,21,22,23,24,25,26,27,28,29], the more important of which are described below. 1.5.2 P r a c t i c a l I n v e s t i g a t i o n s Some previous i n v e s t i g a t i o n s have been p r i m a r i l y concerned w i t h mechanical p r o p e r t i e s and measurements of net dimensional changes i n s t a t i c runs f o r use i n powder metal-lurgy production. Northcott and Leadbeater [20] s i n t e r e d s e v e r a l Fe-Cu mixtures under a v a r i e t y of c o n d i t i o n s . They observed maximum 20 s i n t e r e d d e n s i t i e s i n mixtures c o n t a i n i n g 20% or more of copper, and g r e a t e r shrinkages w i t h f i n e r powders. I n two compacts of 47y m i r o n powder w i t h 4.74% and 14% copper they observed small (1.5 to 3% AV/V0) net expansions a f t e r s i n t e r -i n g f o r one hour at 1100°C. M e t a l l o g r a p h i c s t u d i e s of the s t r u c t u r e s a f t e r i to 4 hours at 1100°C revealed "marked rounding of the i r o n p a r t i c l e s and evidence of t h e i r coalescence even i n the high copper a l l o y s (35% copper)." A f t e r i hour at 1100°C a c l e a r l y marked d i f f u s i o n zone was observed i n the i r o n p a r t i c l e s (due to copper d i f f u s i n g i n t o the i r o n ) . Chadwick, B r o a d f i e l d and Pugh [21] s i n t e r e d 7 5/25 iron-copper mixtures at 1120°C. They observed 1% l i n e a r expansion a f t e r 15 minutes i n a compact made from 2 9y m i r o n w i t h 3 y m copper powders. In a compact prepared from l a r g e r i r o n and copper powders (49y m diameter i r o n and 12y m copper) they observed 9% l i n e a r shrinkage. The two compacts had d i f f e r e n t i n i t i a l d e n s i t i e s . M i c r o s t r u c t u r a l i n v e s t i g a t i o n showed t h a t the s i n t e r e d t s t e u c t u r e f o f a!575/>S25;ocomp.act-was " i r o n - r i c h f e r r i t i c p a r t i c l e s of rounded form i n continuous contact, the intermediate spaces being s u b s t a n t i a l l y f i l l e d by the cop p e r - r i c h phase," (see Figure 5). S i l b e r e i s e n [22], i n an i n v e s t i g a t i o n of "shrinkage compensation" by a l l o y i n g , i n v e s t i g a t e d the expansion of i r o n powder compacts, c o n t a i n i n g up to 8 per cent copper, s i n t e r e d at temperatures between 1000°C and 1250°C. He 21 Figure 5. M i c r o s t r u c t u r e of a 75/25 Fe-Cu compact s i n t e r e d at 1120°C f o r an u n s p e c i f i e d time (not l e s s than 15 minutes) , from Chadwick et al. [21] 50Ox. The copper i s white and the i r o n of v a r i o u s shades, from l i g h t grey to bl a c k . 22 observed i n c r e a s i n g amounts of net expansion w i t h higher copper contents, and p r o g r e s s i v e l y l e s s expansion a f t e r s i n t e r i n g ( f o r a f i x e d time) at higher temperatures above 1100°C. 1.5.3 Dilatometer Studies S e v e r a l i n v e s t i g a t o r s have used d i l a t o m e t e r s to f o l l o w the dimensional changes of Fe-Cu compacts during l i q u i d - p h a s e s i n t e r i n g . Kingery [1.4] s i n t e r e d compacts c o n t a i n i n g 11.3, 22.0 and 43.0 weight per cent copper mixtures at 1150 oC i n a d i l a t o m e t e r w i t h a hydrogen atmosphere, using a short heat-up p e r i o d . A p o r t i o n of hiLssdataaissshown i n Figure 6. The p l o t s of log(AV/V 0) versus l o g time were claimed to be s t r a i g h t l i n e s w i t h i n i t i a l slopes of 1.3 to 1.4 and slopes of 1/3 i n the l a t e r p o r t i o n s . Kingery a l s o i n v e s t i g a t e d the e f f e c t of p a r t i c l e s i z e . He concluded t h a t 'rearrangement 1 and ' s o l u t i o n due to pressure' were the mechanisms causing d e n s i f i -c a t i o n . Kingery apparently d i d not observe any expansion i n h i s compacts during s i n t e r i n g . In 1959 B o c k s t i e g e l [15] observed t h a t i n 20% copper, 80% i r o n powder mixtures the compacts expanded r a p i d l y and e x t e n s i v e l y - uptto 2.7% (AL/Lo) i n the 10 minutes immediately f o l l o w i n g m e l t i n g , and then contracted more s l o w l y . F i g u r e 7 shows h i s r e s u l t s f o r s i n t e r i n g experiments at 1150°C w i t h 23 0021 i i 1 2 5 10 20 50 100 200 500 TIME (MINUTES) Figure 6. Log f r a c t i o n a l d e n s i f i c a t i o n versus l o g time, f o r Fe-Cu compacts c o n t a i n i n g 11.3, 22.0 and 43 weight per cent copper at 1150°C, from Kingery [14] . 24 d i f f e r e n t copper contents. Maximum expansion was r e a l i z e d i n 20% copper mixtures, which a l s o contracted most r a p i d l y . F i g u r e 8 shows the r e s u l t s of a s e r i e s of runs of 7.5% Cu mixtures w i t h various powder s i z e s . They i n d i c a t e t h a t at some c r i t i c a l powder s i z e , near 145y m diameter, maximum expansion occurred. B o c k s t i e g e l [15] was able to show t h a t one cause of the expansion was the d i f f u s i o n of copper i n t o i r o n p a r t i c l e s For example, he observed no expansion during the l i q u i d - p h a s e s i n t e r i n g of i r o n - s i l v e r mixtures, which he r e l a t e d to the low s o l u b i l i t y of s i l v e r i n i r o n . B o c k s t i e g e l [1.5] proposed t h a t shrinkage occurred by s o l u t i o n and r e - p r e c i p i t a t i o n of the s o l i d which, he concluded, was interconnected ( s k e l e t a l ) throughout s i n t e r i n g . He developed equations f o r both the expansion and shrinkage stages. Using them he showed, s c h e m a t i c a l l y , how the r a t e s of the two processes would vary depending on the s p e c i f i c surface area of the i r o n powder and the mass t r a n s p o r t c a p a c i t y of the l i q u i d . Dautzenberg [23] used a d i l a t o m e t e r to i n v e s t i g a t e the l i q u i d - p h a s e s i n t e r i n g of p r e a l l o y e d Fe-Cu powders and an Fe-3% Cu powder mixture. In the l a t t e r mixture he observed expansion followed by c o n t r a c t i o n . He proposed t h a t the expansion and c o n t r a c t i o n were the r e s u l t of the formation and subsequent m e l t i n g of the s o l i d e-phase which e x i s t s between 1084°C and 1096°C. o c oT <0 800 0) o. E <D WO t— 0 3.0 / T e m p e r a t u r e 1 i ^ M e 11 o f I ng f C o p f 3 o i n t ) e r / / 1— T e m / oe r a t J r e i • " M e 1 1 o t i n g : C o p °o i n t \ i '•  G r s S i 2 i n D e n s ' 1 y,i r/cm3: <I50 6.3 i >200 sj/TJ <m \ <20 i 5.3 i \ < 7 \5.3 25 20 W 60 80 100 130 110 0 20 W 60 80 S i n t e r i n g T i m e , M i n u t e s wo m F i g u r e 7. The e f f e c t of copper content on the s i n -t e r i n g behaviour of Fe-Cu compacts of <150u powders at 1150°C from B o c k s t i e g e l [15]. Figure 8. The e f f e c t of powder s i z e on the s i n t e r -i n g behaviour (at 1150°C) of Fe-7.5Cu compacts, from B o c k s t i e g e l [15] . [ ( A L / L ) C U - (AL/L) 0 i n %] i s the dimensional change of an iron-copper compact r e l a t i v e to a pure i r o n compact of the same powder s i z e . 26 In metallographic i n v e s t i g a t i o n s w i t h a hot stage microscope at 1100°C and 1150°C, Dautzenberg observed p e n e t r a t i o n of the i r o n g r a i n boundaries by copper. He remarked t h a t at 1150°C the penetrated g r a i n boundaries had "widened s i g n i f i c a n t l y " [23]. Trudel and Angers [24] a l s o i n v e s t i g a t e d the s i n t e r -i n g of Fe-Cu-C powder mixtures using a d i l a t o m e t e r . They observed s m a l l expansions [0.5% (AL/Lo)] followed by sm a l l srlow c o n t r a c t i o n s [<0.7% (AL/Lo)] i n mixtures c o n t a i n i n g 0 to 12.8% copper and one per cent g r a p h i t e , which were s i n t e r e d at 1120°C f o r h a l f an hour. They observed s i m i l a r expansions when p r e a l l o y e d powders were employed. The l a r g e s t expansion was f o r an Fe-8.1%Cuv mixture. A 12.8% copper mixture expanded l e s s . In t h a t mixture l i q u i d copper was present throughout s i n t e r i n g , as the s o l u b i l i t y l i m i t of copper i n i r o n i s approximately 9.3% at 1120°C. Trudel and Angers [24] a l s o i n v e s t i g a t e d the r a t e of d i f f u s i o n of copper i n t o 60y m diameter Atomet i r o n powder at 1120°C and showed t h a t the d i f f u s i o n d i s t a n c e (penetration depth) was 8 y m a f t e r 30 minutes. Based upon the s o l u b i l i t y of copper i n i r o n at tha t temperature, the maximum expansion r e s u l t i n g from t h a t extent of d i f f u s i o n should be ap p r o x i -mately 2% A V / V 0 . 27 1.5.4 Other Studies Lenel [25] i n v e s t i g a t e d the l i q u i d phase s i n t e r i n g of 80/20 Fe/Cu powder mixtures and presented h i s data as p l o t s of a " d e n s i f i c a t i o n parameter" versus the logarithm of the time at the s i n t e r i n g temperature. Some of h i s data gave s t r a i g h t l i n e p l o t s . However, he was not able to a t t r i b u t e t h a t r e s u l t to any p a r t i c u l a r mechanism of s i n t e r i n g . Lenel a l s o i n v e s t i g a t e d the g r a i n growth of the s o l i d p a r t i c l e s during s i n t e r i n g and concluded t h a t the "Heavy A l l o y Mechanism" of s o l u t i o n - p r e c i p i t a t i o n might have been o p e r a t i n g . Ramakrishnan and Lakshiminarasimhan [26] observed i n c r e a s i n g amounts of shrinkage a f t e r a given time at 1100°C wi t h i n c r e a s i n g copper contents. They a t t r i b u t e d these observations t o i n c r e a s i n g c a p i l l a r y pressure i n the compacts when more l i q u i d was present. 1.5.5 Expansion E f f e c t s B o c k s t i e g a l [15] a t t r i b u t e d the expansion he observed to d i f f u s i o n of copper i n t o i r o n . Dautzenberg [23] suggested t h a t the cause was the f r e e z i n g and mel t i n g of the e-phase between 1084°C and 1096°C. S i l b e r e i s e n [22] proposed t h a t " d e n s i f i c a t i o n p a r a m e t e r " = ( S i n t e r e d d e n s i t y - g r e e n d e n s i t y a b s o l u t e d e n s i t y - g r e e n d e n s i t y x 1 0 0 28 the temperature dependence of the amount of expansion ( a f t e r one hour of s i n t e r i n g ) i n d i c a t e d t h a t the process was not only the r e s u l t of the d i f f u s i o n of copper i n t o i r o n . Trudel and Angers [24] concluded t h a t a p o r t i o n of the expansion was a r e s u l t of g r a i n boundary p e n e t r a t i o n and t h a t the presence of carbon reduced "copper growth" by reducing the g r a i n boundary energy of i r o n and thus the p e n e t r a t i o n of g r a i n boundaries by copper. Berner, Exner and Petzow [12] s i n t e r e d Fe-Cu powder mixtures above 1100°C and performed copper i n f i l t r a t i o n experiments. They noted t h a t the observed expansion i n a given time was more than twice as great as that c a l c u l a t e d to r e s u l t from d i f f u s i o n . They r e l a t e d an observed tempera-t u r e dependence of expansion t o an observed temperature dependence of the d i h e d r a l angle and concluded t h a t some of the expansion was a r e s u l t of the p e n e t r a t i o n of g r a i n boundaries. They a l s o found t h a t l a r g e r copper p a r t i c l e s caused l a r g e r volume in c r e a s e s . No expansion was observed i n i r o n - s i l v e r mixtures and t h i s was a t t r i b u t e d to the high d i h e d r a l angle of the silver-^.iron system. Berner et al. [12] a l s o observed l e s s expansion during i n f i l t r a t i o n than during the l i q u i d - p h a s e s i n t e r i n g of Fe-Cu mixtures, and found t h a t p r e s i n t e r i n g of i r o n compacts before i n f i l t r a t i o n reduced the amount and r a t e of s w e l l i n g during i n f i l t r a t i o n . The e f f e c t of p r e s i n t e r i n g was suggested 29 to be the removal of g r a i n boundaries. They a l s o found t h a t i n f i l t r a t i o n of i r o n w i t h a copper - 4.5% i r o n l i q u i d caused l e s s expansion and a t t r i b u t e d the e f f e c t to the reduced r a t e of s o l u t i o n of i r o n i n the a l l o y e d l i q u i d . Berner et al. pointed out tha t the f o l l o w i n g a d d i -t i o n s to the iron-copper system have been, found by others to reduce the amount of expansion observed during l i q u i d phase s i n t e r i n g [12]: carbon, t i n , phosphorous, tungsten and l e a d . Krantz [27] a l s o performed l i q u i d phase s i n t e r i n g and i n f i l t r a t i o n experiments on Fe-Cu mixtures. He observed t h a t the a d d i t i o n of 1.2% carbon t o 6% copper mixtures reduced the expansion ( a f t e r 30 minutes at 1120°C) from 2.4% (AL/L 0) to 0.2% (AL/Lo). He noted t h a t the expansion was r a p i d and completed i n ten minutes. He suggested t h a t the compacts expanded by the l a t t i c e d i f f u s i o n of copper i n t o i r o n and al s o by. g r a i n boundary d i f f u s i o n . He f u r t h e r hypothesized th a t the e f f e c t of carbon was to reduce the r a t e of g r a i n boundary d i f f u s i o n . B o c k s t i e g e l [15] proposed t h a t i n Fe-Cu-C mixtures the formation of an iron-copper-carbon ternary a l l o y , immiscible w i t h the copp e r - r i c h l i q u i d , increases the " s o l u t i o n -p r e c i p i t a t i o n " of i r o n to such a degree t h a t d e n s i f i c a t i o n overshadows some of the expansion. 30 1.5.6 Shortcomings of Previous Work Much of the p r e v i o u s l y reported work on s i n t e r i n g i n the Fe-Cu system i s d i f f i c u l t to i n t e r p r e t and has l e d to a confusion of t h e o r i e s , due to shortcomings i n e i t h e r experimental technique or i n the r e p o r t i n g of the r e s u l t s . For example: a) In many i n v e s t i g a t i o n s o n l y l o n g s i n t e r i n g e x p e r i m e n t s were c o n d u c t e d , and no e f f o r t was made t o i s o l a t e e a r l y e x p a n s i o n e f f e c t s f r o m l a t e r s h r i n k a g e e f f e c t s . As a r e s u l t , i n t e r p r e t a t i o n s o f t h e e f f e c t o f v a r i a b l e s , s u c h as c o p p e r c o n t e n t and p o w d e r . p a r t i c I e s i z e , a r e o f t e n q u e s t i o n a b l e . b) Powders used i n p r e v i o u s i n v e s t i g a t i o n s have s o m e t i m e s been p o o r l y c h a r a c t e r i z e d w i t h r e s p e c t t o p a r t i c l e s i z e and c o m p o s i t i o n . F o r e x a m p l e , t h e c a r b o n c o n t e n t o f i r o n p o w d e r s , w h i c h m i g h t have an i m p o r t a n t e f f e c t on e x p a n s i o n i n s i n t e r i n g , i s commonly n o t s p e c i f i e d . In a d d i t i o n , most i n v e s t i g a t o r s have used powders w i t h a w i d e p a r t i c l e s i z e d i s t r i b u t i o n , and w i t h a v a r i e t y o f p a r t i c l e s h a p e s . c ) C o m p a c t i n g p r e s s u r e s , and t h e a s - c o m p a c t e d d e n s i t i e s o f Fe-Cu s p e c i m e n s , have v a r i e d w i d e l y f r o m one i n v e s t i g a t i o n t o a n o t h e r ( a n d , s o m e t i m e s , w i t h i n a s i n g l e i n v e s t i g a t i o n ) , w i t h o u t a d e q u a t e c o n s i d e r a t i o n o f t h e i r i m p o r t a n c e t o t h e r e s u l t s o f s i n t e r i n g e x p e r i m e n t s . 31 •d) The t h e r m a l h i s t o r y o f c o m p a c t s p r i o r t o r e a c h i n g a s p e c i f i e d s i n t e r i n g t e m p e r a t u r e , i s commonly n o t d e s c r i b e d . Some i n v e s t i g a t o r s may have " p r e s i n t e r e d " t h e i r c o m p a c t s i n t h e s o l i d s t a t e b e f o r e p e r f o r m i n g t h e l i q u i d - p h a s e e x p e r i m e n t s (a common r o u t i n e i n p a r t s m a n u f a c t u r e ) w h e r e a s o t h e r s may have h e a t e d them d i r e c t l y t o above I084°C f r o m t h e a s - c o m p a c t e d cond i t i o n . Even where t h e y a r e s p e c i f i e d , r a t e s o f h e a t i n g v a r y g r e a t l y f r o m one r e p o r t e d i n v e s t i g a t i o n t o a n o t h e r . Thus a s p e c i m e n h e a t e d t o , s a y , II50°C a t a r a t e o f 5°C p e r m i n u t e C l2] s p e n t 13 m i n u t e s between t h e m e l t i n g p o i n t o f c o p p e r and I I50°C. In c o n t r a s t , a s p e c i m e n h e a t e d t o I 150°C a t a r a t e o f 26°C p e r m i n u t e s Cl-O c o n t a i n e d l i q u i d f o r o n l y t h r e e m i n u t e s b e f o r e t h e f i n a l s i n t e r i n g t e m p e r a t u r e was r e a c h e d . e) Some d i l a t o m e t e r s used i n p r e v i o u s s t u d i e s were i n c a p a b l e o f o b s e r v i n g b o t h e x p a n s i o n and c o n t r a c t i o n i n t h e same s p e c i m e n ; h e n c e , some i n v e s t i g a t o r s have a p p a r e n t l y f a i l e d t o o b s e r v e e x p a n s i o n a l t o g e t h e r . f ) E x p e r i m e n t a l v a r i a b l e s , o t h e r t h a n t h o s e men-t i o n e d a b o v e , have e i t h e r n o t been k e p t u n d e r c o n t r o l o r have been i n a d e q u a t e l y c h a r a c t e r i z e d i n p r e v i o u s r e p o r t e d work. T h e s e i n c l u d e : t h e d e g r e e o f b l e n d i n g o f t h e i r o n and c o p p e r powder m i x t u r e s , t h e s i z e o f t h e c o p p e r powder p a r t i c l e s r e l a t i v e t o t h a t o f t h e i r o n , t h e t e m p e r a t u r e g r a d i e n t s i n 32 t h e s p e c i m e n d u r i n g s i n t e r i n g , t h e h o m o g e n e i t y o f s p e c i m e n d e n s i t y a f t e r c o m p a c t i o n (and t h e r e s u l t i n g i s o t r o p y o f d i m e n s i o n a l c h a n g e s d u r i n g s i n t e r i n g ) . Each o f t h e s e v a r i a b l e s can have i m p o r t a n t e f f e c t s on t h e p r o g r e s s o f l i q u i d - p h a s e s i n t e r i n g , and on t h e i n t e r p r e t a t i o n o f r e s u l t s . g) In o n l y one p r e v i o u s s t u d y o f t h e p r o g r e s s o f l i q u i d - p h a s e s i n t e r i n g i n t h e Fe-Cu s y s t e m L~23H has any s e r i o u s e f f o r t been made t o f o l l o w s t r u c t u r a l c h a n g e s by m e t a l l o g r a p h y . F r e q u e n t l y , a s s u m p t i o n s have been made a b o u t : w e t t i n g r e l a t i o n s h i p s , t h e e x t e n t o f d i f f u s i o n o f c o p p e r i n t o i r o n , and t h e d e g r e e o f c o n t i n u i t y o f t h e s o l i d p h a s e , w i t h o u t s u p p o r t i n g metaI I o g r a p h i c e v i d e n c e . 