UBC Theses and Dissertations

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UBC Theses and Dissertations

Pore formation in metals Hirschfeld, Deidre Ann 1977

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PORE FORMATION IN METALS by DEIDRE ANN HIRSCHFELD B . S c , C a r n e g i e - M e l l o n U n i v e r s i t y , 1975 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n t h e Depa r tment o f METALLURGY 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 r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRIT ISH COLUMBIA Sep tembe r , 1977 © D e i d r e Ann H i r s c h f e l d , 1977 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of Brit ish Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of M e t a l l u r g y The University of Brit ish Columbia Vancouver 8, Canada Date September 27, 1977 ABSTRACT The f o r m a t i o n o f p o r e s i n c a s t i n g s i s dependen t on l o c a l s o l i d i f i c a t i o n r a t e s and t h e gas c o n t e n t i n t h e l i q u i d m e t a l . S e g r e g a t -i o n o f t h e gas i n t h e i n t e r d e n d r i t i c r e g i o n s and t h e p r e s s u r e d r o p due t o s o l i d i f i c a t i o n s h r i n k a g e c o n t r i b u t e t o t h e homogeneous n u c l e a t i o n o f p o r e s . H e t e r o g e n e o u s n u c l e i and o t h e r n o n - n u c l e a t i n g mechan i sms a r e s u f f i c i e n t b u t n o t n e c e s s a r y f o r p o r e f o r m a t i o n . The se c o n c l u s i o n s d e r i v e f r o m e x p e r i m e n t s on p o r e f o r m a t i o n i n i r o n and a l um inum. The f o r m a t i o n o f p o r e s due t o c a r b o n monox ide i n s u p e r c o o l e d i r o n ha s b e e n i n v e s t i g a t e d as a f u n c t i o n o f t h e d e g r e e o f s u p e r c o o l i n g and c o n c e n t r a t i o n s o f c a r b o n and oxygen i n t h e m e l t . P o r e f o r m a t i o n i n A l and A l + Cu a l l o y s , due t o h y d r o g e n , has b e e n i n v e s t i g a t e d u n d e r a v a r i e t y o f s o l i d i f i c a t i o n c o n d i t i o n s . T h i s i n c l u d e s d i r e c t i o n a l s o l i d i f i c a t i o n , d i r e c t i o n a l c a s t i n g , and c a s t i n g i n t o mou ld s a t l o w and h i g h t e m p e r a t u r e s . The s i z e , d i s t r i b u t i o n , and m o r p h o l o g y o f t h e p o r e s ha s b e e n measu red as a f u n c t i o n o f h y d r o g e n c o n t e n t and a l l o y c o m p o s i t i o n , and r e l a t e d t o t h e c a s t i n g c o n d i t i o n s . E x p e r i m e n t s have a l s o been c o n -d u c t e d on A l c o n t a i n i n g A g ^ ^ t o i n v e s t i g a t e m a c r o s e g r e g a t i o n d u r i n g d i r e c t i o n a l s o l i d i f i c a t i o n o f A l a l l o y s . i i TABLE OF CONTENTS CHAPTER PAGE 1 INTRODUCTION 1 1.1 G rowth o f P o r e s 5 1.2 P o r e N u c l e a t i o n 7 1.2.1 Homogeneous N u c l e a t i o n o f P o r e s . . . . 7 1 .2 .2 H e t e r o g e n e o u s N u c l e a t i o n 11 1.2.3 N o n - N u c l e a t i n g Mechan i sms o f P o r e F o r m a t i o n 13 1.3 O b j e c t i v e o f P r e s e n t Work 21 2 EXPERIMENTAL PROCEDURE 23 2.1 A l um inum 23 2 . 1 . 1 P r e p a r a t i o n o f A l , A l + C u , and A l + T i B 2 I n g o t s C o n t a i n i n g Hyd rogen . . 23 2 . 1 . 2 C a s t i n g C o n d i t i o n s 25 2 . 1 . 3 D e n s i t y Measu rement s 29 2 . 1 . 4 Hyd rogen A n a l y s i s 30 2 . 1 . 5 D i r e c t i o n a l S o l i d i f i c a t i o n o f H i g h P u r i t y A l um inum ' 37 2 . 1 . 6 T r a c e r S t u d i e s 39 2 .1 .7 P o l i s h i n g and E t c h i n g o f A l and A l A l l o y s 42 2.2 S u p e r c o o l i n g o f I r o n 44 2 . 2 . 1 M a t e r i a l s and A p p a r a t u s 44 2 . 2 . 2 T e m p e r a t u r e Measurement 44 2 . 