1.6 Objectives of t h i s Work This work was undertaken p r i m a r i l y as an e f f o r t t o overcome some of the confusion which e x i s t s i n the l i t e r a t u r e concerning the o r i g i n of those dimensional changes which occur during l i q u i d - p h a s e s i n t e r i n g of iron-copper powder compacts. By means of s i n t e r i n g experiments i n which the important v a r i a b l e s were under reasonable c o n t r o l , and w i t h thorough supporting metallography, i t was hoped to be able to: a) i d e n t i f y the mechanisms r e s p o n s i b l e f o r both expansion and shrinkage i n the system and, b) d i s t i n g u i s h the i n d i v i d u a l e f f e c t s , on s i n t e r i n g , of such v a r i a b l e s as powder p a r t i c l e 33 s i z e , amount of copper, i n i t i a l d e n s i t y of the compact and the sequence of thermal processing p r i o r t o , and d u r i n g , s i n t e r i n g . In a l l experiments, c l o s e a t t e n t i o n was p a i d to the powder s i z e d i s t r i b u t i o n s , the degree of mixing of the powders, the dimensional changes during c l e a n i n g (deoxidizing) of the compacts and the d e n s i t i e s of compacts at the s t a r t of li q u i d - p h a s e s i n t e r i n g . The heating r a t e through the m e l t i n g p o i n t of copper, and the thermal h i s t o r i e s of samples, were d u p l i c a t e d as c l o s e l y as p o s s i b l e to remove "heating r a t e " e f f e c t s . . The dimensional changes during s i n t e r i n g were measured continuously w i t h a s e n s i t i v e d i l a t o m e t e r and the anisotropy of dimensional changes i n the compacts during s i n t e r i n g <was; c l o s e l y monitored. M i c r o s t r u c t u r a l i n v e s t i g a t i o n s were used t o com-plement d i l a t o m e t r y i n determining the mechanisms of l i q u i d -phase s i n t e r i n g . Chapter 2 MATERIALS/ APPARATUS AND EXPERIMENTAL PROCEDURE 2.1 Metal Powders and Specimen P r e p a r a t i o n The two i r o n powders, one copper powder, and one p r e a l l o y e d Fe-Cu powder described i n Table I I were used i n the experiments. A l l were produced by atomisation processes. Table I I I contains the r e s u l t s of chemical analyses on the as-received powers. The powders were separated i n t o the e i g h t s i z e f r a c t i o n s l i s t e d i n Table IV, using ' T y l e r 1 screens and a 'Rotap' machine. Copper and i r o n powder f r a c t i o n s were then mixed according to the weight percentages given i n Table V. L i q u i d Carbowax 300 was added to a l l the mixtures (Specimen Groups) except PCM-3-1 and PM-2. - I t functioned as both a mixing agent and l u b r i c a n t . Carbowax 300 i s a polyethylene g l y c o l w i t h a f l a s h p o i n t of 196°C and does not hydrolyze. The powder-lubricant mixtures were blended i n a a Paterson-K e l l y t w i n - s h e l l blender f o r one hour. The compositions of the power mixtures f o r v a r i o u s Specimen Groups were c a l c u l a t e d to g i v e iron-copper weight 34 35 Table I I Metal Powders Powder Designation S u p p l i e r Lot No. D e s c r i p t i o n PM Easton Metal Powders RZ 365,746-3 Iron CM Alcan Grade MD154 #2056 Copper ATO Quebec Metal Products Atomet 28 64 Iron PA * Domtar Domtar A-178 P r e a l l o y e d "Fe-7 Cu" * Courtesy of Dr. R. Angers, U n i v e r s i t y de L a v a l , Quebec. 36 Table I I I Composition of Metal Powders Element Powder PM CM ATO PA Fe 99.28 0. 001 99.8 92.3 Cu T 99.35 T T 6.85 C 0.10 NP 0.01 0.04 S i 0.01 0. 001 0.07 0.07 Mn 0.34 NP 0.01 0.08 S 0.003 <0.001 0.005 0.012 P 0.006 0.017 NP NP Sn NP 0.09 0.01 0.01 Zn NP 0.008 NP NP Pb NP 0.015 NP NP T = Trace NP = Not Performed 37 Table IV Powder S i z e F r a c t i o n s Powder S i z e Designation Mesh ("Tyler") P a r t i c l e Diameter Microns (u m) A 105 to 120 0 + 200 74 to 105 1 -200 +230 63 to 74 2 -230 + 27 0 53 to 63 3 -270 + 325 45 to 53 4 -325 + 400 37 to 45 5 -400 + 500 25 to 37 6 -500 <25 UF See Table VI Table V Powder Mixtures used i n Compacts Component M a t e r i a l s Specimen Group Designation P a r t i c l e S i z e Range Iron PM Wt.% Copper CM Wt.; Iron ATO Wt. Fe-7Cu PA Wt.; L u b r i c a n t Carbowax Wt. PCM-A PCM-2-XX(or X) PCM 3-1 PCM-5-XX(or X) PCM-6* ATOCM-5* * PA-2-1 PCM-UF-2 PCM-2-100* PM-2* 105 to 120 53 to 63 45 to 53 25 to 337 <25 25 to 337 53 to 63 See Table VI 53 to 63 53 to 63 76.50 77.23 78.0 77.23 77.23 77.23 89.11 , nn 100 21.51 21.78 22.0 21.78 21.78 21.78 14.92 203.78 9.90 77.23 84.11 1.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 X = d i g i t (run designation) I n d i c a t e s t h a t the Specimen Group comprised a s i n g l e specimen. 00 39 r a t i o s of 90/10, 78/22 and pure i r o n a f t e r 'dewaxing' (removal of the carbowax during the clea n i n g o p e r a t i o n described below). In each of the mixtures, except t h a t used f o r PCM-UF-2, the copper and i r o n powders were both of the same s i z e f r a c t i o n . The s i z e s of the powders i n PCM-UF-2 are l i s t e d i n Table VI. S u i t a b l e weights of powder mixtures were compacted i n a s i n g l e - a c t i n g d i e t o y i e l d c y l i n d r i c a l specimens of 0.5 inches diameter by approximately 0.5 inches i n height. Specimen PCM-2-30 was pressed a t 180 k . s . i . ; a l l other s p e c i -ments were compacted at 60 k . s . i . The dimensions and weight of each compact were measured to allo w c a l c u l a t i o n s of the compacted d e n s i t y (p-'o). A l l compacts were dewaxed and cleaned of oxide by heating i n hydrogen i n a tube furnace, using the two stage thermal c y c l e shown i n Figu r e 9. The treatment was designed to achieve three o b j e c t i v e s : i ) To remove o x y g e n f r o m t h e c o m p a c t s i i ) To c a u s e enough s o l i d s t a t e s i n t e r i n g o f t h e powders t o a l l o w t h e c o m p a c t s t o be d r i l l e d w i t h o u t c r a c k i n g , b u t n o t so much s i n t e r i n g t h a t l a t e r l i q u i d - p h a s e s i n t e r i n g w o u l d be a f f e c t e d a p p r e c i a b l y , and i i i ) To remove a l l Carbowax and r e s i d u e s t h e r e o f . A f t e r c l e a n i n g , the weights and dimensions of the compacts were remeasured to permit the c a l c u l a t i o n of cleaned 40 Table VI S i z e F r a c t i o n s of Powders i n Specimen PCM-UF-2 Si z e Powder Diameter .urn m PM Q. "O CM Q, 6 <25 9.5 5 25 to 37 9.7 4 37 to 45 12.1 3 45 t o 53 11.5 2 53 to 63 7.9 100.0 1 63 to 74 13.1 0 74 to 105 13.1 A 105 to 120 <120 10.0 13.1 8 0 0 h Time (min) F i g u r e 9. Heating c y c l e f o r c l e a n i n g . (Hydrogen flow throughout c y c l e ) . 42 d e n s i t i e s ( P c ) and the weight l o s s e s during c l e a n i n g . To accommodate a thermocouple during d i l a t o m e t r y , a hole was d r i l l e d i n each compact according to F i g u r e 10. 2.2 The Dilatometer 2.2.1 D e s c r i p t i o n The d i l a t o m e t e r , which gave continuous measurements of the length of a compact during l i q u i d - p h a s e s i n t e r i n g , i s shown i n F i g u r e 11. I t c o n s i s t e d of a s t e e l frame which supported a transducer w e l l above the powder compact which was being l i q u i d - p h a s e s i n t e r e d . The compact r e s t e d on a l a v a block p e d e s t a l and was surrounded by an alumina sheath i n s i d e a s i l i c a g l a s s tube. The transducer produced an e l e c t r i c a l output p r o p o r t i o n a l to the amount of upward or downward displacement of an i n t e r n a l i r o n susceptor. The susceptor was l i n k e d to the top of the compact by a slender alumina rod. S i l i c a d i s c s were used between the specimen and the alumina rod, and between the specimen and the l a v a block. The compact was heated by a c o n c e n t r i c i n d u c t i o n c o i l s i t u a t e d o u t s i d e the s i l i c a g l a s s tube. The temperature of the compact was continuously monitored by a 'centred 1 tungsten-rhenium thermocouple which entered the bottom of the compact through the l a v a block p e d e s t a l . Enough clearance was allowed between the t i p of the thermocouple and the top 43 F i g u r e 10. D r i l l e d compact. 44 H 2 gas Transducer Water cooled collar Alumina rod Silica glass tube Induction coil jsilica discs at both ends ot powder compact Alumina tube Lava block Thermocouple F i g u r e 11. The di l a t o m e t e r . 45 of the hole d r i l l e d i n the compact so t h a t a l i n e a r contrac-t i o n of s e v e r a l per cent could occur before the specimen would come to r e s t on the thermocouple. A water-cooling c o l l a r mounted on the top of the s i l i c a g l a s s tube kept the transducer near room temperature during heating of the compacts. The transducer d i d not r e s t on the water c o l l a r ; r a t h e r i t was held i n place by a set screw i n the s t e e l frame through which i t passed. The alumina rod stood f r e e i n s i d e the d i l a t o m e t e r and transducer and was held v e r t i c a l by a small hole i n the water c o l l a r . The susceptor and rod exerted a load on the top of the compact equal to t h e i r weight only (8 grams). The transducer output was l i n e a r w i t h d e f l e c t i o n of the susceptor up t o 0.050" above or below the ' n u l l p o i n t ' of the transducer. The output was connected to a d i f f e r -e n t i a l transformer i n d i c a t o r ( a m p l i f i e r ) which converted i t i n t o thousandths of an i n c h d e f l e c t i o n of the susceptor, and d i s p l a y e d the amount on a VU meter. The a m p l i f e r was a l s o connected t o a chart recorder. The v i s u a l output of the a m p l i f i e r provided a check on the s t r i p chart pen d e f l e c t i o n . The thermocouple was connected to a c o l d j u n c t i o n and thence to the P h i l l i p s i n d uction-furnace c o n t r o l u n i t which contained a thermocouple output s t r i p c h art recorder w i t h a 25 m.v. f u l l - s c a l e d e f l e c t i o n . Independent determina-t i o n s of the temperature at the thermocouple were a l s o made 46 w i t h a potentiometer accurate to 0.01 m.v. C o n t r o l of the power i n p u t to the i n d u c t i o n c o i l was manual. Hydrogen was s u p p l i e d continuously throughout a l l runs. I t was introduced at the top of the transducer i n the d i l a t o m e t e r a t the r a t e of approximately 150 cu. f t . / h r and at approximately 20 p . s . i . The hydrogen was d r i e d w i t h " D r i e r i t e " and phosphorus pentoxidein U-tubes between the supply and the d i l a t o m e t e r . 2.2.2 Advantages of the Dilatometer Design The design of the d i l a t o m e t e r and specimen was such as t o promote: i ) U n i f o r m i t y and symmetry o f h e a t i n g a l o n g t h e l e n g t h , a n d i n t h e p l a n e p e r p e n d i c u l a r t o t h e d i r e c t i o n o f measurement o f t h e c o m p a c t . i i ) R a p i d h e a t i n g t h r o u g h t h e c r o s s -s e c t i o n and l e n g t h o f t h e c o m p a c t . i i i ) S h a l l o w t e m p e r a t u r e g r a d i e n t s i n t h e c o m p a c t s ( l a t e r v e r i f i e d ) . The above are important c o n s i d e r a t i o n s because the changes i n dimension during l i q u i d - p h a s e s i n t e r i n g are r a p i d and extensive. A f u r t h e r advantage of the dil a t o m e t e r was t h a t i t measured both expansion and c o n t r a c t i o n , and d i d so w h i l e e x e r t i n g only a s m a l l load on the compacts (equal t o ap p r o x i -mately 2/3 of the weight of a compact). A l s o the data from the d i l a t o m e t e r were continuously generated. 47 Induction heating was employed because i t o f f e r e d f l e x i b i l i t y i n the c o n t r o l of heating c y c l e s and heated the samples w i t h shallow temperature g r a d i e n t s . I t a l s o o f f e r e d the a b i l i t y to c o o l specimens r a p i d l y from the s i n t e r i n g temperature. 2.3 D i l a t o m e t r i c Technique The d i l a t o m e t e r transducer was c a l i b r a t e d , occasion-a l l y , using a f e e l e r guage of known t h i c k n e s s . R e p e t i t i o n of the procedure, without i n t e r v e n i n g adjustment of the a m p l i f i e r , e s t a b l i s h e d t h a t the d i l a t o m e t e r was accurate to w i t h i n 0.00025" (or approximately 0.05% of the length of the compacts). A c o r r e c t i o n f o r the expansion and c o n t r a c t i o n of the l a v a b l o c k , was made to the curves f o r each run i n the d i l a t o m e t e r . Simulated runs were performed using a 'cored 1 s t e e l block ( i n s t e a d of a compact) through which the alumina rod passed to the l a v a block p e d e s t a l . Curves were generated which gave the p r e c i s e changes i n dimension of the l a v a block. The r e s u l t i n g curves (one f o r each type of heating cycle) were subtracted from the surves f o r s i n t e r i n g runs, to give the true changes i n dimensions of the compacts. The measurement of the length of the compacts a f t e r c l e a n i n g served as the b a s i s f o r converting d i l a t o m e t e r data to %(AL/Lo) ( i . e . the change of length r e l a t i v e to the 48 cleaned l e n g t h ) . At the end of each run measurements were made of the samples using c a l i p e r s . Because the compacts cooled i n the apparatus w i t h the dil a t o m e t e r o p e r a t i n g , the measured change i n leng t h during a run could be compared w i t h the change i n length as recorded by the d i l a t o m e t e r . Four b a s i c heating c y c l e s were used f o r di l a t o m e t e r runs. They are described i n Figures 12 through 15. The c y c l e which i n c l u d e d one hour at 1000°C r e s u l t e d i n extensive s o l i d s t a t e s i n t e r i n g of compacts and was known as 'pre-s i n t e r i n g 1 (see F i g u r e 15). A l l but the 'rap i d ' c y c l e i n -corporated two ho l d i n g stages where the temperature was held constant f o r a few minutes, or more, to reduce temperature gradients i n the compacts. I t was not always p o s s i b l e to reproduce a given heating c y c l e e x a c t l y . During the f i r s t stage of heating (from 20°C to 700°C), i t was not p o s s i b l e t o reduce the power inpu t to the c o i l below a c e r t a i n l e v e l , and some compacts reached 700°C e a r l i e r than others. However, the hol d i n g times and heating r a t e s were reproduced as c l o s e l y as p o s s i b l e at higher temperatures. Thus, some samples reached the melting p o i n t of copper a t sh o r t e r t o t a l times than oth e r s . I f a sample reached the melt i n g p o i n t w i t h i n 60 seconds of the time t h a t temperature was reached i n one of the four t y p i c a l heating c y c l e s (Figures 12 to 15), i t was considered to have been heated w i t h t h a t heating c y c l e . F i g u r e 1 2 . 'Normal 1 heating c y c l e . F i g u r e 13. 'Rapid 1 heating c y c l e . F i g u r e 14. 'Slow' heating c y c l e * Time (min) F i g u r e 15. ' P r e s i n t e r i n g ' heating c y c l e . 53 Because any two compacts, heated w i t h a given c y c l e , e x p e r i -enced e s s e n t i a l l y i d e n t i c a l h o l d i n g times and heating r a t e s above 800°C, the (AL/L 0) versus time curves f o r the two runs could be compared above the m e l t i n g temperature w i t h confidence. The purposes f o r which the various powder specimens were used are described i n Table V I I . Some of the d i l a t o m e t e r runs were simply reproductions of others f o r e s s e n t i a l l y i d e n t i c a l specimens. Where good r e p r o d u c i b i l i t y was i n d i c a t e d , a s i n g l e r e p r e s e n t a t i v e d i l a t o m e t e r p l o t i s presented i n the t h e s i s . 2.4 Other Experimental Procedures Other experimental work comprised: metallography, scanning e l e c t r o n microscopy, e l e c t r o n microprobe a n a l y s i s and chemical a n a l y s i s of the mixtures (Specimen Groups) and compacts generated i n the experiments. Cleaning t e s t s and mixing t e s t s were a l s o performed, as i n d i c a t e d i n Table V I I . Cleaning t e s t s c o n s i s t e d of performing the c l e a n i n g procedure i n one hour stages and weighing the compacts between stages. The object was to determine when most of the weight l o s s was complete and how e f f e c t i v e was the oxygen removal. Mixing t e s t s c o n s i s t e d simply of o b t a i n i n g chemical analyses on the top and bottom halves of compacts which had Table V I I D i s p o s i t i o n of Samples Sample Dilatometry Run Performed AL/Lo Curve Produced Cleaning Test Mixing Test (chem. a n a l y s i s ) Metallography Other Chemical A n a l y s i s PCM-2-3 N N N Y N N PCM-2-4 N N Y N Y N PCM-2-5 Y N N N Y N PCM-2-6 Y N N N Y N PCM-2-10 Y Y N N N N PCM-2-12 Y N N N N. •. N PCM-2-13 Y N N N N N PCM-2-14 N N N N Y N PCM-2-15 Y N N Y , N N PCM-2-17 Y N N N Y N PCM-2-18 Y Y N N Y N PCM-2-19 Y Y N N N N PCM-2-20 Y Y N N Y N PCM-2-21 Y N N N Y N PCM-2-22 Y Y N N Y N PCM-2-23 Y Y N N N N PCM-2-24 Y Y N Y N N PCM-2-25 Y Y N N N N PCM-5-6 Y Y N N N N PCM-5-7 N N Y N N N PCM-5-9 N N N Y N N PCM-5-12 Y Y N N N N PCM-5-13 Y Y N N Y N Y = yes, N = no CONTINUED Table V I I (Continued) Sample Dilatometry Run Performed A L / L o Curve Produced Cleaning Test Mixing Test (chem. a n a l y s i s ) Metallography Other Chemical A n a l y s i s PM-2 PCM-3-1 PCM-6 PCM-A PCM-UF-2 PCM-2-30 PCM-2-100 PA-2-1 ATOCM-5 Y N Y Y Y Y Y Y Y Y N Y Y Y Y Y Y Y N Y N N N N N N N N N N N N N N N N N N Y Y N N Y Y N N N Y Y N N Y Y Y Cn 5 6 been cleaned but not l i q u i d - p h a s e s i n t e r e d . Comparison of the r e s u l t s of chemical analyses i n d i c a t e d the degree of homogeneity of the mixing i n the compacts. P r i o r to s e c t i o n i n g f o r me t a l l o g r a p h i c work, the compacts were immersed i n an epoxy r e s i n under a vacuum. The r e s i n was "drawn" i n t o , and f i l l e d , the interconnected pores i n the compact. A f t e r the epoxy had hardened the samples were p o l i s h e d and the epoxy " f i l l e r " maintained the i n t e g r i t y of the interconnected pores. Chapter 3 OBSERVATIONS AND RESULTS 3.1 Observations: Blending, Compacting and Cleaning 3.1.1 Blending and Compacting Figu r e 16 i s a photomicrograph of a pressed and cleaned sample from Specimen Group PCM-2-XX. E x c e l l e n t mixing of the i r o n and copper powders i s evident. T y p i c a l l y , never more than four copper p a r t i c l e s were found to be "nearest neigh-bours" i n the s e c t i o n s examined; i . e . there was minimal c l u s t e r i n g of the copper p a r t i c l e s . M e t a l l o g r a p h i c s t u d i e s of other samples and Specimen Groups (PCM-6, PCM-A, PCM-5-XX and ATOCM-5) i n d i c a t e d a s i m i l a r l y high degree of mixing. Mixing t e s t s (chemical analyses) i n d i c a t e d t h a t the v a r i a t i o n of copper content from compact to compact, and along the length of i n d i v i d u a l compacts of the same Specimen Groups, was l e s s than three per cent of the amount of copper present. Table V I I I shows th a t f o r a given composition of powder mixture (containing l u b r i c a n t ) , the compacted d e n s i t i e s were lower f o r f i n e r powders. The v a r i a t i o n of compacted d e n s i t y 57 58 Figure 16. S e c t i o n of compact PCM-2-14 a f t e r c l e a n i n g . lOOx. The voids appear b l a c k , the copper i s the l i g h t e s t c o n s t i t u e n t observed. The grey areas are i r o n . ( N i t a l etch) 59 Table V I I I Average Density of Fe-2 2Cu-lG.afbow/ax Compacts Pressed at 60 k . s . i . Specimen Group Pressed Density Po (gm/cc) Powder S i z e u m (diam.) Clean Density Pc (% of t h e o r e t i c a l ) PCM-A 6.61 105 to 120 68.7 PCM-2-XX 6.34 53 to 63 70.7 PCM-5-XX 6.21 25 27 25 to 37 70.3 PCM-6 6.17 <25 68.7 60 f o r samples i n a given Specimen Group was l e s s than 0.8%. Sample PCM-3-1 was of a powder s i z e intermediate between those of Specimen Groups PCM-2-XX and PCM-5-XX but contained no Carbowax. PCM-3-1 compacted to a d e n s i t y of only 6.04 gm/cc i n d i c a t i n g t h a t the use of a l u b r i c a n t l e d t o higher compacted d e n s i t i e s f o r a given compacting pressure. 3.1.2 Cleaning Table IX shows i t h ^ complete r e s u l t s f o r the compacted d e n s i t i e s , cleaned d e n s i t i e s and changes i n weight, length and diameter of compacts which were cleaned. A l l compacts (except PM-2 which was pure i r o n powder) expanded appreciably as a r e s u l t of c l e a n i n g . The expansion was not a s s o c i a t e d w i t h the presence of l u b r i c a n t . The specimen PCM-3-1, which d i d not contain Carbowax 300, a l s o expanded during c l e a n i n g . Because no expansion was observed i n PM-2 which d i d not c o n t a i n copper; whereas a l l samples c o n t a i n i n g copper expanded, the expansion i s a s s o c i a t e d w i t h the presence of copper. Moreover, the amount of expansion was c l o s e l y r e l a t e d to the amount of copper powder i n the mixtures. The compacts d i d not expand i s o t r o p i c a l l y during c l e a n -i n g . The r a t i o of the expansion i n length t o t h a t i n the diameter ( A L % / A D %) ranged from 1.44 to 2.48 f o r d i f f e r e n t c c samples (see Table I X ) . The more f r e q u e n t l y used specimens, Table IX Compacting and Cleaning Sample Number Compacting Pressure Compacted Density Po Loss i n Weight (cleaning) Expansion i n Diameter AD % c Expansion i n Length AL % c Cleaned Density, p c Cleaned Density, % t h e o r e t i c a l * (AL %/AD %) c c k. s. £ ; gm/cc % % a *o gm/cc % PCM-2-3 60 6. 27 1.39 2.44 3.50 5.70 70.3 1.44 PCM-2-4 60 6.28 1.46 2.36 3.50 5.71 70.4 1.48 PCM-2-5 60 6.32 1.34 1.98 3.33 5.74 70.8 1.68 PCM-2-6 60 6.34 1.35 1.98 3.33 5.84 72.0 1.68 PCM-2-10 60 6.29 1.36 2.28 4.27 5.70 70.3 1. 87 PCM-2-12 60 6.33 1.61 2.74 4.15 5.67 69.9 1.51 PCM-2-13 60 6.28 1.40 2.59 3.94 5.66 69.8 1. 52 PCM-2-14 60 6.33 1.35 2.36 4.37 5.70 70.3 1.85 PCM-2-15 60 6.35 1.32 2.44 4.28 5.73 70.6 1.75 PCM-2-17 60 6.26 1.38 2.23 5. 00 5.62 69.3 2.24 PCM-2-18 60 6.35 1.37 2.28 4.95 5.70 70.3 2.17 PCM-2-19 60 6.34 1.35 2.13 5.01 5.71 70.4 2.35 PCM-2-20 60 - 6.33 1.37 2.28 4.09 5.74 70.8 1.79 PCM-2-21 60 6.44 1.38 2.28 4. 87 5.79 71.4 2.13 PCM-2-22 60 6.41 1.35 2.28 4.35 5.79 71.4 1.91 PCM-2-23 60 6.42 1.42 2.23 4.09 5. 81 71.6 1. 83 PCM-2-24 60 6.37 1.41 2.28 4337 5.75 70.9 1.98 PCM-2-25 60 6.41 1.39 2.28 4.11 5. 80 71.5 1.80 Average — 6.34 1.38 2.34 4.21 5.73 70.6 1.80 ND = not done CONTINUED t h e o r e t i c a l d e n s i t y based on: 7.87 gm/cc f o r pure i r o n 8.11 gm/cc f o r 78% Fe, 22% Cu mixture 7.97 gm/cc f o r 90% Fe, 10% Cu mixture 8.09 gm/cc f o r PA-2-1 mixture ** t o t a l l o s s , see Table X Table IX (Continued) Sample Number Compacting Pressure Compacted Density Po Loss i n Weight (cleaning) Expansion i n Diameter AD % c Expansion i n Length AL % c Cleaned Density, p c Cleaned Density, % t h e o r e t i c a l (AL c%/AD c%) k . s . i . gm/cc &% " ? % % gm/cc o 'o . PCM-5-6 PCM-5-7 PCM-5-9 PCM-5-12 PCM-5-13 60 60 60 60 60 6.25 6.19 6.21 6.22 6.18 1.47 .. 1.45** 1.48 1.51 1.65 1.67 1.98 ND 1.52 1.52 3. 80 3.82 ND 3.35 3.35 5.76 5.63 NND 5.74 5.70 71.0 69.4 70.8 70.3 2.27 1.93 2.20 2.20 Average — 6.21 1. 51 1.67 3.58 5.71 70.3 2.15 PM-2 PCM-3-1 PCM-6 PCM-A PCM-UF-2 PCM-2-30 PCM-2-100 PA-2-1 , ATOCM-5 60 60 60 60 60 180 60 60 60 5.95 6.04 6.17 6.61 6.46 7.43 6.21 6.36 6.28 0.35 0. 40** 1.68 2. 06 1. 51 1.11 1.16 1.36 1.19 0 1.98 1.52 3.04 1.94 1.98 1.52 1.75 1.52 0.13 3.78 2.60 5. 86 3.65 3.71 2.76 3.02 1 3.77 5.92 5.57 5.75 5.75 5.90 6.81 5.79 5. 88 5.82 75.2 68.7 68.7 68.7 72.7 84.0 72.7 72.7 71. 8 1.71 1.92 1.88 1.87 1. 82 1.73 2.48 63 from Groups PCM-2-XX and PCM-5-XX had average ( A L C % / A D C % ) r a t i o s of 1.8 and 2.2 r e s p e c t i v e l y . Each r a t i o i s the r e s u l t of four measurements which were accurate to 0.10% of the sample l e n g t h . Thus, a r a t i o of ( A L % / A D %) which i s reported o c as 2.00 could l i e between 1.65 and 2.45 due to the inaccuracy of the measurement technique. Even w i t h i n t h i s l i m i t a t i o n however, the r e s u l t s i n d i c a t e g r e a t e r anisotropy of expansion i n the f i n e r powders. Although length and diameter expansions* were unequal f o r a l l Fe-Cu compacts, i t should be noted t h a t no shape d i s t o r t i o n occurred during c l e a n i n g . A f t e r c l e a n i n g , the diameter and length of a given compact were uniform w i t h i n 0.001 inches. Weight losses during c l e a n i n g r e s u l t e d from the removal of Carbowax by v o l a t i l i z a t i o n and from the r e d u c t i o n of oxides on the metal powder p a r t i c l e s . The r e s u l t s i n Table IX were c o n s i s t e n t w i t h expectations; i . e . a) S a m p l e s PM-2 and PCM-3--I- c o n t a i n e d no Carbowax; t h e i r w e i g h t l o s s e s o f a p p r o x i -m a t e l y 0.4$ were due t o o x i d e r e d u c t i o n . b) S a m p l e s o f Group PCM-2-XX c o n t a i n e d I wt. % o f C a r b o w a x , and t h e i r w e i g h t l o s s e s were a c c o r d i n g l y a p p r o x i m a t e l y 1.4$. The v a r i a t i o n s a r e a t t r i b u t e d m a i n l y t o 64 n o n - u n i f o r m i t y o f m i x i n g o f Carbowax w i t h t h e p o w d e r s . c ) PCM-A o r i g i n a l l y c o n t a i n e d 2% C a r b o w a x , some o f w h i c h was s q u e e z e d o u t o f t h e compact d u r i n g p r e s s i n g ( o b s e r v e d ) . d) S p e c i m e n s o f Group PCM-5-XX and s a m p l e PCM-6 were made fr o m v e r y f i n e p o w d e r s , t h e o r i g i n a l o x i d e c o n t e n t s o f w h i c h were h i g h e r t h a n i n Group PCM-2-XX , e) B e c a u s e PCM-2-30 was comp ao^e'd r a t ^a h i g h p r e s s u r e , i t l o s t more o f i t s \% , Carbowax t h a n o t h e r s p e c i m e n s d u r i n g p r e s s i n g . A l l c o m p a c t s e x h i b i t e d c l e a n e d d e n s i t i e s o f 70.5% ± 2.5% o f t h e o r e t i c a l s o l i d d e n s i t y , e x c e p t i n t h e s p e c i a l c a s e s o f PCM-2-30 ( p r e s s e d a t 180 k . s . i . ) and PM-2 ( p u r e i r o n ) . The r e s u l t s of cl e a n i n g t e s t s on samples which con-t a i n e d 1% Carbowax (PCM-5-7 and PCM-2-4) and one which con-t a i n e d no Carbowax (PCM-3tl) are given i n Table X. The data i n d i c a t e d t h a t a l l the Carbowax and most of the oxygen i n the samples was l o s t i n the f i r s t hour of clea n i n g at 60 0°C. The r e s u l t s of c l e a n i n g at 7 00°C f o r longer times than those used i n the standard procedure was an almost n e g l i g i b l e amount of a d d i t i o n a l weight l o s s (0.01%). 65 Table X Weight Loss During Cleaning Sample Weight l o s s a f t e r 1 hour at 600°C T o t a l cumulative l o s s a f t e r 1 hr. at 600°C & 2 hrs. at 700°C T o t a l cumulative weight l o s s a f t e r 1 hr. at 600°C, 3 h r s . at 700°C PCM-5-7 1.25% 1.44% 1.45% PCM-2-4 1.30% 1.40% N/P PCM-3-1 0.31% 0.39% 0.40% N/P .= not performed 66 3.2 Changes i n Weight and Dimensions During Dilatometry Table XI contains data showing the t o t a l changes i n dimensions of samples heated i n the d i l a t o m e t e r . Three f i g u r e s are recorded: i ) % A L S M , t h e change i n l e n g t h as m e a s u r e d w i t h c a l i p e r s . i i ) % AD^^, t h e c hange i n d i a m e t e r as m e a s u r e d w i t h c a l i p e r s . i i i ) % Al_2p, t h e c hange i n l e n g t h r e c o r d e d by t h e d i l a t o m e t e r . The changes were c a l c u l a t e d as a percentage of the appropriate dimension of the sample a f t e r c l e a n i n g . A s i n g l e c a l i p e r measurement was accurate to ±0.0005" or ±0.10% of the sample length (or diameter). Since % ALg M and % AD_„ are d i f f e r e n c e s between two measurements. ±0.20% SM ' e r r o r was p o s s i b l e due to measuring u n c e r t a i n t y . The d i l a t o -meter was accurate to ±0.05% of the sample length . Thus the disagreement between % ALg M and % ALg D f o r a s i n g l e run could have been as high as ±0.25% due to the u n c e r t a i n t y of the measuring technique alone. For t h i s reason, an agreement between % AL^„ and % AL O T N, f o r a s i n g l e run, of b e t t e r than SM SD' • ' 0.50% was considered to be acceptable. D i f f e r e n c e of 0.50% to 0.75% were a l s o considered reasonable i n those cases where a compact might have shrunk s u f f i c i e n t l y to come to r e s t on the thermocouple during the c o o l i n g stage at the end of the run. This could cause a sample to l i f t o f f the l a v a 6 7 block base. I f the i n t e r f e r e n c e w i t h sample shrinkage by the thermocouple occurred, the e f f e c t would be confined t o the c o o l i n g stage of the run ( a f t e r the power was cut o f f ) ; i . e . , the d i l a t o m e t e r curve would not be a f f e c t e d up to the p o i n t where heating of the sample was terminated. The d i f f e r e n c e between % AD.,., and % A L 0 . . f o r a given SM SM ^ sample could have been as l a r g e as ±0.40% (even i f the sample shrank i s o t r o p i c a l l y ) due to measuring u n c e r t a i n t y . The observed d i f f e r e n c e s were greater than ±0.40% f o r a p p r o x i -mately two-thirds of the runs; i . e . the shrinkage was g e n e r a l l y not i s o t r o p i c . About one out of three samples shrank more i n the diameter than i n the l e n g t h , w h i l e the others showed more shrinkage i n the l e n g t h than i n the diameter. Approximately h a l f of the samples e x h i b i t e d a taper i n the diameter; however, the d i f f e r e n c e between the l a r g e s t and s m a l l e s t diameter measured i n a sample was l e s s than 3% of the average diameter. A l l the samples which had tapers were l a r g e r at the bottom,, w i t h the one exception of PCM-UF-2. None of the samples i n which copper remained s o l i d , during the whole run, e x h i b i t e d any s i g n i f i c a n t taper i n the diameter. I t i s t h e r e f o r e probable t h a t the t a p e r i n g r e s u l t e d from sagging of the compacts, due to t h e i r weight (plus t h a t of the transducer c o r e ) , i n the presence of a l i q u i d phase. A l l samples l o s t weight during s i n t e r i n g i n the d i l a t o m e t e r (see Table X I ) . Comparison of Runs PCM-2-12 Table XI T o t a l Change i n Dimensions and Weight During Dilatometry Specimen Melted 1 A8SM % A LSM % A LSD Measuring method showing most (Run) ( c a l i p e r s ) ( c a l i p e r s ) d ilatometer shrinkage % % O C a l i p e r s Dilatometer PCM-2-12 N -0. 96 -0.82 — — PCM-2-13 N -0. 89 -1.49 — — PCM-2-21 Y -1.15 -2.iZ6 -- — — PCM-2-18 Y -1.34 -2.22 -2.38 — Y PCM-2-17 Y -0.7 5 -1.34 — — — PCM-2-22 Y -0.45 -0.63 -S0998 — Y PCM-2-20 Y -2.31 -2.82 -S2S83 — Y PCM-A Y -1.03 -2.48 - T 2 . 4 1 Y — PCM-6 Y -6.37 -5.74 -6.05 — Y PCM-UF-2 Y -1.94 -2.33 -2.61 — Y PCM-5-6 Y -5.54 -5.45 -5.33 Y — PCM-2-10 Y -2.98 -2.32 -3.00 — Y PCM-2-19 Y -3.17 -3.00 -2.82 Y — PCM-5-13 Y -6.30 -6.16 -5.86 Y — PCM-5-12 Y -6. 00 -5.57 -5.18 Y — PCM-2-23 Y -2.75 -3.02 -2.82 Y — PCM-2-24 Y -1.04 -1.68 -1.86 — Y PCM-2-25 Y -2.23 -2.39 -2.29 Y — PCM-2-30 Y + 0.67 -0. 81 -0.38 Y — PCM-2-100 Y + 0.08 -0.40 -0.68 — Y PA-2-1 Y -3.44 -3.99 -3. 85 Y — ATOCM-5 Y -3.77* -5.31* -5.01* Y — PM-2 Y -0.38 -0.47 -0.54 — Y Oxidized at end of run Y = yes 00 Table XI (Continued) Specimen %AL -%AL %AD -%AL Sinte r e d Weight l o s s (Run) SM °fl SD " SM °aJjSD d e n s i t y , . p g %AW s % Q. "O gm/cc Q. "O PCM-2-12 -- -- 5.84 0.09 PCM-2-13 — -- 5.85 0.14 PCM-2-21 -- -- 6.11 0.11 PCM-2-18 + 0.16 5.98 0.18 PCM-2-17 — 5.77 0.19 PCM-2-22 + 0.35 • + 0.52 5.75 0.24 PCM-2-20 + 0.01 + 0.52 6.20 0.42 PCM-A -0.07 +1.38 6.02 0.42 PCM-6 + 0.31 -0.32 6.93 0.61 PCM-UF-2 + 0.28 +0.67 6.26 0.47 PCM-5-6 -0.12 -0.21 6.79 0.64 PCM-2-10 +0.68 + 0.02 6.17 0.54 PCM-2-19 -0.18 -0.35 6.26 0.58 PCM-5-13 -0.30 -0.44 6.89 0.51 PCM-5-12 -0.39 -0.82 6.85 0.54 PCM-2-23 -0.20 +0.07 6.30 0.53 PCM-2-24 + 0.18 • +0.82 5.97 0.41 PCM-2-25 -0.10 +0.06 6.19 0.44 PCM-2-30 -0.43 +1.05 6.74 0.45 PCM-2-100 +0 .28 +0.76 5.78 0.76 PA-2-1 -0.14 + 0.41 6.56 0.41 ATOCM-5 -0.30* +1.54* — -0.20* . PM-2 + 0.07 -0.09 — 0.21 70 through PCM-2-20 i n d i c a t e s t h a t the weight losses increased w i t h longer s i n t e r i n g times i n the d i l a t o m e t e r , reaching a l e v e l of approximately 0.40% t o 0.60% f o r runs of up t o two hours. However, about 0.15 to 0.20% change i n weight occurred a f t e r s h o r t times ( i . e . before m e l t i n g , see Table XI; Runs PCM-2-12 and PCM-2-13). I t i s l i k e l y t h a t much of the e a r l y weight l o s s was due to r e d u c t i o n of oxides ( s t a b l e below 700°C) i n the compacts. As f u r t h e r evidence of t h a t , the pure i r o n specimen, PM-2, l o s t 0.2% weight a f t e r 18 minutes of the normal heating c y c l e . Of the 0.2 t o 0.4% weight l o s s a f t e r m e l t i n g , some was due to the evaporation of copper out of the compacts. Copper was observed to have condensed on the s i l i c a d i s c s and the alumina sheath i n the di l a t o m e t e r . However, chemical analyses i n d i c a t e d t h a t not more than 0.2% weight l o s s r e s u l t e d from copper l o s s . The remainder of the weight l o s s up to 0.2% i s considered t o have been due t o l o s s of oxygen from the compacts during s i n t e r i n g above the me l t i n g p o i n t of copper. The f i g u r e s f o r weight l o s s and dimensional changes f o r the run ATOCM-5 should not be compared w i t h other samples. The hydrogen flow f a i l e d at the end of the run, w h i l e the sample was being cooled.to room temperature. As a r e s u l t , the sample o x i d i z e d to a considerable extent, and expanded. The d i l a t o m e t e r data, however, were u s e f u l because the hydrogen supply d i d not f a i l u n t i l the c o o l i n g stage. 71 3.3 D i l a t o m e t r i c Data f o r a Pure Iron Compact The c o r r e c t e d d i l a t o m e t e r p l o t f o r the pure i r o n compact, PM-2, i s reproduced i n F i g u r e 17. The "normal" heating c y c l e used f o r t h i s specimen i s shown i n F i g u r e 12. At no stage was any l i q u i d phase formed. In common w i t h a l l d i l a t o m e t e r p l o t s i n t h i s work, Fi g u r e 17 presents the percentage change i n length of the compact ( r e l a t i v e to i t s as-cleaned length) versus the time a f t e r which heat was f i r s t a p p l i e d to the specimen i n the d i l a t o m e t e r . For the sake of c l a r i t y , the f i n a l p o r t i o n of the experimental d i l a t o m e t e r p l o t s , which showed the shrinkage of specimens when heating ceased, has been omitted from the curves presented i n t h i s work. While the compact was being heated to 500°C (point J i n F i g u r e 17) a r a p i d increase i n l e n g t h occurred due to thermal expansion. The amount of expansion at 500°C i s c l o s e l y comparable to what would be p r e d i c t e d f o r f u l l y dense pure i r o n , based on published thermal expansion c o e f f i c i e n t s . Above about 500°C, s o l i d - s t a t e s i n t e r i n g became r e l a t i v e l y r a p i d i n compacts of approximately 55y m i r o n powder. The marked decrease i n the slope of the d i l a t o m e t e r p l o t thus r e f l e c t e d the modifying i n f l u e n c e of s i n t e r i n g shrinkage on thermal expansion. A 'net 1 expansion was observed between 500°C and 750°C. Between p o i n t s K and L F i g u r e 17. Dilatometer p l o t f o r PM-2; a pure i r o n specimen heated w i t h a normal c y c l e . 73 (see F i g u r e 17), the temperature was he l d constant at 750°C f o r approximately three minutes during which time s i n t e r i n g shrinkage continued, and a s m a l l 'net' c o n t r a c t i o n was observed. Between 750°C and approximately 900°C (point M, Fig u r e 17) the a to y transformation occurred i n the low-carbon i r o n , and the di l a t o m e t e r p l o t r e f l e c t e d the change from a higher to lower thermal expansion c o e f f i c i e n t . Further heating t o 1000°C (point N, Figu r e 17) produced expansion i n the y - i r o n . Because s o l i d s t a t e s i n t e r i n g at low tempera-tures i n the y phase i s i n t r i n s i c a l l y slow (associated w i t h a low s e l f - d i f f u s i o n c o e f f i c i e n t ) , l i t t l e s i n t e r i n g shrinkage occurred simultaneously i n t h a t r e g i o n . For the same reason, h o l d i n g the specimen at 1000°C (between p o i n t s N and P, Fig u r e 17) produced l i t t l e dimensional change. Further heating to 1155°C (point Q, Figure 17) caused some net expansion. At a constant temperature of 1155°C, the r a t e of s o l i d s t a t e s i n t e r i n g was s u f f i c i e n t l y high t o produce a measureable amount of shrinkage i n a few minutes. 3.4 S i n t e r i n g Behaviour of Fe-22 Cu Mixtures 3.4.1 Dilatometer P l o t f o r PCM-2-20 The c o r r e c t e d d i l a t o m e t e r p l o t f o r an Fe-22% Cu mixed-powder compact, PCM-2-20, heated w i t h the 'normal' c y c l e , 74 i s presented i n Figure 18. The form of the curve i s t y p i c a l of t h a t obtained f o r a l l Fe-Cu specimens, i n which the f i v e stages of dimensional change i n d i c a t e d i n Figu r e 18 were observed. I n f l e c t i o n s i n the curve, at po i n t s A, B, C and D represent times at which one dominant process of expansion or c o n t r a c t i o n e i t h e r ended or gave way to a new and more r a p i d process. P o i n t s A, B, C and D are simply maxima or minima i n the d i l a t o m e t e r curve, f o r which an eq u i v a l e n t was observed i n p l o t s f o r a l l Fe-Cu compacts. The f i r s t e i g h t d i l a t o m e t r i c runs l i s t e d i n Table X I I (PCM-2-12 to PCM-2-22) were e s s e n t i a l l y reproductions of p o r t i o n s of Run PCM-2-20, i n which the heating was terminated at various s h o r t e r i n t e r v a l s . The end-points of these runs are i n d i c a t e d i n Figu r e 19. The purpose of those runs was to provide specimens f o r m e t a l l o g r a p h i c examination i n order to study the s t r u c t u r a l changes a s s o c i a t e d w i t h the var i o u s stages of dimensional change observed i n Fi g u r e 18. The short runs i n d i c a t e d that the e a r l y p o r t i o n s of Run PCM-2-20 could be reproduced w i t h an accuracy of ±0.15% AL/L 0. 3.4.2 S o l i d State Dimensional Changes Stage I i n Fi g u r e 18 i s analogous to the f i r s t p a r t of the dil a t o m e t e r curve f o r the pure i r o n specimen of comparable powder s i z e (up to p o i n t J i n Figu r e 17) i n which simple thermal expansion i n i t i a l l y occurred. Because copper 75 Table X I I Runs which were Reproductions of the E a r l y Stages of PCM-2-20. A l l Runs were Made w i t h the 'Normal' Heating Cycle. Sample Run was stopped a t : time (mins) temperature (°C) PCM-2-12 9.8 1000° PCM-2-13 12.6 1040° PCM-2-6 13.5 1060° PCM-2-5 14.0 1070° PCM-2-21 14.1 107 5° PCM-2-18 •14.. 