2 . 3 P r o c e d u r e . . . . 46 2 . 2 . 4 A d d i t i o n o f Ca rbon and Oxygen 48 2 . 2 . 5 E x a m i n a t i o n and A n a l y s i s 48 3 RESULTS . . 49 3.1 E f f e c t o f C a s t i n g C o n d i t i o n s on P o r o s i t y i n A l and A l A l l o y s . . . . 49 3 .1 .1 P o r o s i t y i n P u r e A l um inum C a s t i n g s . . . . 49 i i i CHAPTER PAGE 3 . 1 . 2 E f f e c t o f Mou l d T e m p e r a t u r e on P o r o s i t y . . . . . . 53 3 . 1 . 3 D i r e c t i o n a l l y C a s t A l and A l + 3% Cu 57 3.2 D i r e c t i o n a l S o l i d i f i c a t i o n o f H i g h P u r i t y A l um inum 62 3.3 D i r e c t i o n a l S o l i d i f i c a t i o n o f A l + A g 1 1 0 . . . . 76 3.4 P o r o s i t y i n S u p e r c o o l e d I r o n 84 4 DISCUSSION 100 4 . 1 P o r o s i t y i n D i r e c t i o n a l l y S o l i d i f i e d A l and A l + A g 1 1 0 100 4 . 1 . 1 P o r o s i t y i n H i g h P u r i t y A l um inum . . . . 100 4 . 1 . 2 D i r e c t i o n a l S o l i d i f i c a t i o n o f A l + A g 1 1 0 105 4 . 2 The E f f e c t o f C a s t i n g C o n d i t i o n s on P o r o s i t y . . 106 4 . 2 . 1 P o r o s i t y and S e g r e g a t i o n 107 4 . 2 . 2 I n f l u e n c e o f Hyd r ogen C o n t e n t on P o r o s i t y 108 4 . 2 . 3 The E f f e c t o f G r a i n S i z e and Second Pha se M a t e r i a l 109 4 . 2 . 4 The Rimmed D i s t r i b u t i o n o f P o r e s 110 4 . 2 . 5 U n i f o r m D i s t r i b u t i o n o f P o r e s . . . . I l l 4 . 2 . 6 K i n k i n g a t G r a i n B o u n d a r i e s I l l 4 . 3 N u c l e a t i o n o f P o r e s i n S u p e r c o o l e d I r o n . . . . 112 4 . 3 . 1 . N o n - M e t a l l i c I n c l u s i o n s . . . . . . . . 113 4 . 3 . 2 E f f e c t o f Oxygen and Ca rbon C o n t e n t 113 4 . 3 . 3 S u p e r c o o l i n g 114 5 CONCLUSIONS 117 6 SUGGESTIONS FOR FUTURE WORK 118 REFERENCES 119 i v L I ST OF TABLES T a b l e No. Page 2.1 A l and A l A l l o y s c a s t i n t o g r a p h i t e mou ld s 26 2.2 D i r e c t i o n a l l y s o l i d i f i e d c a s t s i n f i b e r f a x t u b e s 27 2 .3 D e n s i t y measurements o f A l i n w a t e r and m e r c u r y 31 2.4 A l and A l + Cu c a s t f o r h y d r o g e n a n a l y s i s 34 3.1 P o r o s i t y i n A l c a s t i n g 50 3.2 O b s e r v a t i o n s o f p o r o s i t y i n A l and i t s a l l o y s c a s t i n t o h e a t e d g r a p h i t e mou ld s . . . . 55 3.3 P o r o s i t y i n d i r e c t i o n a l c a s t i n g s 59 3.4 P o r o s i t y i n d i r e c t i o n a l l y s o l i d i f i e d h i g h p u r i t y a l um inum 63 3.5 P o r o s i t y i n s u p e r c o o l e d i r o n 86 v L I S T OF FIGURES F i g u r e No. Page 1.1 S o l u b i l i t y o f H yd r ogen i n A l a s a f u n c t i o n o f T e m p e r a t u r e f r o m Ohno e t a l (2) 9 1.2 R e l a t i o n s h i p be tween p o r e s and s o l i d - l i q u i d i n t e r f a c e g r o w t h a t (a) s l o w , (b) f a s t , and ( c ) i n t e r m e d i a t e g r o w t h r a t e s (10) 6 1.3 S e g r e g a t i o n o f h y d r o g e n m s t e e l a s a f u n c t i o n o f f r a c t i o n s o l i d i f i e d f r o m F r e d r i k s s o n ^ - " - 3 ) . C u r v e a i s h y d r o g e n s o l u b i l i t y p r e s u m i n g homo-geneous n u c l e a t i o n , c u r v e b i s t h e s o l u b i l i t y f o r homogeneous n u c l e a t i o n w h i c h has been a d j u s t e d f o r t h e p r e s s u r e d r o p a c c o r d i n g t o s o l i d i f i c a t i o n s h r i n k a g e , and c u r v e c i s t h e s e g r e g a t i o n o f h y d r o g e n i n t h e l i q u i d a c c o r d i n g t o e q n . 