5 1100° PCM-2-17 16.0 1155° PCM-2-22 •17.4 1155° PCM-2-20 122.2 1155° 3.0 F i g u r e 19. E a r l y stages of Run PCM-2-20, showing where samples i n Table XII were stopped. 78 has a higher thermal expansion c o e f f i c i e n t than i r o n i n t h i s temperature range, and because there i s a degree of c o n t i n u i t y of copper i n p o r t i o n s of Fe-Cu compacts, a gr e a t e r amount of expansion i s seen i n the l a t t e r . Toward the end of Stage I , s o l i d s t a t e s i n t e r i n g shrinkage occurred s i m u l -taneously w i t h thermal expansion, and the net r a t e of dimen-s i o n a l change d e c l i n e d . At p o i n t A i n Figure 18 (the end of Stage I ) , the s i n t e r i n g shrinkage r a t e exceeded the thermal expansion r a t e and the curve assumed a negative slope. During Stage I I (from A to B i n Figure 18), s o l i d s t a t e s i n t e r i n g was so r a p i d t h a t a marked net c o n t r a c t i o n of the specimen was seen, even though the temperature was r i s i n g . At B, where a temperature of 1065°C to 1070°C was a t t a i n e d , the r a t e of c o n t r a c t i o n f u r t h e r i n c r e a s e d , as a r e s u l t of the onset of melting of copper, which i s discussed below. Thus the behaviour of the pure i r o n and the Fe^-22% Cu compacts was s t r i k i n g l y d i f f e r e n t above about 800°C w i t h the same heating c y c l e . I t i s apparent t h a t s o l i d s t a t e s i n t e r i n g of Fe-Cu powder mixtures was much more r a p i d than t h a t of pure i r o n powder of the same p a r t i c l e s i z e and app r o x i -mately the same i n i t i a l d e n s i t y . While t h i s o bservation i s i n t e r e s t i n g , i t has not been explored i n d e t a i l i n the present i n v e s t i g a t i o n ? x 79 3.4.3 The Onset of M e l t i n g The o p t i c a l m i c r o s t r u c t u r e s of PCM-2-12 (Figure 20) and PCM-2-13 (not shown), which were heated to 1000°C and 1040°C r e s p e c t i v e l y , were i d e n t i c a l and not d i s t i n g u i s h a b l e from those of comparable as-cleaned Fe-Cu-specimens (see Se c t i o n 3.1). Me t a l l o g r a p h i c s e c t i o n s of the compacts PCM-2-6, PCM-2-25 and PCM-2-21 (which ,were heated to 1060°C, 1070°C and 1075°C) were taken p a r a l l e l to the l o n g i t u d i n a l a x i s , through the centre of the compacts, as shown i n Figure 21. A photomicrograph of the f i n a l s t r u c t u r e of PCM-2-6 at l o c a t i o n A (near the top of the compact, see F i g u r e 21) which was t y p i c a l of the s t r u c t u r e at a l l l o c a t i o n s i n the sample i s reproduced i n Fig u r e 22. D i s c r e t e p a r t i c l e s of copper are c l e a r l y v i s i b l e . Many had been deformed (during compaction) but none had melted as a r e s u l t of heating to 1060°C (at the specimen c e n t r e ) . F i g u r e 22 a l s o r e v e a l s the t y p i c a l l y i r r e g u l a r shape of the Easton i r o n powder p a r t i c l e s , some of which contained i n t e r n a l pores. Fig u r e 23 i s a photomicrograph of the s t r u c t u r e at l o c a t i o n B i n sample PCM-2-21, the centre of which reached 1075°C. The copper had obv i o u s l y melted and flowed through the compact. The same s t r u c t u r e was observed at a l l l o c a t i o n s i n t h a t sample; i . e . a l l the copper had melted by the time the centre of the compact reached 1075°C. Figure 20. A s e c t i o n of sample PCM-2-12 at lOOx. The specimen was heated to 1000°C w i t h a normal heating c y c l e . The l i g h t areas are copper; the dark grey areas, i r o n ; and the black areas, v o i d s . ( N i t a l etch) Figure 21. Se c t i o n of compacts used f o r metallography. A, B, C and D are d i f f e r e n t l o c a t i o n s which were examined by o p t i c a l microscopy. at i t s centre. ( N i t a l etch) Figure 23. A s e c t i o n i n sample PCM-2-21 at 300x, taken at l o c a t i o n "B". The specimen was heated 1075°C a t its centre. ( N i t a l etch) 82 In sample PCM-2-5, the centre of which was heated to 1070°C, the extent of m e l t i n g of the copper v a r i e d w i t h p o s i t i o n i n the compact. This could be detected macro-s c o p i c a l l y (see Figure 24) and m i c r o s c o p i c a l l y (see Figures 25 to 31). Near the top of the specimen, at p o s i t i o n s A and B, no copper had melted (see Figures 25 and 26). At p o s i t i o n s C and D some, or a l l , of the copper had melted and begun to flow through the compact (see Figures 27 and 28). Pores were created where copper formerly e x i s t e d as d i s c r e t e p a r t i c l e s , and many of the o r i g i n a l voids between i r o n p a r t i c l e s were f i l l e d . More extensive flow of l i q u i d copper was evident at p o s i t i o n s C and D (Figures 29 and 30), and the pores l e f t behind by the moving copper were even l a r g e r than at p o s i t i o n B (Figure 31). The v a r i a t i o n s observed i n the macrostructure (Figure 24) can thus be i n t e r p r e t e d i n terms of v a r i a t i o n s i n the s i z e and d i s t r i b u t i o n of pores. The voids are l a r g e s t i n a c e n t r a l zone of the compact ( i d e n t i f i e d as Area #1 i n Figure 24), i n d i c a t i n g t h a t t h a t zone reached the m e l t i n g temperature f i r s t . Copper i n regions near the top and bottom of the compact (Areas #2V,and #3 i n Figure 24) was e i t h e r not melted or was melted f o r a s h o r t e r time than i n the c e n t r a l region (Area #1). Thus a temperature g r a d i e n t e x i s t e d i n the compact when the centre was at 1070°C. Figure 24. Macrophoto of PCM-2-5 at 5x. The specimen was heated 1070°C at the centre, the s e c t i o n was taken almost through the centre of the compact ( i . e . .035" away). ( N i t a l etch) Figure 25. A s e c t i o n i n sample PCM-2-5 at 300x, taken at p o s i t i o n 'A'. ( N i t a l etch) Figure 27. A s e c t i o n i n sample PCM-2-5 a t 300x, taken at l o c a t i o n *C'. ( N i t a l etch) Figure 28. A s e c t i o n i n specimen PCM-2-5 at 300x, taken at p o s i t i o n 'D'. ( N i t a l etch) Figure 29. A s e c t i o n i n specimen PCM-2-5 at lOOx, taken at l o c a t i o n 'C. ( N i t a l etch) Figure 31. A section i n specimen PCM-2-5 at lOOx, taken at p o s i t i o n 'B1. 87 3.4.4 Temperature Gradients i n the Dilatometer Specimens To summarize, the observations i n Se c t i o n 3.4.3: a) T h e r e was no m e l t i n g o f c o p p e r a n y w h e r e i n a compact h e a t e d t o I060°C a t t h e c e n t r e . b) T h e r e was m e l t i n g o f c o p p e r e v e r y w h e r e i n a compact h e a t e d t o I075°C a t t h e ce nt r e . c ) T h e r e was m e l t i n g o f c o p p e r i n a r e g i o n w h i c h r e a c h e d I070°C. I t should a l s o be noted t h a t i n d i l a t o m e t r i c runs, the s t a r t of r a p i d Stage I I I c o n t r a c t i o n (Point B ) , which was as s o c i a t e d w i t h the melting of copper, c o n s i s t e n t l y occurred when the centre of the compact reached 1065°C t o 1070°C. The tempera-ture gradients (both v e r t i c a l and d i a m e t r a l ) , i n compacts are t h e r e f o r e u n l i k e l y to have exceeded 5 to 10°C at the melting temperature of the copper. M e l t i n g of the copper i n the compacts was observed to occur at temperatures between 1065°C and 1070°C. The e q u i l i b r i u m melting p o i n t of pure copper i s 1084.5°C. I t i s t h e r e f o r e evident t h a t the compacts contained one or more i m p u r i t i e s which lowered the me l t i n g p o i n t of copper t o below 1070°C. M e t a l l i c i m p u r i t i e s can account f o r only 2 to 3 88 degrees of the decrease. The only remaining p o s s i b i l i t y i s t h a t oxygen was present i n the compacts at m e l t i n g . In S e c t i o n 3.2, a weight l o s s of up to 0.2 weight % by the compacts, f o l l o w i n g m e l t i n g , was a t t r i b u t e d to the removal of r e s i d u a l oxygen during l i q u i d - p h a s e s i n t e r i n g . According to the Cu-0 phase-diagram, 0.2 weight % oxygen i n copper lowers the l i q u i d u s temperature of copper to 1068°C and, as l i t t l e as 0.002% oxygen lowers the s o l i d u s tempera-ture to 1066°C. I t i s t h e r e f o r e concluded t h a t the presence of oxygen i n the compacts caused the low observed me l t i n g p o i n t of the copper. 3.4.5 M i c r o s t r u c t u r a l Changes Coincident w i t h M e l t i n g In Run PCM-2-5 melt i n g of the copper was observed to have been complete near the centre of the compact which reached 1070°C. Att a c k on the i r o n powder by the l i q u i d copper was e x t e n s i v e , as revealed i n Figures 27, 28 and 32. Extremely f i n e i r o n p a r t i c l e s (some l e s s than one micron i n diameter) , "'.are observed i n the m i c r o s t r u c t u r e at the i n s i d e of pores and i n the dense i n t e r v o i d areas (see Figure 28). These p a r t i c l e s were apparently formed by the d i s i n t e g r a t i o n of l a r g e r p a r t i c l e s through the a c t i o n of l i q u i d copper. Because copper i s capable of d i s s o l v i n g up to 3.7% i r o n at the m e l t i n g temperature, i t i s l i k e l y t h a t a d i s s o l u t i o n process was i n v o l v e d i n the d i s i n t e g r a t i o n . Figure 32. A s e c t i o n i n specimen PCM-2-5 at 1200x. ( N i t a l etch) 90 Some d i f f u s i o n of copper along the g r a i n boundaries i s evident i n Figures 2 8 and 32 as dark etching bands w i t i n the i r o n p a r t i c l e s . Gamma i r o n i s capable of d i s s o l v i n g 7.9% copper at 1070°C. In the lower l e f t corner of F i g u r e 32 i s what may have been a g r a i n boundary (or p o s s i b l y , a s o l i d neck between two i r o n p a r t i c l e s ) which was penetrated or d i s s o l v e d by l i q u i d copper. However, the m a j o r i t y of g r a i n boundaries d i d not co n t a i n l i q u i d copper, and few instances of copper having penetrated i r o n g r a i n boundaries were observed i n th a t sample. A few s o l i d contacts between i r o n p a r t i c l e s were observed i n PCM-2-5 but most p a r t i c l e s were found t o be i s o l a t e d from t h e i r neighbours by t h i n f i l m s of copper which formed between p a r t i c l e s as a r e s u l t of the p e n e t r a t i o n and/or d i s s o l u t i o n of necks. The s t r u c t u r e of the compacts imme-d i a t e l y f o l l o w i n g melting was, then, a d i s p e r s i o n of i r o n -r i c h p a r t i c l e s i n a copper m a t r i x , r a t h e r than the continuous i r o n s k e l e t o n which i s expected i n a s o l i d - s t a t e s i n t e r e d aggregate. 3.4.6 S t r u c t u r a l Changes i n Stages I I I , IV and V Figur e 33 i s a high m a g n i f i c a t i o n photomicrograph of PCM-2-18 which was stopped at the end of Stage I I I (at 1100°C). Pools of copper are observed i n the g r a i n boundaries 91 Figure 33. A s e c t i o n i n sample PCM-2-18, 1200x. ( N i t a l etch) 92 of the l a r g e r i r o n p a r t i c l e s ( i n Figure 33), i n d i c a t i n g t h a t the p e n e t r a t i o n of i r o n g r a i n boundaries by l i q u i d copper was underway i n Stage I I I . D i f f u s i o n of copper along the i r o n g r a i n boundaries (shown by the darker areas adjacent to the boundaries) and volume d i f f u s i o n of copper i n t o the i r o n , are both evident. \v-...ra 34 ohows c • J ~ r Fig u r e 34 shows another area i n the same sample. The t h i n f i l m s of copper separating adjacent i r o n p a r t i c l e s were t y p i c a l of the sample and i n d i c a t e that the d i h e d r a l angle of the system was zero. The s t r u c t u r e at the end of Stage I I I can s t i l l be described as 'dispersed'. Figures 35 to 38 are photomicrographs of PCM-2-17 which was stopped approximately half-way through Stage IV expansion. The p o l i s h i n g and etching technique f o r the samples i n Fi g u r e 35 to 38 were i d e n t i c a l to those used i n preparing previous samples. The d i f f e r e n c e s between the Stage I I I and Stage IV s t r u c t u r e s are con s i d e r a b l e . P e n e t r a t i o n of a few i r o n g r a i n boundaries by copper was i n progress i n the l a r g e p a r t i c l e i n the centre of Figure 35. Both g r a i n boundary and volume d i f f u s i o n of copper i n t o the i r o n were underway and were more advanced than i n the Stage I I I s t r u c t u r e s . Figures 36 and 37 show tha t the s t r u c t u r e was s t i l l 'dispersed'. Small p a r t i c l e s of i r o n were fewer and of l a r g e r average diameter than they were i n samples which were stopped i n Stage I I I . The disappearance of the s m a l l e s t F i g u r e 37. A s e c t i o n i n PCM-2-17, 300x. ( N i t a l etch) 95 Fi g u r e 38. A s e c t i o n i n PCM-2-22, 1200x. ( N i t a l etch) 96 p a r t i c l e s i n d i c a t e s the operation of a s o l u t i o n - r e p r e c i p i t a -t i o n p a r t i c l e growth process during Stages I I I and IV. Not only small p a r t i c l e s , but a l s o areas of small radius of curvature i n l a r g e r p a r t i c l e s d i s s o l v e d p r e f e r e n t i a l l y and r e p r e c i p i t a t e d on surfaces of l a r g e r r a d i u s . F i g u r e s 38 and 39 show s e c t i o n s of PCM-2-22 which had almost completed Stage IV expansion when the run was stopped. The average p a r t i c l e s i z e a f t e r s i n t e r i n g was l a r g e r than i n samples stopped at e a r l i e r times, due to the s o l u t i o n -r e p r e c i p i t a t i o n mechanism (compare Figures 39 and 37). The e l i m i n a t i o n of sm a l l ia?on p a r t i c l e s was v i r t u a l l y complete by the end of Stage IV. Many p a r t i c l e s e x h i b i t e d n o n - s p h e r i c a l shapes r e s u l t i n g from the d i s i n t e g r a t i o n of the o r i g i n a l (more s p h e r i c a l ) i r o n p a r t i c l e s by g r a i n boundary p e n e t r a t i o n . The long t h i n f i l m s of copper sep a r a t i n g s o l i d surfaces i n the lower r i g h t area of the photograph i n Figu r e 3 8 are t y p i c a l of the specimen, and are b e l i e v e d to have r e s u l t e d from the complete p e n e t r a t i o n of an a u s t e n i t e g r a i n boundary (or boundaries). The presence of copper i n such a narrow c a p i l l a r y i n d i c a t e s t h a t the d i h e d r a l angle of the system was zero l a t e i n Stage IV. As expected, the d i f f u s i o n of copper i n t o the i r o n was even more advanced i n PCM-2-22 than i n shor t e r - r u n samples. However, the p r o p o r t i o n of l i g h t etching r e g i o n s , which were not a l l o y e d to more than 1% copper was s t i l l l a r g e , i n d i c a t i n g 97 98 t h a t d i f f u s i o n was by no means complete. V i r t u a l l y a l l the g r a i n boundaries i n the i r o n had been penetrated by copper i n PCM-2-22. Figures 40 and 41 are photomicrographs of PCM-2-20, which was h e l d f o r 100 minutes at 1155°C. The degree of i r o n p a r t i c l e - i r o n p a r t i c l e c o n t i g u i t y (contact) was so extensive t h a t there can be no doubt t h a t the d i h e d r a l angle had become gre a t e r than zero at t h i s stage i n s i n t e r i n g . In f a c t , the i r o n - r i c h p a r t i c l e s i n PCM-2-20 e x i s t e d as a s o l i d s k e l e t o n w i t h copper f i l l i n g many of the voids i n the sk e l e t o n . The i r o n ' p a r t i c l e s ' i n PCM-2-20 were more s p h e r i c a l , and had even more re g u l a r surfaces than those i n PCM-2-22. They were a l s o shaped i n such a manner as t o allow high packing e f f i c i e n c y , as i f the Kingery ' s o l u t i o n due to pressure' mechanism had operated i n Stage V before <j> became p o s i t i v e . There i s f u r t h e r evidence of coalescence of i r o n p a r t i c l e s i n Figure 40 where l a r g e necks had formed between adjacent i r o n p a r t i c l e s . The f o l l o w i n g observations were made concerning the e v o l u t i o n of the pore s t r u c t u r e during Stages I I I , IV and V: A) In S t a g e II ( s e e F I g u re- 4-2) : v o i d s were s m a l l and e v e n l y d i s t r i b u t e d t h r o u g h t h e c o m p a c t . Figure 40. A s e c t i o n i n PCM-2-20, 300x. ( N i t a l etch) 100 Figure 42. A s e c t i o n i n PCM-2-6, lOOx. ( N i t a l etch) 101 B) In l a t e S t a g e I I I ( s e e F i g u r e . .43): t h e c o p p e r had m e l t e d and d i s p e r s e d t h r o u g h t h e compact l e a v i n g l a r g e v o i d s where t h e o r i g i n a l c o p p e r p a r t i c l e s e x i s t e d . T h ose v o i d s s e p a r a t e d d e n s e r a r e a s o f i r o n and c o p p e r ( i n t e r v o i d a r e a s ) . C) In l a t e S t a g e IV ( F i g u r e s . 44 and 4 5 ) : t h e p o r e s t r u c t u r e was s t r i k i n g l y d i f f e r e n t f r o m t h a t i n l a t e S t a g e I I I . The p o r e s were more e v e n l y d i s t r i b u t e d t h r o u g h o u t t h e c o m p a c t . Few l a r g e i n t e r v o i d a r e a s r e m a i n e d . D) In l a t e S t a g e V ( s e e F i g u r e 4 6 ) : t h e p o r e s were l e s s numerous and s m a I l e r t h e n i n l a t e S t a g e IV. L a r g e i n t e r v o i d a r e a s a g a i n e x i s t e d and e x t e n s i v e c o a l e s c e n c e between i r o n p a r t i c l e s was e v i d e n t . The change i n pore d i s t r i b u t i o n i n Stage IV, to a more r e g u l a r d i s t r i b u t i o n , i s considered to have r e s u l t e d from the d i s s o l u t i o n of c l u s t e r s of small i r o n p a r t i c l e s i n the denser i n t e r v o i d areas during Stages I I I and IV. When those c l u s t e r s d i s s o l v e d , c a p i l l a r i e s between small p a r t i c l e s Figure 43. A s e c t i o n i n PCM-2-18, lOOx. ( N i t a l etch) Figure 44. A s e c t i o n i n PCM-2-22 (Area #1), lOOx. ( N i t a l etch) F i g u r e 46. A s e c t i o n i n PCM-2-20, lOOx. ( N i t a l etch) 104 (which tended to hold l i q u i d i n them) were removed. The l i q u i d r e t r e a t e d from those areas l e a v i n g v o i d s . Large i r o n p a r t i c l e s f a i l e d to c o l l a p s e i n t o the voids and f i l l them, because the p a r t i c l e s surrounding the voids had bridged. The uneven d i s t r i b u t i o n of voids at the end of Stage V l i k e l y r e s u l t e d from the c l o s i n g of those same voids (which were probably smaller than those which formed when copper melted) as a r e s u l t of Stage V shrinkage. Figure 47 shows the e x t e r n a l surface of PCM-2-18 (which was stopped i n Stage I I I ) . The photograph r e v e a l s i r o n p a r t i c l e s w i t h copper between them. The i r o n p a r t i c l e faces were f l a t t e n e d i n the compaction process. The surface of the copper made a small angle w i t h the surface of the i r o n p a r t i c l e s , i n d i c a t i n g t h a t w e t t i n g was complete or almost complete ( i . e . 6 = 0°) i n Stage I I I . Figure 48 shows the surface of PCM-2-20 ( a f t e r 100 minutes at 1155°C) and i n d i c a t e s t h a t the wetting of the i r o n by the copper was incomplete i n l a t e Stage V. The copper had 'beaded' to meet the i r o n surfaces at r e l a t i v e l y l a r g e angles. F i g u r e 49 i s an absorbed e l e c t r o n image of PCM-2-20. The photograph i n d i c a t e s t h a t the i r o n p a r t i c l e s were i n s o l i d contact w i t h each other around the l a r g e pores ( i . e . b r i d g e d ) , thus causing those voids to remain i n the s t r u c t u r e . F i g u r e 50 i s a copper X^r.ay image of the same area which Figure 47. Surface of PCM-2-18 at 2000x; viewed through a scanning e l e c t r o n microscope. The oxide 'flowers' formed during storage a f t e r l i q u i d - p h a s e s i n t e r i n g . F igure 48. Surface of PCM-2-20 at 1040x, as viewed w i t h a scanning e l e c t r o n microscope. F i q u r e 49. Absorbed e l e c t r o n image of a s e c t i o n i n PCM-2-20; 100Ox. The l i g h t grey areas are i r o n , the dark grey areas are copper. The white area i s a v o i d . F i q u r e 50. Copper X-ray image of a s e c t i o n i n PCM-2-20; lOOOx. The l i g h t areas are high m copper c o n c e n t r a t i o n . The black area ( l e f t side) i s a v o i d . This s e c t i o n i s the same as t h a t shown i n Figure 49. 107 i n d i c a t e s t h a t the copper d i d not form a continuous f i l m on the i n s i d e of the pores. The wetting was, then, incomplete as the surface morphology i n Figu r e 48 i n d i c a t e d . Figure 50 a l s o i n d i c a t e s t h a t a l l o y i n g of the i r o n w i t h the copper was not complete at the centres of l a r g e i r o n p a r t i c l e s a f t e r 100 minutes a t 1155°C. Analyses made w i t h the e l e c t r o n microprobe analyzer i n d i c a t e d t h a t the centre of l a r g e i r o n p a r t i c l e s i n PCM-2-20 contained approximately 4% copper. A simlLlarminvestigation of pores i n PCM-2-22 and PCM-2-18 d i d not r e v e a l whether or not wetting was complete because d i f f u s i o n zones (at the surface of i r o n p a r t i c l e s a b u t t i n g voids) made i t impossible to determine whether l i q u i d copper coated the i r o n surfaces or i f the observed copper concen t r a t i o n was due to d i f f u s i o n . I t i s concluded t h a t the wetting was v i r t u a l l y complete during Stages I I I and IV, wh i l e at the end of Stage V i t was incomplete. 3.4.7 Summary of M e t a l l o g r a p h i c Observations A summary of the observations made i n the previous s e c t i o n f o l l o w s : 108 Macroscopic Occurrences M e t a l l o g r a p h i c Observations Copper melts, Stage I I I begins Stage I I I c o n t r a c t i o n i s overcome by Stage IV expansion 10 11 12 13 14 15 Copper disperses through the compact wetting i r o n p a r t i c l e s . Voids remain where copper had e x i s t e d as powder p a r t i c l e s . Small i r o n p a r t i c l e s are broken loose by l i q u i d copper. Gra i n boundary d i f f u s i o n begins. Iron s k e l e t o n i s broken up by s o l u t i o n of i r o n i n copper. Necks are penetrated or d i s -solved by copper. Dense i n t e r v o i d areas formed. Grain boundary p e n e t r a t i o n becomes apparent. D i f f u s i o n (grain boundary and volume) i s more advanced. D i f f u s i o n at s o l i d contacts i s observed. P e n e t r a t i o n of g r a i n boundaries becomes ext e n s i v e . More s o l i d - s o l i d contacts are observed than during Stage I I I . Small i r o n p a r t i c l e s d i s -appearing . I r r e g u l a r p a r t i c l e shapes observed. Iro n p a r t i c l e s have more re g u l a r s u r f a c e s . 109 Macroscopic Occurrences 16) IT). Stage V contraction 18')) begins and af t e r 100 minutes: 1-93)) 210')) 2?1T 22*) 23) 22m Metallographic Observations Pores are more evenly d i s t r i -buted than during Stage I I I ; no high density regions remain. Iron p a r t i c l e s are not a s o l i d skeleton. Small iron p a r t i c l e s are gone. Iron p a r t i c l e s have equiaxed shapes and regular surfaces. Iron p a r t i c l e s are shaped for dense packing. Iron p a r t i c l e s are i n the form of a s o l i d skeleton. There i s only p a r t i a l wetting of i r o n by the copper. Pore d i s t r i b u t i o n i s uneven. Di f f u s i o n i s not complete i n group PCM-2-XX. 3.5 Proposed Mechanisms of Sintering i n Fe-22 Cu Mixtures Metallographic and dilatometric observations for PCM-2-20 (see Figure 51) permit an analysis of the mechanisms operating during liquid-phase s i n t e r i n g i n the present study. Immediately following melting of copper, the s k e l e t a l iron structure formed by p r i o r s o l i d - s t a t e s i n t e r i n g i s destroyed by d i s s o l u t i o n of i r o n i n the copper and necks between s o l i d p a r t i c l e s are penetrated. I n i t i a l l y wetting i s complete and the dihedral angle i s zero. Coincident with, 3.0 F i g u r e 51. Dilatometer p l o t f o r Run PCM-2-20. (Normal he a t i n g , 22% copper, 53 to 63u powders). i — • o I l l and f o l l o w i n g d i s i n t e g r a t i o n of the i r o n s k e l e t o n , Stage I I I shrinkage occurs. Stage I I I i s a p e r i o d i n which the compact undergoes extensive shrinkage (1.32 %(AL/L 0) i n PCM-2-20) at a r a t e which i s high and v i r t u a l l y constant. Only one d e n s i f i c a t i o n process seems to be capable of producing such r a p i d , extensive and l i n e a r c o n t r a c t i o n ; i . e . rearrangement. The rearrangement process which Kingery [3] proposed as the f i r s t d e n s i f i c a t i o n mechanism to operate i n l i q u i d phase s i n t e r i n g i n v o l v e s repacking of the s o l i d p a r t i c l e s w i t h the a i d of viscous flow, which has a l i n e a r r a t e . A l l other suggested d e n s i f i c a t i o n processes have a r a t e which i s expected to decrease w i t h time. Stage I I I i s thus considered to be the r e s u l t of rearrangement of s o l i d p a r t i c l e s to a higher packing e f f i c i e n c y than t h a t which e x i s t e d at m e l t i n g . The process i s aided by the d i s s o l u t i o n of i r o n by copper. That breaks i r r e g u l a r i r o n p a r t i c l e s i n t o s m a l l e r , more equiaxed fragements, and a l s o reduces the volume of the s o l i d i r o n by 0.80%(AV/Vo), or 0.26%(AL/Ln). However, unless the d i h e d r a l angle i s zero i n the system at t h i s stage, s o l i d - s o l i d contacts would r a p i d l y reform, and s o l i d p a r t i c l e s could not s l i d e past each other to repack. The complete wetting i n Stage I I I a l s o gives r i s e to the c a p i l l a r y pressure which d r i v e s rearrangement. The sharp t r a n s i t i o n from Stage I I I c o n t r a c t i o n to Stage IV expansion i n PCM-2-20 (see Figure 51) suggests t h a t 112 shrinkage may not be complete when Stage IV expansion s t a r t s , but t h a t i t continues i n t o Stage IV and i s 'masked' by the r a p i d expansion processes. Even when shrinkage due to r e -arrangement i s complete, other important c o n t r a c t i o n processes would be expected to continue as long as the d i h e d r a l angle was zero and there was continued existence of a c a p i l l a r y pressure. For example, Kingery's ' s o l u t i o n due to pressure' mechanism would be expected to operate beyond the end of rearrangement, although the theory p r e d i c t s t h a t the r a t e of shrinkage would be lower. However, i n Stage IV any i n t r i n s i c shrinkage from ' s o l u t i o n due to pressure' i s apparently masked by i n t e r v e n i n g expansion processes. The net expansion i n Stage IV i s both extensive and extremely r a p i d . The observed r a t e of expansion i n PCM-2-20 i s 1.50 % ( A L / L 0 ) per minute e a r l y i n Stage IV. In view of the f a c t t h a t some i n t r i n s i c shrinkage i s masked by expansion, the r a t e of the expansion processes i n Stage IV must be very high. D i f f u s i o n and g r a i n boundary p e n e t r a t i o n i n the i r o n w i l l produce net expansion only i f the s o l i d p a r t i c l e s are c l o s e l y packed; i . e . only a f t e r rearrangement i s e s s e n t i a l l y complete. Any d i f f u s i o n of copper i n t o i r o n which occurs, before rearrangement i s complete, w i l l lower the observed r a t e of c o n t r a c t i o n by i n c r e a s i n g the volume of the s o l i d . G r ain boundary p e n e t r a t i o n which occurs before rearrangement i s complete may, or may not, cause a decrease i n the net r a t e 113 of shrinkage. That depends on the d e n s i t y of packing i n the r e g i o n surrounding a p a r t i c l e i n which a g r a i n boundary i s penetrated. Furthermore, net expansion i s observed only i f the r a t e of expansion i s greater than the r a t e of simultaneous shrinkage. The net l i n e a r expansion i n Stage IV i n PCM-2-20 was 3.29%-(AL/Lo). To convert t h a t f i g u r e to volume expansion, an estimate must be made of the d i a m e t r a l dimensional change during Stage IV. Reference to Table XI r e v e a l s t h a t there was 0.52% more shrinkage i n diameter than i n length during the f u l l course of the run PCM-2-20. I f i t i s assumed th a t the same d i f f e r e n c e p r e v a i l e d i n Stage IV, then the net d i a m e t r a l expansion i n Stage IV was 3.29-0.52 = 2.77% (AD/D 0). This leads to a conservative estimate of 9.09%^(AV/Vo) net expansion i n PCM-2-20 during Stage IV. The maximum expansion which can r e s u l t from complete s a t u r a t i o n of the i r o n (with 9 wt % copper) i s 9%' (AV/Vo). In f a c t a l l o y i n g was found to be i n an e a r l y s t a t e of progress at the end of Stage IV ( i n sample PCM-2-22). Even PCM-2-20 was not i n a l l o y e q u i l i b r i u m a f t e r 100 minutes a t 1155°C. I t can be estimated t h a t not more than 2% volume expansion was the d i r e c t r e s u l t of the d i f f u s i o n of copper i n t o i r o n i n Stage IV. I t f o l l o w s t h a t at l e a s t 7% volume expansion was caused by one or more other processes. In f a c t , as noted above, simultaneous c o n t r a c t i o n was o c c u r r i n g i n Stage IV. 114 Thus the amount of i n t r i n s i c expansion a s s o c i a t e d w i t h pro-cesses other than a l l o y i n g of the i r o n was substantially g r e a t e r than 7 volume per cent. Metallography d i d not i n d i c a t e the o p e r a t i o n of any process other than g r a i n boundary p e n e t r a t i o n which could cause such expansion, and no other can be suggested which i s c o n s i s t e n t w i t h the observations. Thus the evidence c l e a r l y p o i n t s to g r a i n boundary p e n e t r a t i o n as the cause of most of the Stage IV,expansion. The m e t a l l o g r a p h i c observation t h a t v i r t u a l l y a l l the Y~i r° n g r a i n boundaries had been penetrated, by l a t e Stage IV ( i n PCM -2-22), i s f u r t h e r evidence t h a t p e n e t r a t i o n caused expansion. The end of Stage IV was appar-e n t l y c o i n c i d e n t w i t h the completion of the p e n e t r a t i o n of a l l the a v a i l a b l e g r a i n boundaries i n the group PCM-2-XX. A g r a i n boundary i s l i k e l y f i r s t penetrated by a mechanism of d i f f u s i o n i n t o the boundary, progressing u n t i l a f i l m of copper e x i s t s . That f i l m then expands by the movement of more copper i n t o the boundary (driven by c a p i l l a r y p r e s s u r e ) . A penetrated boundary i s i n f a c t a small c a p i l l a r y and as such i s a favourable l o c a t i o n f o r l i q u i d t o gather a t . In t h i s manner the penetrated boundary grows i n width to some e q u i l i b r i u m t h i c k n e s s . However, l i q u i d i s a l s o h e l d i n other areas of the compact by c a p i l l a r y f orces and the d i s p o s i t i o n of l i q u i d i s c o n t r o l l e d by some s o r t of h y d r a u l i c e q u i l i b r i u m . 115 The s e l e c t i v e d i s s o l u t i o n of small p a r t i c l e s i n the i n t e r v o i d areas, and from the i n s i d e of pores during Stage IV, removes some of the s m a l l e s t l i q u i d - h o l d i n g c a p i l l a r i e s . That allows l i q u i d to move to those g r a i n boundaries which are penetrated, or i n the act of being penetrated. As a r e s u l t voids form, where c l u s t e r s of small p a r t i c l e s p r e v i o u s l y e x i s t e d , and l i q u i d i s fr e e d to ssupply' the g r a i n boundary c a p i l l a r i e s . The only reported values f o r d i h e d r a l angle i n the s o l i d i r o n - l i q u i d copper system l i e i n the range of 27 to 35 degrees (see Table I ) . I f these values p r e v a i l e d under the co n d i t i o n s of l i q u i d phase s i n t e r i n g i n Stage IV and e a r l i e r , n e i t h e r g r a i n boundary p e n e t r a t i o n nor the observed 'dispersed' m i c r o s t r u c t u r e could be explained. In f a c t , i n the present work, and i n s t u d i e s described by a number of previous i n v e s -t i g a t o r s „[12,14,23], i t i s apparent t h a t a zero d i h e d r a l angle p r e v a i l e d , at l e a s t w h i l e the compacts were undergoing r a p i d expansion. However, a f t e r prolonged l i q u i d - p h a s e s i n t e r i n g ( w e l l i n t o Stage V) i n the present work, a continuous i r o n - r i c h s k e l e t o n i s observed, and a p o s i t i v e d i h e d r a l angle was i n d i c a t e d . I t may t h e r e f o r e be concluded t h a t : a) t h e d i h e d r a l a n g l e c h a n g e s d u r i n g t h e c o u r s e o f s i n t e r i n g u n d e r t h e c o n d i t i o n s o f t h e p r e s e n t (and many p r e v i o u s l y r e p o r t e d ) e x p e r i m e n t s , b) r e p o r t e d v a l u e s o f t h e d i h e d r a l a n g l e i n t h e Fe-Cu s y s t e m a r e b a s e d on m e a s u r e m e n t s on s p e c i m e n s w h i c h had been ' s i n t e r e d ' i n t o t h e e q u i v a l e n t o f S t a g e V. 1 1 6 Any change i n d i h e d r a l angle must be as s o c i a t e d w i t h a change i n Y S L or Y S S ' S p e c i f i c a l l y , the present r e s u l t s i n d i c a t e t h a t Y S S > 2 Y S L i n the e a r l y stages of l i q u i d - p h a s e s i n t e r i n g , and tha t e i t h e r Y O T increases or v„„ decreases, o L i 0 0 such as t o make Y s s become equal t o 2 y S L i n Stage V. There are s e v e r a l p o s s i b l e explanations f o r these changes: a) Yg[_ i n c r e a s e s , o r Y55 d e c r e a s e s - , as t h e i r o n becomes a l l o y e d w i t h t h e c o p p e r . T h i s i s n o t c o n s i s t e n t w i t h t h e o b s e r v a t i o n t h a t c o p p e r d i f f u s e s r a p i d l y a l o n g g r a i n b o u n d a r i e s . A c c o r d i n g t o t h a t , a r a p i d i n c r e a s e i n Y s l o r d e c r e a s e i n Y55 s h o u l d o c c u r as e a r l y as S t a g e I I I . b) y 5 j_ i n c r e a s e s as a r e s u l t o f t h e d i s s o l u t i o n o f i r o n by l i q u i d c o p p e r . M i c r o p r o b e a n a l y s i s showed t h a t t h e c o p p e r becomes s a t u r a t e d w i t h i r o n i n e a r l y S t a g e I I I . T h e r e -f o r e t h i s e x p l a n a t i o n i s a l s o u n s a t i s f a c t o r y . I t i s a l s o c o n t r a r y t o t h e o b s e r v a t i o n s o f Van V l a c k ( s e e S e c t i o n 1 . 3 . 2 ) . c) Yg|_ i n c r e a s e s , o r Y55 d e c r e a s e s as a r e s u l t o f t h e i n t r o d u c t i o n o f an i n t e r f a c e - a c t i v e i m p u r i t y s p e c i e s , o r as a r e s u l t o f t h e p r o g r e s s i v e e l i m i n a t i o n o f some o t h e r i n t e r f a c e - a c t i v e s p e c i e s , d u r i n g t h e c o u r s e o f S t a g e V. S i n c e i t i s u n l i k e l y t h a t an i m p u r i t y i s added t o t h e s y s t e m d u r i n g s i n t e r i n g , the former case i s u n l i k e l y . However i f oxygen i s considered as the i n t e r f a c e - a c t i v e im-p u r i t y , e x p l a n a t i o n (c) becomes a strong p o s s i b i l i t y . I t i s 117 p o s s i b l e t h a t not a l l the oxides i n the compacts were completely reduced i n the course of c l e a n i n g at 600°C and 700°C. As was noted i n S e c t i o n 3.2.2 up t o 0.2 per cent oxygen may have been removed by hydrogen i n the d i l a t o m e t e r during Stages I I I to V ( i n a normal heating c y c l e ) . I t was a l s o noted i n S e c t i o n 3.4.4 t h a t s u f f i c i e n t oxygen was present at the time of me l t i n g of the copper t o lower to l i q u i d u s temperature to below 1070°C. There i s , t h e r e f o r e , evidence t h a t oxygen was present i n the specimens during Stages I I I , IV and V i n concentrations which s t e a d i l y decreased w i t h time. I t has been shown by O'Brien and Chaklader [30] t h a t small oxygen concentrations i n l i q u i d copper are s u f f i c i e n t to markedly reduce the value of Y s l at a l i q u i d - c o p p e r / s a p p h i r e i n t e r f a c e . Moreover i t i s known t h a t the presence of an e l e c t r o n e g a t i v e impurity i n a l i q u i d metal tends to reduce the values of both y T T 7 and y C T [31] . I t i s thus argued, •LiV o Ju r e l e v a n t to the experiments reported i n t h i s t h e s i s , t h a t enough oxygen i s present i n the Fe-Cu specimens during Stages I I I and IV to produce a zero d i h e d r a l angle. In the Fe-Cu system l e s s than a 19% r e d u c t i o n of Y s l (for constant Y S S ) would be necessary to s a t i s f y the requirement t h a t 2Y 0T - Yn C Ojj OO based on (J) = '35°. I t i s f u r t h e r argued t h a t at a l a t e r p e r i o d i n l i q u i d - p h a s e s i n t e r i n g (Stage V ) , oxygen i n the copper becomes reduced t o such a l e v e l t h a t 2 Y s l i s > Y s s ( i . e . a p o s i t i v e d i h e d r a l angle e x i s t s ) and coalescence of s o l i d g r a i n s (or p a r t i c l e s ensues. 118 A p o s s i b l e refinement i n t h i s e x planation i n v o l v e s the lowering of Y s l due to d e p o s i t i o n of oxygen at the l i q u i d s o l i d i n t e r f a c e s as copper d i f f u s e s i n t o i r o n . The f l u x of copper across the i n t e r f a c e would decrease as the i r o n became saturated w i t h copper, and oxygen would become dis p e r s e d more uniformly through the l i q u i d , a l l o w i n g Y p t t o i n c r e a s e . As the p e n e t r a t i o n of g r a i n boundaries nears comple-t i o n , Stage IV expansion slows and c o n t r a c t i o n processes begin to dominate as Stage V i s entered. The dominant c o n t r a c t i o n process e a r l y i n Stage V i s considered to be one of p a r t i c l e shape change by ' s o l u t i o n due to pressure.' This mechanism, which probably begins operating when rearrangement i s almost complete, i s only e f f e c t i v e w h i l e the i r o n - r i c h p a r t i c l e s are s t i l l d i spersed; i . e . w h i l e the d i h e d r a l angle i s zero. A c c o r d i n g l y , ' s o l u t i o n due to pressure' continues i n Stage V u n t i l the value of a) f o r the system becomes p o s i t i v e . The r a t e of the process i s i n i t i a l l y high but decreases w i t h time. The rounding of the i r o n p a r t i c l e s , which was observed i n PCM -2-20, i s a r e s u l t of the combined a c t i o n o f : a) the Heavy A l l o y Mechanism, and b) ' s o l u t i o n due to pressure', which causes r e p r e c i p i t a t i o n of s o l i d at surfaces which are not under compressive s t r e s s . During some i n t e r v a l i n Stage V the change of d i h e d r a l angle from zero to a p o s i t i v e value took p l a c e . The t r a n s i t i o n was probably gradual because cf> became p o s i t i v e at f i r s t a t 119 some areas, then l a t e r at ot h e r s , depending on the l o c a l oxygen concentrations and d i s t r i b u t i o n i n the compact. Presumably, at the same time the contact angle was becoming p o s i t i v e and the d r i v i n g f o r c e f o r s o l u t i o n due to pressure was decreasing. The f i n a l stage of shrinkage i n Stage V was dominated by a process of coalescence of the s o l i d p a r t i c l e s , which was p o s s i b l e once the d i h e d r a l angle became p o s i t i v e , and which continued u n t i l the end of the runs^ Expansion due to d i f f u s i o n continued during a l l of Stage V ( i n the PCM-2-XX group) but wi t h a low and s t e a d i l y decreasing r a t e (as the i r o n p a r t i c l e s became saturated w i t h copper). In the preceding d i s c u s s i o n i t has been proposed t h a t Stages I I I , IV and V are the r e s u l t of the op e r a t i o n of s e v e r a l shrinkage and expansion mechanisms. The d e t a i l e d shapes of the curves are determined by: a) t h e a b s o l u t e r a t e s o f e a c h s h r i n k a g e and e x p a n s i o n p r o c e s s w h i l e i t i s ope r a t i ng, b) t h e e x t e n t o f t h e s h r i n k a g e o r c o n -t r a c t i o n w h i c h e a c h p r o c e s s i s c a p a b l e o f p r o d u c i ng, c) t h e d e g r e e t o w h i c h t h e d i f f e r e n t p r o -c e s s e s ' o v e r l a p * i n t i m e ; i . e . o c c u r s i m u I t a n e o u s I y . Thus i t has been proposed t h a t the c o n t r a c t i o n processes of rearrangement, ' s o l u t i o n due to pressure' and coalescence occur as a continuous sequence (with some p o s s i b l e overlap) from the onset of mel t i n g ( s t a r t of Stage I I I ) t o the completion of a 120 s i n t e r i n g run. S i m i l a r l y , the overlapping expansion processes of copper d i f f u s i o n i n t o i r o n , and g r a i n boundary p e n e t r a t i o n , may s t a r t w i t h the onset of copper m e l t i n g . The d i f f u s i o n process continues i n t o Stage V. I f the above arguments are c o r r e c t , i t should be p o s s i b l e to i n t e r p r e t the observed e f f e c t of a number of experimental v a r i a b l e s on the dimensional changes which occur i n l i q u i d - p h a s e s i n t e r i n g . 3.6 E f f e c t of S i n t e r i n g V a r i a b l e s 3.6.1 General Table X I I I contains d e t a i l s of those d i l a t o m e t e r runs which were performed to i n v e s t i g a t e the e f f e c t s of s e v e r a l v a r i a b l e s on the progress of l i q u i d - p h a s e s i n t e r i n g . A l l the runs were designed t o be compared to the standard runs ( f o r each of two p a r t i c l e s i z e s ) , PCM-2-20 and PCM-5-6. As discussed i n S e c t i o n 2.3, heating c y c l e s of a given type were not always completely r e p r o d u c i b l e , due to the v a r i a b l e and u n c o n t r o l l a b l e response of d i f f e r e n t compacts to i n d u c t i o n heating below 700°C. However, c o n s i s t e n t heating r a t e s and ho l d i n g times were used above 700°C. A c c o r d i n g l y , i n some samples copper melted a t e a r l i e r t o t a l times than others. The d i l a t o m e t r y curves f o r Table X I I I Runs Showing E f f e c t s of V a r i a b l e s on Liquid-Phase S i n t e r i n g Mechanisms Specimen Heating Cycle Fe/Cu Rat i o ..Length of Run (mins.) Holding Temperature Th, °C Time at Th (mins.) Powder S i z e of Iron** Powder Size of Copper** Other Components PCM-2-20 PCM-A PCM-6 PCM-5-6 PCM-UF-2 N N N N N 78/22 78/22 78/22 78/22 78/22 122 123 55 121 124 1155 1155 1155 1155 1155 105 108 40 106 108 2 A 6 5 UF* 2 A 6 5 2 • Standard Standard Unfractior E f f e c t of - p a r t i c l e s i z e led powder PCM-2-10 PCM-2-19 PCM-5-13 PCM-5-12 P P P P 78/22 78/22 78/22 78/22 202 202 212 186 1155 1155 1155 1155 116 116 126 110 2 2 5 5 2 2 5 5 71 71 72 62 Mins. at 1000°C E f f e c t of p r e s i n t e r i n g PCM-2-23 PCM-2-24 PCM-2-25 PCM-2-30 PCM-2-100 PA-2-1 ATOCM-5 R S N N N N N 78/22 7 8/22 78/22 78/22 90/10. 78/22 78/22 111 63 121 123 123 123 101 1155 1155 1110 1155 1155 1155 1155 106 33 105 107 108 108 85 2 2 2 2 2 2 5 2 2 2 2 2 2 5 m 0 -P - o -CD M-l iH W heating r a t e heating r a t e h o l d i n g temp, i n i t i a l d e n s i t y copper content p r e a l l o y e d powder Atomet 28 powder P = P r e s i n t e r e d N = Normal R = Rapid See Table VI See Table V 122 the l i q u i d - p h a s e p o r t i o n s of runs are, t h e r e f o r e , best compared r e l a t i v e to t h a t p o i n t on the curves at which the m e l t i n g of copper occurred. Where important d i f f e r e n c e s i n heating r a t e s occurred they w i l l be noted i n the t e x t . The Stage I I I slopes quoted i n the f o l l o w i n g s e c t i o n s were measured over the second quarter of t h a t stage. The Stage IV slopes were measured over the f i r s t t h i r d of the net expansion i n Stage IV. 3.6.2 E f f e c t of P a r t i c l e S i z e and P a r t i c l e S i z e D i s t r i b u t i o n With decreasing p a r t i c l e s i z e (other parameters being held e s s e n t i a l l y c o n s t a n t ) , the previous d i s c u s s i o n would p r e d i c t : A) E a r l i e r and more r a p i d i n t r i n s i c e x p a n s i o n due t o p e n e t r a t i o n o f i r o n g r a i n b o u n d a r i e s by l i q u i d , b e c a u s e t h e g r a i n b o u n d a r i e s have a smaI I e r a v e r a g e l e n g t h ( t h i s assumes t h a t t h e c o n c e n t r a t i o n o f g r a i n b o u n d a r i e s i s e s s e n t i a l l y i n d e p e n d e n t o f p a r t i c l e s i z e o v e r t h e r a n g e s t u d i e d ) . B) No e f f e c t on t h e amount o f e x p a n s i o n c a p a b l e o f b e i n g c a u s e d by g r a i n b o u n d a r y p e n e t r a t i o n ( b a s e d on t h e same a s s u m p t i o n as i n (A') ) . C) More r a p i d and more e x t e n s i v e e x p a n s i o n due t o d i f f u s i o n o f c o p p e r i n i r o n w i t h i n S t a g e IV due t o t h e s h o r t e r d i f f u s i o n p a t h i n smaI Ie r pa r t i c I e s . D) No e f f e c t on t h e i n t r i n s i c r a t e o r e x t e n t o f r e a r r a n g e m e n t s h r i n k a g e (assumes p a r t i c l e s h a p e i s n o t r e l a t e d t o s i z e ) . E) E a r l i e r and more r a p i d s h r i n k a g e by t h e ' s o l u t i o n due t o p r e s s u r e ' m e chanism. 123 F) More r a p i d s h r i n k a g e due t o c o a l e s c e n c e . G) E a r l i e r and more e x t e n s i v e i n t e r f e r e n c e between s h r i n k a g e by r e a r r a n g e m e n t and e x p a n s i o n p r o c e s s e s ( f o l l o w s f r o m (A) and (CO). H) A s h a r p e r t r a n s i t i o n between S t a g e IV and S t a g e V ( f o l l o w s f r o m (E) and ( F ) ) . J ) V a r i a b l e i n t e r f e r e n c e between e x p a n s i o n p r o c e s s e s and s h r i n k a g e p r o c e s s e s ( o t h e r t h a n r e a r r a n g e m e n t ) i n l a t e S t a g e IV, d e p e n d i n g e o n t h e e f f e c t s o f p a r t i c l e s i z e on t h e Tetat-ive r a t e s o f t h e o p p o s i n g p r o c e s s e s . In a l l the above p r e d i c t i o n s , i t i s assumed t h a t the oxygen content of compacts, and i t s change w i t h s i n t e r i n g time, are not s i g n i f i c a n t l y a f f e c t e d by the powder p a r t i c l e s i z e . Runs PCM-A, PCM-2-20, PCM-5-6 and PCM-6, which i n -volved normally-heated specimens of decreasing powder p a r t i c l e s i z e , are described by Figures 51 tto 5:4. Some r e l e v a n t data taken from the d i l a t o m e t e r p l o t s are contained i n Table XIV. I t was observed t h a t , w i t h decreasing p a r t i c l e s i z e : 1) The r a t e and amount o f n e t s h r i n k a g e o b s e r v e d i n S t a g e I I I d e c r e a s e d . T h i s i s c o n s i s t e n t w i t h p r e d i c t i o n ( G ) . More-o v e r , t h e f o r m a t i o n o f e-phase j u s t a b ove t h e m e l t i n g p o i n t o f c o p p e r m i g h t be more c o m p l e t e w i t h f i n e r powders b e f o r e t h e p e r i t e c t i c t e m p e r a t u r e was r e a c h e d . B e c a u s e t h a t r e d u c e s t h e amount o f l i q u i d a v a i l a b l e t o t h e s y s t e m f o r a s m a l l i n t e r v a l o f t i m e , i t m i g h t a l s o p a r t i a l l y a c c o u n t t i ' f o r t h e l o w e r i n i t i a l s a t e o f S t a g e I 1 1 s h r i n k a g e w i t h f i n e r powder s p e c i m e n s . 2) The r a t e and amount o f e x p a n s i o n i n S t a g e IV f i r s t i n c r e a s e d ( b e t w e e n PCM-A and PCM-2-20), and t h e n d e c r e a s e d , ( c o n s i s t e n t F i g u r e 52. Dilatometer p l o t f o r Run PCM-A. (Normal heating, 22% copper, 105 to 120y powders) % A L / L 0 F i g u r e 54. Dilatometer p l o t f o r Run PCM-6. (Normal heating, 22% copper, <25u powders.) Table XIV E f f e c t of P a r t i c l e S i z e and P a r t i c l e S i z e D i s t r i b u t i o n on the Dimensional Changes of Compacts During Liquid-Phase S i n t e r i n g Sample PCM« A PCP.eM-AO PCM-2-20 PCM-5-6 PGM°-"6r - PCM-UF-2 Powder S i z e microns 105 to 120 53 to 63 25 to 37 <25 see Table X I I I A % (AL/Lo) +1.87 +1.35 + 1.30 +1.22 +1.30 B %(AL/Lg) - .30 - .35 + .22 0 - .10 C %(AL/Lo) - 2 . 0 2 - 1 . 6 7 - .95 - .75 - .95 D %(AL/Lo) - .15 +1.62 +1.02 + .20 +1.60 B to C %(AL/Lo) 1.72 1.32 1.17 .75 .85 C to D %(AL/Lo) 1. 87 3.29 1.97 .95 2.55 Slope I I I [%(AL/Lo)]/min - 2 . 5 - 2 . 0 - 1 . 8 - .4 - 1 . 3 Slope IV [% (AL/Lo)]/min + 1.0 +1.8 +1.5 +1.0 +1.5 128 w i t h p r e d i c t i o n J a b o v e ) . W i t h t h e c o a r s e s t powder i n t h e s e r i e s , t h e r a t e s o f t h e e x p a n s i o n p r o c e s s e s a r e so low r e l a t i v e t o t h o s e o f c o n t i n u i n g c o n t r a c t i o n p r o c e s s e s t h a t t h e r a t e and n e t e x t e n t o f o b s e r v e d e x p a n s i o n i n S t a g e IV a r e b o t h s m a l l . W i t h v e r y f i n e p o w d e r s , b o t h e x p a n s i o n and c o n -t r a c t i o n p r o c e s s e s a r e r a p i d . T h i s l e a d s t o e a r l y i n t e r a c t i o n b etween t h e o p p o s i n g p r o c e s s e s , and a s h a r p t r a n s i -t i o n between S t a g e s IV and V. 3) The r a t e s o f n e t c o n t r a c t i o n t h r o u g h o u t S t a g e V were h i g h e r , c o n s i s t e n t w i t h p r e d i c t i o n s (E) and ( F ) a b o v e . Specimen PCM-UF-2 was prepared from u n f r a c t i o n e d i r o n powder, i t contained p a r t i c l e s both coarser and f i n e r than those of PCM-2-20 and PCM-6, r e s p e c t i v e l y (see Table V I ) . The response of the specimen to l i q u i d - p h a s e s i n t e r i n g , which i s shown i n Figu r e 55 and Table XIV, was g e n e r a l l y intermediate between th a t of specimens PCM-5-6 and PCM-2-20; i . e . the specimen behaved e s s e n t i a l l y as though i t had an average p a r t i c l e s i z e of 37 to 53 microns. 3.6.3 E f f e c t of P r e s i n t e r i n g Another group of Fe-22 Cu specimens, PCM-5-12, PGM-5-13 , PCM-2-10, and PCMS2--19 ,-were>heated using the p r e s i n t e r i n g c y c l e of Fig u r e 15, i n c o r p o r a t i n g a p e r i o d of roughly one hour at 1000°C. The d i l a t o m e t r i c r e s u l t s are i n Figures 56 through 59 and the data are summarised i n Table XV. 2 o 1 1 1 1 1 l I I l — I I I i 1 1 — " 0 20 40 60 80 100 120 140 Time (min) F i g u r e 55. Dilatometer p l o t f o r Run PCM-UF-2. (Normal he a t i n g , 22% copper, unfra c t i o n e d i r o n powder.) 130 3. CN r o H I o LT) +J I s m U CN 04 3 c E i -Pi 04 u o E ^ o u +J CN O CN H D-. -T3 M OJ OJ M -P OJ —> 0J +J • S A W O - H 5-1 4-> W OJ CO OJ TS H M £ • H P-i O Q w O . in OJ u •iH &4 °-|/~lV % powders.) 2.0 h < - I.Oh -2.0 F i g u r e 58. 80 100 120 Time (min) 140 Dilatometer p l o t f o r Run PCM ( P r e s i n t e r e d , 22% copper, 53 powder.) (Spurious data) 2.0h .0 0 0 20 40 60 F i g u r e 59. • • I I I I I I I 1 — 80 100 120 140 160 Time (min) Dilatometer p l o t f o r Run PCM-2-19. ( P r e s i n t e r e d , 22% copper, 53 to 63u powder.) 134 Table XV E f f e c t of P r e s i n t e r i n g on Dimensional Changes of Compacts During Liquid-Phase S i n t e r i n g Sample PCM-2-10 PCM-2-19 PCM-5-13 PCM-5-12 Powder s i z e microns 53 to 63 53 to 63 25 to 37 25 to 37 A %(AL/Lo) + 1.25 +1.37 +1.32 +1.25 B %(AL/Lo) -1.22 -1.52 -1.25 -1.13 C %(AL/Lo) -1.50 -1. 64 -1.35 -1.35 D %(AL/Lo) +1.25 +1.40 - .11 + .45 B to C %(AL/L o) .28 .12 .10 .22 C to D %(AL/Lo) 2.75 3.04 1.24 1.80 Slope I I I [%(AL/Lo)]/min - .3 - .1 - .1 - .3 Slope IV [%(AL/Lo)]/min +1.0 +1.0 +1.2 • + .7 135 A l l these specimens experienced s u b s t a n t i a l s o l i d -s t a t e s i n t e r i n g shrinkage at 1000°C. At the end of Stage I I , a l l underwent s l i g h t thermal expansion as the temperature was increased to 1070°C. A f t e r the onset of copper m e l t i n g , the d i l a t o m e t r i c behaviour was s i m i l a r to tha t of specimens pre-v i o u s l y d iscussed, w i t h the notable exception t h a t Stage I I I was very s m a l l . Specimens PCM^5-12 and PCM-5-13 were held at 1000°C f o r 62 minutes and 74 minutes r e s p e c t i v e l y . In method of pre p a r a t i o n and processi n g , they were otherwise i d e n t i c a l to PCM-5-6 (Figure 56 and Table XIV) which was he l d a t 1000°C f o r only three minutes. Comparing the s i n t e r i n g behaviour of the three specimens a f t e r copper melted, i t i s seen t h a t w i t h increased p r e s i n t e r i n g at 1000°C: A) S t a g e I I I s h r i n k a g e was s m a l l e r and had a l o w e r r a t e . B) S t a g e IV e x p a n s i o n was l e s s e x t e n s i v e . C) The r a t e o f S t a g e IV e x p a n s i o n was l o w e r ( c o m p a r i n g t h e a v e r a g e o f PCM-5-12 and PCM-5-13 w i t h t h a t o f PCM-5-6). E x a c t l y the same trends are seen when specimens PCM-2-10 and PCM-2-19 are compared w i t h t h e i r non-presintered e q u i v a l e n t , PCM-2-20. One of the consequences of longer p r e s i n t e r i n g at 1000°C i s t h a t more s u b s t a n t i a l necks are e s t a b l i s h e d at i r o n -i r o n p a r t i c l e contacts. That delays the rearrangement process 1 3 6 i n Stage I I I , by increasing the time required for molten copper to penetrate the necks of the s o l i d - s t a t e sintered skeleton. Soon afte r rearrangement i s underway, the rate of the expansion processes i s high, and Stage IV (net expansion) st a r t s before much rearrangement shrinkage i s observed. However, rearrange-ment i s b a s i c a l l y a rapid shrinkage process and i t s delayed, but continued, operation i n Stage IV has the e f f e c t of reducing the Stage IV slope, as well as reducing the amount of net expansion which i s seen i n Stage IV. The observed effects of presintering are a l l thus s a t i s f a c t o r i l y explained, i n terms of a delay i n the onset of rearrangement shrinkage. Another e f f e c t of longer presintering should be to remove more of the residual oxygen i n the compact before liquid-phase s i n t e r i n g processes begin to operate. That should r e s u l t i n the attainment of a p o s i t i v e (non-zero) dihedral angle i n the system at an e a r l i e r time aft e r melting ( i . e . e a r l i e r i n Stage V) and a possible reduction i n the rate of Stage V shrinkage. That predicted e f f e c t , i f present at a l l , was small. However, i f the oxygen content of the presintered compacts was s i g n i f i c a n t l y lower than that of non-presintered, equivalent specimens at the onset of melting, i t i s also possible that the aforementioned delay i n the s t a r t of Stage III (and i t s subsequent influence on Stage IV) i s due to the 137 increase i n the l i q u i d u s temperature of Cu-0 a l l o y s w i t h de-creasin g oxygen content. The e f f e c t of lower oxygen content would be to cause m e l t i n g to occur over a l a r g e r range of temperature; i . e . the amount of l i q u i d i n the system would increase more g r a d u a l l y as the temperature was increased above 1065-1070°C. 3.6.4 E f f e c t of Heating Rate and Holding Temperature Figures 6 0 and 61 and Table XVI (samples PCM-2-20, PCM-2-23 and PCM-2-24) show the e f f e c t s of heating r a t e on the progress of l i q u i d - p h a s e s i n t e r i n g i n FE-22Cu ' specimens. In other r e s p e c t s , the three samples were e s s e n t i a l l y i d e n t i c a l , i n terms of method of pr e p a r a t i o n and processing. The f o l l o w i n g observations accompanied more r a p i d heating of the compacts: i ) S t a g e I I I s h r i n k a g e was more e x t e n s i v e . i i ) The r a t e o f S t a g e 111 s h r i n k a g e was h i g h e r . i i i ) The r a t e o f S t a g e IV e x p a n s i o n was h i g h e r . i v ) The amount o f S t a g e IV e x p a n s i o n was s i i g h t I y r e d u c e d . Observations i ) and i i ) can be p a r t l y explained i n terms of e-phase formation. A higher heating r a t e decreases the time the compacts spend i n the temperature range i n which e-phase forms. Therefore the amount of l i q u i d a v a i l a b l e e a r l y 2.0 F i g u r e 60. Dilatometer p l o t f o r Run PCM-2-23. (Rapid h e a t i n g , 22% copper, 53 to 63y powders.) F i g u r e 61. Dilatometer p l o t f o r Run PCM-2-24. (Slow heating, 22% copper, 53 to 631% powders.) The dashed curve shows the r e s u l t s - f o r Run PCM-2-20. 1 4 0 Table X V I E f f e c t of Heating Rate and Holding Temperature on Dimensional Changes During Liquid-Phase S i n t e r i n g Sample PCM-2-23 PCM -2 -20 PCM-2-24 PCM-2-25 Heating r a t e Rapid Normal Slow Normal Holding temperature degrees centigrade 1 1 5 5 1 1 5 5 1 1 5 5 1 1 1 0 A % ( A L / L o) + 1 . 3 0 + 1 . 3 5 + 1 . 3 0 + 1 . 3 5 B %(AL/Lo) + . 7 7 - . 3 5 - . 5 5 - . 2 7 C %(AL/Lo) - 1 . 2 5 - 1 . 6 7 - 1 . 6 2 - 1 . 3 0 D % ( A L / L 0 ) + 1 . 3 5 + 1 . 6 2 + 1 . 6 5 + 2 . 0 2 B to C % ( A L / L 0 ) 2 . 0 2 1 . 3 2 1 . 0 7 1. 03 C to D % ( A L / L 0 ) 2 . 6 0 3 . 2 9 3 . 2 7 3 . 3 2 Slope I I I [% ( A L/Lo)]/min - 4 . 5 - 1 . 5 - . 6 - 1 . 0 ;Slope I V [ % ( A L / L Q ) ] / m i n + 2 . 7 + 1 . 7 + 1 . 5 + 2 . 3 141 i n Stage I I I i s increased by more r a p i d h eating. A l s o , because the expansion processes are d i f f u s i o n c o n t r o l l e d , whereas rearrangement i s not, at high r a t e s of heating i t i s p o s s i b l e f o r more rearrangement to occur before the expansion processes are r a p i d enough to provide appreciable i n t e r f e r e n c e . However, shrinkage processes which i n v o l v e s o l u t i o n p r e c i p i t a t i o n are slow compared t o g r a i n boundary p e n e t r a t i o n . Thus w i t h high heating r a t e s , more r a p i d expansion i s 'seen' i n Stage IV because there i s l e s s simultaneous shrinkage. In the fast-heated sample there i s i n s u f f i c i e n t time f o r much expansion due to d i f f u s i o n to occur before Stage IV ended. Thus, l e s s net expansion i s 'seen' ( i n Stage IV) f o r t h a t run than f o r the slower-heated samples. Because more expansion by d i f f u s i o n remained to occur i n Stage V f o r the f a s t heated specimen, i t e x h i b i t e d a lower net r a t e of shrinkage i n Stage V. Run PCM-2-25 was made w i t h a f i n a l h o l d i n g temperature of 1110°C i n s t e a d of 1155°C. Comparison of Runs PCM-2-25 and PCM-2-20 (see F i g u r e 62 and Table XVI) shows the d i f f e r e n c e i n s i n t e r i n g behaviour. The d i f f e r e n t r a t e s of shrinkage i n Stage I I I were due to a s l i g h t l y lower heating r a t e above the m e l t i n g p o i n t i n PCM-2-25. The major d i f f e r e n c e between the two curves i s th a t the amount of Stage I I I shrinkage i s l e s s i n the sample heated^ to 1110°C. A p o s s i b l e cause i s tha t more e-phase was F i g u r e 62. Dilatometer p l o t f o r Run PCM-2-25. (Normal heating, 22% copper, held ^ at 1110°C.) The dashed curve shows ^ the r e s u l t s f o r Run PCM-2-20. 143 formed i n the more slowly heated sample (PCM-2-25), thus l e s s l i q u i d was a v a i l a b l e f o r the rearrangement process. The s l i g h t l y lower r a t e of Stage V shrinkage i n the sample heated to 1110°C was l i k e l y due to l e s s r a p i d coalescence because of lower d i f f u s i o n r a t e s . 3.6.5 E f f e c t of I n i t i a l Density Comparison of Runs PCM-2-30 and PCM-2-20 (see Figure 63) and Table XVII) shows the e f f e c t of i n i t i a l compact d e n s i t y on s i n t e r i n g behaviour. Although otherwise s i m i l a r to PCM-2-20 i n method of pr e p a r a t i o n and pro c e s s i n g , PCM-2-30 was compacted to a much higher i n i t i a l d e n s i t y (85%, versus 71% of t h e o r e t i c a l a f t e r c l e a n i n g ) . The more dense specimen contracted much l e s s i n Stage I I I , which would be expected because there was l e s s p o s s i b i l i t y of accommodation by the rearrangement process. The ra t e s of shrinkage i n Stage I I I and of expansion i n Stage IV were e s s e n t i a l l y independent of the s t a r t i n g d e n s i t y . I t was observed t h a t f o r the more dense specimen, PCM-2-30, c o n t r a c t i o n was i n i t i a l l y more r a p i d i n Stage V. Ass o c i a t e d w i t h t h a t was the sharper t r a n s i t i o n between Stages IV and V. I t i s speculated t h a t these e f f e c t s are r e l a t e d to the stage i n s i n t e r i n g at which y S L i s i n c r e a s i n g due to removal of oxygen from the system. The denser compact i s penetrated by hydrogen ( i n the dilatometer) w i t h more d i f f i c u l t y . Water JL _L J ^JL 0 2 0 4 0 6 0 8 0 Time (min) 100 ~--l20 140 F i g u r e 63. Dilatometer p l o t f o r Run PCM-2-30. (Normal he a t i n g , 22% copper, high i n i t i a l density.) The dashed curve shows the curve f o r Run PCM-2-20. 145 Table XVII E f f e c t of I n i t i a l Density on Dimensional Changes During Liquid-Phase S i n t e r i n g Sample PCM-2-30 PCM-2-20 I n i t i a l d e n s i t y (p c) gm/cc 6.81 5.74 A %(AL/Lo) +1.30 +1.35 B %(AL/Lo) + .20 - .35 C %(AL/Lo) - .16 -1.67 D %(AL/Lo) + 3.22 +1. 62 B t o e %(AL/Lo) .36 1.32 C to D %(AL/Lo) 3.38 3.29 Slope I I I [ % ( A L / L Q ) ] / m i n -1.4 -1.5 Slope IV [ % ( A L/Lo)]/min +1.6 +1.7 146 vapour produced by the r e d u c t i o n of oxides at i n t e r n a l pores a l s o f i n d s i t more d i f f i c u l t to escape from the denser compact. Thus, i t i s suggested t h a t the major increase i n y„T (which fail causes the d i h e d r a l angle to become > 0) occurred at a l a t e r time i n Stage V f o r PCM-2-30 than f o r PCM-2-20. Once the d i h e d r a l angle has become p o s i t i v e , shrinkag can proceed only by coalescence. The coalescence process i s expected to be more r a p i d i n a lower d e n s i t y compact. Thus, l a t e r i n Stage V the lower d e n s i t y specimen i s observed t o shr i n k more r a p i d l y . 3.6.6 E f f e c t of Copper Content Sample PCM-2-100 contained 10% copper and had a compacted d e n s i t y which was 2.9% (of t h e o r e t i c a l ) higher than t h a t of PCM-2-20 (which contained 22% copper). Comparison of Figures 51 and 64 and Table XVIII i n d i c a t e s t h a t the sample w i t h l e s s copper: I ) C o n t r a c t e d l e s s i n S t a g e I I I i i ) C o n t r a c t e d more s l o w l y i n S t a g e I I I i i i ) E x panded l e s s i n S t a g e IV i v ) E xpanded more s l o w l y i n S t a g e IV v) C o n t r a c t e d much l e s s s l o w l y i n S t a g e V A l l of these observations are c o n s i s t e n t w i t h mech-anisms proposed e a r l i e r . With l e s s l i q u i d phase present, voids w i t h i n the l i q u i d are l a r g e r and the d r i v i n g f o r c e f o r shrinkag 2 . 0 J L _ 140 Time (min) F i g u r e 64. Dilatometer p l o t f o r Run PCM-2-100. (Normal heating, 10% copper.) £ 148 Table XVIII E f f e c t of Copper Content on Dimensional Changes During L i q u i d f P h a s e i S i n t e r i n g . * Sample PCM-2-1000 PCM-2-20 Copper % 10 22 A %(AL/Lo) +1.30 +1.35 B %(AL/Lo) + .70 - .35 C %(AL/Lo) - .10 -1.67 D % (AL/Lo) +1.53 +1.62 B to C %(AL/L 0) .60 1.32 C to D %(AL/Lo) 1.63 3.29 Slope I I I [%(AL/Lo)/min] -1.6 -1.5 : Slope IV [% (AL/L 0)/min] + .7 +1.7 149 by rearrangement or s o l u t i o n due to pressure i s decreased. With l e s s l i q u i d to d i s t r i b u t e at p a r t i c l e "contacts', i n penetrated g r a i n boundaries and over p a r t i c l e s u r f a c e s , one would expect: a) l e s s growth due to grain-boundary p e n e t r a t i o n , b) l e s s shape-change of p a r t i c l e s by the s o l u t i o n - p r e c i p i t a t i o n process, and c) a higher e f f e c t i v e v i s c o s i t y i n the s o l i d - l i q u i d aggregate. I t should be noted t h a t i n PCM-2-100 there was only 10% l i q u i d at the onset of m e l t i n g . As copper d i f f u s e d i n t o the i r o n , the amount of l i q u i d decreased. In e a r l y Stage V, the amount of l i q u i d would have become reduced to <9% by weight and the very low net r a t e of c o n t r a c t i o n i n t h a t stage i s e a s i l y understood. 3.6.7 E f f e c t of a D i f f e r e n t Type of I r o n Powder Sample ATOCM-5 contained Atomet 28 i r o n , whereas PCM-5-6, which was otherwise prepared and processed i n the same way, contained Eastono^iron powderj i^r Comparison-of the two runs (see Table XIX and Figures 53 and 65) i n d i c a t e s t h a t the Atomet i r o n compact expanded l e s s i n Stage IV and, contracted l e s s r a p i d l y i n Stage V. In other respects the d i l a t o m e t r i c curves, f o r the two specimens, were s i m i l a r . Based on m e t a l l o g r a p h i c observations the Atomet 28 powder p a r t i c l e s were i n i t i a l l y more r e g u l a r i n shape. They may have contained fewer g r a i n boundaries, at the s i n t e r i n g 150 Table XIX E f f e c t of Powder Type on Dimensional Changes During L i q u i d Phase S i n t e r i n g Sample PCM-5-6 ATOCM-5 Iron powder type Easton Atomet 28 A % (AL/Lo) +1.30 +1.30 B + .22 - .10 C %(AL/Lo) - .95 -1.27 D %(AL/Lo) +1.02 + .15 B to C %(AL/Lo) 1.17 1.17 C to D %(AL/Lo) 1.97 1.42 Slope I I I [%(AL/Lo)]/min -2.0 * - .7 Slope IV [%(AL/Lo)]/min +1.5 +1.4 * Not i n d i c a t i v e of average r a t e i n Stage I I I . I 1 1 I I I I I I I I I -I 0 2 0 4 0 6 0 8 0 100 120 Time (min) F i g u r e 6 5 . Dilatometer p l o t f o r Run ATOCM-5. (Normal heating, 25 to 37y Atomet i r o n powderi) 152 temperature, than the equi v a l e n t Easton powder p a r t i c l e s . That could e x p l a i n a smaller amount of expansion by g r a i n boundary p e n e t r a t i o n i n Stage IV. I t i s a l s o b e l i e v e d t h a t the Easton powder was more e x t e n s i v e l y fragmented by l i q u i d i n Stage I I I , p r o v i d i n g a higher d e n s i t y of small p a r t i c l e s (and a higher shrinkage rate) f o r s o l u t i o n - p r e c i p i t a t i o n processes i n l a t e r stages. 3.6.8 Use of P r e a l l o y e d Powder Sample PA-2-1 was prepared from p r e a l l o y e d i r o n powder co n t a i n i n g 6.9% copper. S u f f i c i e n t pure copper powder was added to b r i n g the Fe/Cu r a t i o i n the compact to 78/22. Com-pa r i s o n of PA-2-1 w i t h PCM-2-20 (see Figures 66 and 51) shows the e f f e c t of p r e a l l o y e d i r o n on the progress of l i q u i d - p h a s e s i n t e r i n g . PCM-2-20 and PA-2-1 contained the same s i z e of powders. R e l a t i v e to PCM-2-20, the p r e a l l o y e d i r o n powder specimen rearranged more e x t e n s i v e l y i n Stage I I I and e x h i b i t e d almost no Stage IV expansion. The net c o n t r a c t i o n r a t e i n Stage V was a l s o very low. ThoseobbservationsaaiL=eceonsistehtwwithtthe~ f o l l o w i n g : i ) B e c a u s e t h e I r o n i s n e a r l y s a t u r a t e d w i t h c o p p e r f r o m t h e b e g i n n i n g , t h e r e i s l i t t l e f l u x o f c o p p e r a c r o s s t h e l i q u i d - s o l i d i n t e r f a c e . As a r e s u l t , o x y g e n p r e s e n t i n t h e l i q u i d c o p p e r 2.0 154 does n o t become s e g r e g a t e d a t t h e i n t e r -f a c e . The d i h e d r a l a n g l e a c c o r d i n g l y rema i n s p o s i t i v e . i i ) B e c a u s e o f ( i ) , g r a i n b o u n d a r y p e n e t r a t i o n , and t h e e x p a n s i o n a s s o c i a t e d w i t h i t c a n n o t o c c u r . F u r t h e r , t h e amount o f e x p a n s i o n p o s s i b l e by volume d i f f u s i o n i s v e r y l i m i t e d . i i i ) E x t e n s i v e r e a r r a n g e m e n t , i n S t a g e I I I , i s p o s s i b l e b e c a u s e o f t h e n e a r t o t a l a b s c e n c e .of i n t e r f e r e n c e f r o m e x p a n s i o n p r o c e s s e s . i v ) Beyond S t a g e I I I , t h e o n l y s h r i n k a g e mechanism a v a i l a b l e i s c o a l e s c e n c e . Thus t h e r a t e o f o b s e r v e d s h r i n k a g e i s v e r y low. Chapter 4 COMPARISON WITH PREVIOUS WORK 4.1 Comparison w i t h the Results of Kingery A s t r i k i n g o bservation i n t h i s work was the extensive (up to 2.02% (AL/Lo)) and r a p i d shrinkage of Fe-Cu compacts which occurred immediately f o l l o w i n g m e l t i n g of copper, p a r t i c u -l a r l y i n non-presintered specimens. Figure 67 contains l o g -log p l o t s of Stage I I I data f o r s e v e r a l compacts, i n c l u d i n g those which e x h i b i t e d the lowest and highest r a t e s of shrinkage. The change i n length ( A L ) ,of<fa specimen, as a percentage of the l e n g t h , L Q M / at the onset of m e l t i n g , has been p l o t t e d against ( t - t m ) , the time i n t e r v a l a f t e r m elting occurred. In o b t a i n i n g length values from the d i l a t o m e t r i c data a c o r r e c t i o n was made f o r the thermal expansion which occurs as a specimen i s heated above 1070°C i n Stage I I I . The c o r r e c t i o n was s m a l l . The l o g - l o g p l o t s of the data g i v e s t r a i g h t l i n e s w i t h slopes ranging from 0.80 to 1.50 f o r the e a r l y p a r t s of Stage I I I . I f i t i s assumed th a t there i s a s i n g l e dominant (contraction) process o p e r a t i n g e a r l y i n Stage I I I , then the k i n e t i c s of t h a t process are represented by: 155 156 F i g u r e 67. Log f r a c t i o n a l change i n l e n g t h of compacts ( r e l a t i v e to L Q M , the length at m e l t i n g ) , v versus l o g ( t _ t m ) / the time from m e l t i n g , during Stage I I I shrinkage. 157 AL/Lo = 1/3(AV/Vo) = k t n (4.1) where n = (1.15 + 0.35). The r e s u l t s are c o n s i s t e n t w i t h the p r e d i c t i o n s of Kingery's model f o r shrinkage by rearrangement. Other s i n t e r i n g shrinkage mechanisms g i v e values of the time exponent, n, ranging from 0.2 to 0.5. I t may t h e r e f o r e beareas.bhabl>y concluded t h a t the process of rearrangement dominated Stage I I I behaviour. For a number of l i q u i d - p h a s e s i n t e r i n g system, other than Fe-Cu, Eremenko [6] has reported values of n ranging from 1 t o 3 f o r the f i r s t stage of c o n t r a c t i o n , which he i d e n t i f i e d as rearrangement. Kingery [3] s i n t e r e d 7 8/22 Fe-Cu powder mixtures based on i r o n powders of <35u m i n diameter, at 1150°C. F i g u r e 6 contains Kingery's l o g - l o g p l o t s of h i s r e s u l t s . Because the slopes of the p l o t s i n the e e a r l y ' stages of s i n t e r i n g were i n the range 1.3 t o 1.4, Kingery concluded t h a t he was observing shrinkage due to rearrangement. His e a r l i e s t observations f o r the 15.8 and 33.1y m powders were 'at 10 minutes'. However i t i s not c l e a r from h i s published work [14] whether that was 10 minutes a f t e r m e l t i n g , or a f t e r the attainment of 1150°C i n the specimen. Fig u r e 68 shows the d i l a t o m e t r i c p l o t from the present work f o r Run PCM-6, which a l s o contained < 2 5 u m powder p a r t i c l e s . The onset of melting i s shown as t ; the temperature reached 1155°C at t . Ten minutes a f t e r t , the specimen i s at t , , ; s m , ±K 158 _L JL 0 20 40 Time (min) 60 F i g u r e 68. Dilatometer curve f o r Run PCM-5-6. The dotted l i n e shows p r e d i c t e d dimensional changes i f Stage IV expansion d i d not occur. 159 i . e . already at the s t a r t of Stage V c o n t r a c t i o n . F u r t h e r -more by t l k the specimen has already undergone net shrinkage r e l a t i v e to i t s o r i g i n a l (as-cleaned) room-temperature s i z e . Because Kingery based volume or d e n s i t y changes on as-compacted ('green') measurements, he would see c o n t r a c t i o n ( d e n s i f i c a t i o n ) a t the e q u i v a l e n t of time t ^ i n a specimen comparable to PCM-6. In f a c t , i t i s more l i k e l y t h a t Kingery's time base was the attainment of 1150°C ( i n h i s specimens) i n which case h i s f i r s t o b servation would have been even l a t e r i n Stage V than t ^ . I t i s t h e r e f o r e reasonable to conclude t h a t Kingery, perhaps by v i r t u e of h i s d i l a t o m e t r i c technique, completely f a i l e d to detect the Stage I I I c o n t r a c t i o n and the Stage IV expansion observed i n the present work. I t f u r t h e r f o l l o w s t h a t h i s l o g - l o g p l o t s , as i n d i c a t o r s of the processes o c c u r r i n g during l i q u i d - p h a s e s i n t e r i n g , are meaningless. The dimensional, or d e n s i t y changes p l o t t e d f o r a given time are the r e s u l t of the cumulative e f f e c t of the s e v e r a l processes which have operated up to t h a t time. I t i s p o s s i b l e to make rough c o r r e c t i o n s to Kingery's data which remove the e f f e c t s of Stage IV expansion (dotted l i n e F i g u r e 68). On the b a s i s of observed expansion i n PCM-6 an a d d i t i o n of 0.038 (AV/V-'o) has been made to the amount of shrinkage at each of Kingery"s data p o i n t s f o r a 15.8y m powder specimen. The r e s u l t s are shown i n F i g u r e 69, a p l o t of the o r i g i n a l and 'corrected' data. The c o r r e c t e d curve, which 160 > < 1.000 0.500 0.100 0.050 c ~ 0.020 o 0.010 0.005 0.002 • 1 1 1 1 1 r— -AV • • • / / -- / 22.0 % Cu • 15.8 f~Lm • • i • i 1 »— 5 10 20 50 100 200 500 Time From Melting (min) F i g u r e 69. Kingery's data f o r f r a c t i o n a l d e n s i f i c a t i o n versus s i n t e r i n g time ( s o l i d l l i n e ) and the same data c o r r e c t e d to e l i m i n a t e the e f f e c t of Stage IV expansion (dotted l i n e ) . The c o r r e c t i o n was based on the r e s u l t s f o r <25u powder i n the present work. 161 should roughly i n d i c a t e the mechanisms causing shrinkage i n Kingery's specimen, shows two p o r t i o n s . The f i r s t has a slope of 3/4, which i s between the values of n f o r rearrangement and ' s o l u t i o n due to pressure'. However, even the c o r r e c t e d curve has questionable meaning. The time base of the p l o t may f a r from c o i n c i d e w i t h the s t a r t of the p a r t i c u l a r shrinkage process t h a t i s operating during the time range of the data i n Figu r e 69, even i f only one such process i s i n v o l v e d . I t should be noted t h a t Kingery used three i r o n powders i n h i s Fe-Cu experiments, two of which were described only as "magnetic i r o n powder" [14]. The composition of the powders was not gi v e n , but "magnetic" i r o n powders t y p i c a l l y c o n t a in 0.6 to 1.0 per cent carbon. As discussed i n S e c t i o n 1.5.5 and elsewhere i n t h i s chapter, carbon a d d i t i o n s to Fe-Cu compacts can reduce, or e l i m i n a t e , t h a t l a r g e component of expansion which i s a s s o c i a t e d w i t h g r a i n boundary penetra-t i o n by l i q u i d copper. I t i s t h e r e f o r e p o s s i b l e t h a t at l e a s t some of Kingery's samples experienced much l e s s expansion during s i n t e r i n g than d i d those of the present work. However, even i f h i s specimens d i d not e x h i b i t any Stage IV expansion, h i s i n t e r p r e t a t i o n of - h i s - d i l a t o m e t r i c data i s s t i l l i n v a l i d . From the d i l a t o m e t r i c data f o r Fe-Cu runs i n t h i s work, i t i s not p o s s i b l e to i d e n t i f y the time or specimen dimensions at which Stage V ' s t a r t s ' . A c c o r d i n g l y , the k i n e t i c s 162 of Stage V shrinkage cannot be analysed on the b a s i s of l o g -l o g p l o t s of the data. However, me t a l l o g r a p h i c and other observations i n d i c a t e d t h a t ' s o l u t i o n due t o pressure' was the dominant process causing shrinkage during Stage IV and e a r l y Stage V. Therefore both of Kingery's models f o r l i q u i d - p h a s e s i n t e r i n g shrinkage (rearrangement and ' s o l u t i o n due to pressure') apply to the s i n t e r i n g of iron-copper compacts. As demonstrated above, however, Kingery's c l a i m t h a t h i s own experiments w i t h Fe-Cu are i n support of those t h e o r i e s , i s q u i t e u n j u s t i f i e d . 4.2 Comparison w i t h B o c k s t i e g e l ' s Work The observations i n t h i s work of the e f f e c t of p a r t i c l e s i z e on Stage IV and Stage V behaviour are i n c l o s e agreement w i t h those of B o c k s t i e g e l [15] (see F i g u r e 8). B o c k s t i e g e l d i d not observe Stage I I I c o n t r a c t i o n i n any of h i s experiments. This i s r e a d i l y explained i f he h e a v i l y p r e s i n t e r e d a l l h i s specimens p r i o r t o using them f o r d i l a -tometry. D e t a i l s of h i s specimen p r e p a r a t i o n are not described [1.5]; however, the Stage IV expansions which he observed are s i m i l a r to those observed i n the present work f o r p r e s i n t e r e d samples of roughly comparable powder s i z e (e.g. 3.04% (AL/Lo) i n a 53 to 63ti m powder Fe-20 Cu specimen, PCM-2-19) . B o c k s t i e g e l * s compacts presumably expanded l e s s because of lower (copper) l i q u i d content - (7.5% Cu). 163 B o c k s t i e g e l a t t r i b u t e d a l l the expansion i n Fe-Cu compacts to d i f f u s i o n of copper i n t o i r o n . I t has been shown i n t h i s work th a t a more important r o l e i s played by the l i q u i d p e n e t r a t i o n of g r a i n boundaries. B o c k s t i e g e l a l s o proposed t h a t a s o l i d i r o n - r i c h s k e l e t o n e x i s t e d throughout l i q u i d - p h a s e s i n t e r i n g , and t h a t shrinkage was by a s o l u t i o n -r e p r e c i p i t a t i o n process (described i n S e c t i o n 1.4.2). Several problems are a s s o c i a t e d w i t h the mechanism proposed. F i r s t , metallography has shown i n the present work tha t the i r o n i s not i n the form of a s o l i d s k e l e t o n throughout much of the s i n t e r i n g process. Second, B o c k s t i e g e l 1 s theory r e q u i r e s complete w e t t i n g , which was shown not to e x i s t l a t e i n Stage V i n t h i s work. T h i r d , the mechanism i n v o l v e s the long range t r a n s p o r t of a considerable mass of i r o n i n a short time. Yet there i s no obvious d r i v i n g f o r c e f o r such t r a n s p o r t or f o r d i s s o l u t i o n and r e p r e c i p i t a t i o n . 4.3 Comparison w i t h Other Previous Work The Heavy A l l o y Mechanism which was proposed by Lenel causes small i r o n p a r t i c l e s to d i s s o l v e and r e p r e c i p i t a t e on l a r g e r ones. The mechanism was observed to operate i n i r o n -copper i n the present work. The process i s not considered to cause d e n s i f i c a t i o n s i n c e the removal of small p a r t i c l e s and growth of l a r g e ones does not n e c e s s a r i l y lead to complete d e n s i f i c a t i o n without high l i q u i d contents [18]. In f a c t , 164 the mechanism can cause d e n s i f i c a t i o n only i n compacts where the s m a l l p a r t i c l e s hold l a r g e r ones apart. I f the small p a r t i c l e s e x i s t at i n t e r s t i t i a l s i t e s , between l a r g e r ones, no d e n s i f i c a t i o n would r e s u l t from t h e i r removal. As i n t h i s work, other i n v e s t i g a t o r s [14,17,20,21] observed t h a t the f i n a l s i n t e r e d s t r u c t u r e of Fe-Cu compacts (of about 20% copper) were rounded i r o n - r i c h p a r t i c l e s i n continuous contact, w i t h copper f i l l i n g the i n t e r s t i t i a l spaces of the network. No previous i n v e s t i g a t o r , however, has proposed why such a s t r u c t u r e should form f o l l o w i n g g r a i n boundary p e n e t r a t i o n and/or rearrangement. In t h i s work the cause i s considered to be a changing d i h e d r a l angle. 4.4 Grain Boundary P e n e t r a t i o n and the D i h e d r a l Angle Berner et at. [12] observed g r a i n boundary penetra-t i o n during l i q u i d - p h a s e s i n t e r i n g and concluded t h a t i t was the cause of expansion during the l i q u i d - p h a s e s i n t e r i n g of Fe-Cu mixtures. They measured the d i h e d r a l angles observed i n s e c t i o n s of specimens which had been s i n t e r e d at v a r i o u s temperatures. According to the technique [7,9] the most f r e q u e n t l y observed v a l u e , cj)^, i s the t r u e value of cj> f o r a system at a given temperature. Thus according to t h e i r data (see Figure- 70) cf> f o r the system at 1180°C was 21°. In the present work v i r t u a l l y a l l g r a i n boundaries were penetrated by copper during l i q u i d - p h a s e s i n t e r i n g . That 165 0 10 20 30 i.0 50 60 70 80 DIHEDRAL ANGLE 6 [DEG] F i g u r e 70. 166 means t h a t the d i h e d r a l angle was zero during Stages I I I and IV. Because Berner et at. [12] a l s o observed g r a i n boundary-p e n e t r a t i o n (and explained expansion i n terms of g r a i n boundary p e n e t r a t i o n ) , i t must be concluded t h a t the d i h e d r a l angle was a l s o zero i n t h e i r system.totfet theirhmeasurements of a) i n d i c a t e d t h a t i t was p o s i t i v e . Faced w i t h t h a t dilemma, they explained the penetra-t i o n of g r a i n boundaries by c l a i m i n g t h a t the value of a) v a r i e d (as low as z e r o ) , f o r d i f f e r e n t g r a i n boundaries at the same temperature. They based t h a t c l a i m on the f a c t t h a t the d i s -t r i b u t i o n of <J> (as measured i n the mi c r o s t r u c t u r e s ) , changed a s s y m e t r i c a l l y w i t h temperature (see Figure 70). In f a c t , when the most f r e q u e n t l y observed angle (tfV^ ) had a low v a l u e , the d i s t r i b u t i o n was log-normal, whereas the d i s t r i b u t i o n was normal when d) was l a r g e . That the d i s t r i b u t i o n should m change i n such a manner i s e x a c t l y what one would expect, when a) i s the true d i h e d r a l angle. I f d) i s the t r u e (and only) d i h e d r a l angle of a system (and measurements are made of the d i h e d r a l angles observed i n s e c t i o n s taken at random through a three-dimensional matrix) , then the lower i s <J>, the higher i s the frequency of ob s e r v a t i o n of angles l e s s than <j) (= d>m) . Thus the explanation of Berner et al. [12] t h a t cf> was zero f o r some boundaries and not others i s not s a t i s f a c t o r y . I t must t h e r e f o r e be concluded t h a t t h e i r method of measurement of d i h e d r a l angles was erroneous. 167 Berner et al. [12] observed t h a t p r e s i n t e r i n g of i r o n powder compacts decreased the amount of s w e l l i n g during i n f i l t r a t i o n w i t h copper. In t h i s work a s i m i l a r observation was made of the e f f e c t of p r e s i n t e r i n g on the r a t e of expansion ( i n Stage IV) of Fe-22 Cu compacts. This e f f e c t i s a s s o c i a t e d w i t h a delay i n the rearrangement process (due to growth of l a r g e r necks during the p r e s i n t e r i n g treatment) such t h a t there i s more prolonged i n t e r f e r e n c e w i t h Stage IV expansion. The e f f e c t of a d d i t i o n s which reduce expansion i n the Fe-Cu system ( i . e . C, Sn, Mn, P, W) [12] may>:be to reduce the t o t a l amount of oxygen i n the system, or the amount which i s concentrated at s o l i d - l i q u i d i n t e r f a c e s , b y a c t i n g as 'ge t t e r s ' . That would increase the d i h e d r a l angle and reduce or prevent g r a i n boundary p e n e t r a t i o n . Some expansion i n h i b i t o r s (e.g. W) are not good oxygen ' g e t t e r s ' , however. I t i s suggested t h a t they cause an increase i n y or decrease i n Y s s , perhaps as a consequence of segregating to i n t e r f a c e s . Chapter 5 CONCLUSION 5.1 Summary The dimensional and d e n s i t y changes which occur when Fe-Cu powder compacts are s i n t e r e d above the mel t i n g p o i n t of copper can be i n t e r p r e t e d i n terms of the oper a t i o n and i n t e r -a c t i o n of f i v e processes: a) r e a r r a n g e m e n t b) s o l u t i o n due t o p r e s s u r e c ) c o a I e s c e n c e p r o c e s s e s c a u s i n g c o n t r a c t i on d) p e n e t r a t i o n o f s o l i d y-Fe g r a i n b o u n d a r i e s by- l i q u i d c o p p e r e) d i f f u s i o n o f up t o 9 wt % c o p p e r i n t o s o l i d i r o n p r o c e s s e s c a u s i n g e x p a n s i on The shrinkage processes a) to c) occur i n the sequence i n d i c a t e d but they overlap each other i n time to a degree which i s determined by powder p a r t i c l e s i z e , the thermal h i s t o r y of the compact and other parameters. The two expansion 168 169 processes d) and e) a l s o occur together, although expansion due to g r a i n boundary p e n e t r a t i o n terminates e a r l i e r . More-over, the expansion processes operate simultaneously w i t h the f i r s t two of the c o n t r a c t i o n processes. These i n t e r a c t i o n s give r i s e , t y p i c a l l y , to a three-stage sequence of net dimen-s i o n a l changes i n a compact during l i q u i d - p h a s e s i n t e r i n g . The sequence i s : I) R a p i d n e t s h r i n k a g e I I ) R a p i d and e x t e n s i v e n e t e x p a n s i o n I I I ) N et s h r i n k a g e a t a r a t e w h i c h d e c r e a s e s w i t h t i m e A s s o c i a t e d w i t h these dimensional°hehanges. i s the occurrence of the f o l l o w i n g s t r u c t u r a l changes: a) When c o p p e r m e l t s , s o l i d - s t a t e s i n t e r e d c o n t a c t s between i r o n p a r t i c l e s a r e e^liiiirh lima ibed bby aaccofnb iina tiuonoc f d d l s s o I u-t i o n o f i r o n i n c o p p e r and p e n e t r a t i o n o f g r a i n b o u n d a r i e s by l i q u i d . b) G r a i n b o u n d a r i e s i n then-i-ronr.r&ch ' so I i d c o n t i n u e t o be p e n e t r a t e d by l i q u i d , and t h e s y s t e m c o n s i s t s o f s o l i d p a r t i c l e s ' d i s p e r s e d ' i n l i q u i d , u n t i l t h e f i n a l s h r i n k a g e s t a g e i s r e a c h e d . c ) D u r i n g t h e f i n a l s h r i n k a g e s t a g e a s o l i d s k e l e t o n i s r e - e s t a b l i s h e d and s h r i n k a g e o c c u r s by c o a l e s c e n c e o n l y . When compacts are p r e s i n t e r e d f o r prolonged periods i n the s o l i d s t a t e (e.g. at 1000°C), l a r g e r necks are e s t a b l i s h e d between i r o n p a r t i c l e s . The complete d i s s o l u t i o n of necks a f t e r copper melts i s thus delayed, and there i s e a r l i e r 170 i n t e r f e r e n c e between shrinkage by rearrangement and the expan-s i o n processes. The r e s u l t i s a reduced net c o n t r a c t i o n i n the e a r l y stage of l i q u i d - p h a s e s i n t e r i n g , and reduced net expansion subsequently. The i n i t i a l c o n t r a c t i o n stage may even become 'masked' e n t i r e l y . The change from a ' d i s p e r s e d - s o l i d ' to a coalesced s o l i d system i n the l a t e stages of s i n t e r i n g i s as s o c i a t e d w i t h a change i n the value of Y S L f the s o l i d - l i q u i d i n t e r f a c i a l energy. I t i s suggested t h a t YOT i s lowered by the presence of oxygen i n the compact, e s p e c i a l l y when the a v a i l a b l e oxygen has segregated to the s o l i d - l i q u i d i n t e r f a c e s during the p e r i o d i n which copper i s d i f f u s i n g i n t o i r o n . The contact and d i h e d r a l angles i n the system are both zero w h i l e t h i s condi-t i o n p r e v a i l s . Grain boundary p e n e t r a t i o n by l i q u i d copper i s thus p o s s i b l e . When the oxygen co n c e n t r a t i o n i n the compact, or the extent of segregation at s o l i d l i q u i d i n t e r f a c e s becomes reduced, a f t e r longer times of s i n t e r i n g , Y s l increases to the extent t h a t both the contact and d i h e d r a l angles become p o s i t i v e and coalescence of s©ld!d p a r t i c l e s ensues. This e x p l a n a t i o n has not been advanced by any previous i n v e s t i g a t o r s of l i q u i d phase s i n t e r i n g and may be capable of e x p l a i n i n g some reported observations f o r other systems besides Fe-Cu. Small a d d i t i o n s of phosphorous and carbon are reported to markedly reduce the;:amount of expansion which occurs when Fe-Cu powder compacts are l i q u i d - p h a s e s i n t e r e d . 171 This can be explained i f the a d d i t i o n s act as i n t e r n a l de-oxidants i n the system. They may thus prevent the attainment of a zero d i h e d r a l angle and prevent the expansion which i s otherwise a s s o c i a t e d w i t h g r a i n boundary p e n e t r a t i o n . The e f f e c t , on the progress of l i q u i d - p h a s e s i n t e r -i n g , of v a r i a b l e s such as powder p a r t i c l e s i z e , r a t e of heating through the smeltihgg temperature, i n i t i a l d e n s i t y of com-pacts , copper content of the powder mixture and p r e a l l o y i n g of the i r o n powder can be i n t e r p r e t e d i n terms of the p r e d i c t -able e f f e c t s of each v a r i a b l e on the s e v e r a l shrinkage and c o n t r a c t i o n processes and t h e i r mutual i n t e r a c t i o n s . The p r i n c i p a l c o n t r i b u t i o n s made, by t h i s work to an understanding of li q u i d - p h a s e s i n t e r i n g , are as f o l l o w s : I ) D i mens i ona,bh§hanges r a rehcfeheeBesai I (bf o f t h e o p e r a t i o n and i n t e r a c t i o n o f up t o f i v e d i f f e r e n t p r o c e s s e s . 2) W h i l e t h e d i h e d r a l a n g l e i s z e r o , c o n d i t i o n s f o r r a p i d d e n s i f i c a t i o n by r e a r r a n g e m e n t and s o l u t i o n due t o p r e s s u r e e x i s t . However, a z e r o d i h e d r a l a n g l e a l s o p r o m o t e s s i m u l -t a n e o u s e x p a n s i o n by t h e p e n e t r a t i o n o f s o l i d g r a i n b o u n d a r i e s . Thus i n a " c o m p l e t e l y w e t t i n g " s y s t e m ( i . e . 6 = 0) w i t h (J) = 0°, n e t s h r i n k a g e w i l l n o t n e c e s s a r i l y be o b s e r v e d i n a g i v e n s i n t e r i n g p e r i o d . . 3) The c o n t a c t and d i h e d r a l a n g l e s i n some l i q u i d - p h a s e s i n t e r i n g s y s t e m s ( ' i n c l u d i n g Fe-Cu) may change f r o m z e r o t o p o s i t i v e v a l u e s d u r i n g t h e c o u r s e o f s i n t e r i n g , as t h e r e s u l t o f a change i n t h e c o n c e n t r a t i o n o r d i s t r i -b u t i o n o f an i m p u r i t y s p e c i e s i n t h e s y s t e m . The e f f e c t i s a ch a n g e f r o m r a p i d s h r i n k a g e p r o c e s s e s t o t h e s l o w e r p r o c e s s o f c o a l e s c e n c e . 172 4) L a r g e c h a n g e s i n l i q u i d - p h a s e s i n t e r -i n g b e h a v i o u r c a n r e s u l t f r o m s m a l l c h a n g e s i n c o m p a c t i n g and s i n t e r i n g p a r a m e t e r s , due t o t h e c o m p l e x i n t e r -a c t i o n s o f t h e s e v e r a l s h r i n k a g e and e x p a n s i o n p r o c e s s e s , w h i c h a r e a f f e c t e d by t h e same p a r a m e t e r s . T h a t e x p l a i n s t h e marked d i f f e r e n c e i n b e h a v i o u r r e p o r t e d f o r a g i v e n s y s t e m by d i f f e r e n t p r e v i o u s i n v e s t i g a t o r s . 5.2 Future Work I t i s suggested t h a t f u t u r e work i n the Fe-Cu system be d i r e c t e d towards an i n v e s t i g a t i o n of the e f f e c t s of " a d d i t i o n s " (e.g. W, Sn, P) on the values of 8 and <j>, the wetting and d i h e d r a l angles, of the Fe-Cu system. S e s s i l e drop e x p e r i -ments, under c o n d i t i o n s of a c o n t r o l l e d oxygen p o t e n t i a l , i n which small " a d d i t i o n s " are made to the copper l i q u i d should lead to a b e t t e r understanding of how oxygen and other a d d i t i o n s e f f e c t Y S L« The wetting and d i h e d r a l angles may both be measured i n s e s s i l e drop experiments t o g i v e d i r e c t r e s u l t s f o r the e f f e c t of a d d i t i o n s . A r e - a n a l y s i s of e x i s t i n g l i t e r a t u r e on l i q u i d - p h a s e s i n t e r i n g , i n systems other than Fe-Cu, w i t h a view to determin-i n g whether oxygen plays an important r o l e , may le a d to a b e t t e r understanding of the li q u i d - p h a s e s i n t e r i n g of those systems and may r e s o l v e apparently c o n t r a d i c t o r y o b s e r v a t i o n s . REFERENCES [1] Metals Handbook, 1973 e d i t i o n , V o l . 8, p. 293, American Soc i e t y f o r Metals, Metals Park, Ohio. [2] Smith, C.S. Trans. A.I.M.E., 1948, V o l . 175, pp. 15-51. [3] Kingery, W.D. J. Appl. Physics,, 1959, V o l . 3, pp. 301-3T, i „ 306. [4] Gessinger, G.H., H.F. F i s c h m e i s t e r and H.L. Lukas. Powder Metallurgy, 1973, V o l . 16, pp. 119-127. [5] T a y l o r , J.W. Progress in Nuclear Engineering Series3 1959, S e r i e s 5, V o l . 2, pp. 398-416, Pergamon Pr e s s , New York. [6] Eremenko, V.N., Yu. V. N a i d i c h and I.A. Lavrimenko. Liquid Phase Sintering3 1970, pp. 19-21. Consultants Bureau, New York. [7] J . White. S i n t e r i n g and Related Phenomena, 1973, pp. 81-108, Plenum Press, New York. [8] Sundquist, B.E. Acta. Met., 1964, V o l . 12, pp. 67-86. [9] VanvV.Mc.'k, L.H. Trans. A.I.M.E., 1951, V o l . 3, pp. 251-259. [10] Whalen, J . and M. Humenik. Proc. 18th Annual Powder Metallurgy Technical Conference, 1962, pp. 85-98, Metal Powder I n d u s t r i e s Fed., New York. [11] Hough, R.R. and R. R o l l s . Metl Trans., 1971, V o l . 2, pp. 2471-76. 173 174 [12] Berner, D., H.E. Exner and G. Petzow. Modern Developments in Powder Metallurgy, 1974, V o l . 6, pp. 237-50, Metal Powder I n d u s t r i e s Fed., P r i n c e t o n , N.J. [13] Inman, M.C. and H.R. T r i p l e r . Met. Revs., 1963, V o l . 8, pp. 105-166. [14] Kingery, W.D. J. Appl. Physios, 1959, V o l . 3, pp. 307-10. [15] B o c k s t i e g e l , G. Stahl und Essen, 1959, V o l . 79, No. 17, pp. 1187-1201. [16] P r i c e , G.H.S., C.J. S m i t h e l l s and S.V. W i l l i a m s . J. Inst. Met., 1938, V o l . 62, pp. 239-264. [17] L e n e l , F.V. Trans. A.I.M.E., 1948, V o l . 175, pp. 878-95. [18] Kingery, W.D. Ceramic Fabrication Processes, 1958, pp. 131-143, J . Wiley and Sons Inc., New York. [19] Warren, R. J. Mat Sci., 1968, V o l . 3, pp. 471-85. [2.0] N o r t h c o t t , L. and C.J. Leadbeater. Special Report No. 38, pp. 142^-50, Iron and S t e e l I n s t . , London, 1947. [21] Chadwick, R., E.R. B r o a d f i e l d , and S.E. Pugh. Special Report No. 38, pp. 151-157, Iro n and S t e e l I n s t . London, 1947. [2.2] S i l b e r e i s e n , H. Powder Metallurgy, 1961, pp. 611-27, I n t e r s c i e n c e , New York. [23] Dautzenberg, N. Archiv f.d. Eisenhuttenwesen, 1970, V o l . 41, No. 10, pp. 1005-10. [24] T r u d e l , Y. and R. Angers. Procs. 1973 I n t ' l . Powder Metallurgy Conf., 1973, pp. 306-322, Metal Powder I n d u s t r i e s Fed. and Am. Powder Metallurgy I n s t . , P r i n c e t o n , N.J. [25] L e n e l , F.V. The Physics of Powder Metallurgy, 1951, pp. 238-53, McGraw H i l l Book Co., New York. 175 [26] Ramakrishnan, P. and R. Lakshminarasimhan. Int'l. Journal of Powder Metallurgy3 1967, V o l . 3, No. 2, pp. 63-68. [27] Krantz, T. I n t ' l . Journal of Powder Metallurgy, 1969, V o l . 5, No. 2, pp. 33-43. [28] Matsumura, G. Planseeberiohte Pur Pulvermetallurgie, 1961, V o l . 9, pp. 33-35. [29] Gumme.son, P.U. and L. Forss. Proa. 11th Annual Meeting, 1955, pp. 55-65, Metal Powder A s s o c i a t i o n , New York. [30] O'Brien, T.E. and A.C.D. Chaklader. J. Amer. Ceram. Soc, 1974, V o l . 57, No. 8, pp. 329-32. [31] Chaklader, A.C.D. Met a l l u r g y Department, The U n i v e r s i t y of B r i t i s h Columbia, 1974, p r i v a t e communication. 

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