6 . P o i n t d marks t h e p o i n t whe re homo-geneous n u c l e a t i o n t h e o r e t i c a l l y t a k e s p l a c e . The s o l i d i f i c a t i o n r a t e i s 108 cm/min and t h e i n i t i a l h y d r o g e n c o n c e n t r a t i o n i s 14 ppm 10 1.4 S c h e m a t i c d i a g r a m o f m o d e l f o r p o r e f o r m a t i o n i n a l uminum a c c o r d i n g t o Uda and Ohno (2) 14 1.5 P r e s s u r e i n t h e r e s i d u a l l i q u i d o f a 1 cm r a d i u s c y l i n d e r o f i r o n . The f r a c t u r e p r e s s u r e n e c e s s a r y f o r homogeneous n u c l e a t i o n i n t e r -s e c t s t h e c a l c u l a t e d p r e s s u r e l i n e a t c o r e r a d i i b e t w e e n 1 0 ~ 6 - 1 0 - 7 cm. F rom C a m p b e l l ( 3 ° ) . . . . 18 1.6 Assumed m o t i o n o f e n t r a i n e d b u b b l e s i n a t i l t e d mou ld f r o m B u r n s and B e e c h ( 2 3 ) 20 2.1 S c h e m a t i c r e p r e s e n t a t i o n o f a p p a r a t u s f o r i n j e c t i n g s t eam i n t o A l b a t h 24 2.2 S c h e m a t i c r e p r e s e n t a t i o n o f t h e c a s t i n g a r r a n g e m e n t f o r d i r e c t i o n a l l y c a s t i n g o t s 28 2.3 S c h e m a t i c r e p r e s e n t a t i o n o f c y l i n d r i c a l c o p p e r mou ld f o r h y d r o g e n a n a l y s i s s amp le s 32 v i F i g u r e No. Page 2.4 Hyd r ogen c o n t e n t a s a f u n c t i o n o f i m m e r s i o n t i m e f o r two i m m e r s i o n - p o u r i n g t e m p e r a t u r e s . . . . . . 35 2.5 R e l a t i o n s h i p be tween h y d r o g e n c o n t e n t and p o u r i n g t e m p e r a t u r e 36 2.6 S c h e m a t i c r e p r e s e n t a t i o n o f a p p a r a t u s u s ed i n d i r e c t i o n a l s o l i d i f i c a t i o n o f h i g h p u r i t y a l uminum 38 2.7 T e m p e r a t u r e p r o f i l e o f v e r t i c a l f u r n a c e u sed i n d i r e c t i o n a l s o l i d i f i c a t i o n e x p e r i m e n t s . The s o l i d l i n e i s f o r t h e f u r n a c e w i t h o u t s amp le s and t h e d o t t e d l i n e s show t e m p e r a t u r e o f t h e s amp le s s o l i d i f i e d i n t h e up and down d i r e c t i o n s 40 2.8 S c h e m a t i c r e p r e s e n t a t i o n o f s u p e r c o o l i n g a p p a r a t u s . . 45 2.9 T e m p e r a t u r e - t i m e r e c o r d show ing s u p e r c o o l i n g o f samp le #2 f o r c y c l e s 4 ,5 and 6 . . 47 3.1 C a s t i n g 4 - 2 9 - 1 s how ing r immed p o r o s i t y and t h e g r a i n s t r u c t u r e . P o r e d i s t r i b u t i o n f o r bands A and B a r e shown i n F i g u r e 3.2 (4 X ) 51 3.2 P o r e d i s t r i b u t i o n a c r o s s c a s t i n g 4 - 2 9 - 1 demon-s t r a t i n g r immed p o r o s i t y . A c o r r e s p o n d s t o Band A and B t o Band B i n F i g u r e 3 . 1 . P o r e s c o u n t e d w e r e t h o s e v i s i b l e a t 5 X m a g n i f i c a t i o n 52 3.3 K i n k e d g r a i n b o u n d a r i e s i n A luminum c a s t i n g 4 - 2 9 - 1 i n d i c a t e d by a r r o w . The k i n k s a r e a s s o c i a t e d w i t h p o r e s (11 X) 54 3.4 C a s t s t r u c t u r e o f A l + 0 .25 w/o T i B 2 c a s t i n t o 625°C m o u l d . N o t e p o r o s i t y i n r immed c o n f i g u r a t i o n ( 2 .1 X ) 54 3.5 I n t e r d e n d r i t i c p o r e i n A l + 1% Cu c a s t i n g 6 -10 -4 (150 X) 58 3.6 P o r o s i t y i n c a s t 5 - 2 6 - 2 , A l + 3% C u , s how ing p o r e s i n t h e i n t e r d e n d r i t i c s e cond pha se (8 X ) . . . . 58 3.7 A l d i r e c t i o n a l l y c a s t i n t o f i b e r f a x t u b e on a Cu c h i l l , c a s t i n g 5 - 4 -2 p o r e d i s t r i b u t i o n f o r bands A and B a r e shown i n F i g u r e 3.8 ( 2 . 33 X) 60 v i i F i g u r e No. Page 3.8 P o r e d i s t r i b u t i o n a c r o s s F i g u r e 3 . 7 . A i s p o r e c o u n t f o r Band A and B f o r Band B. The p o r e c o u n t i s f o r v i s i b l e p o r e s a t 5 X m a g n i f i c a t i o n 61 3.9 U n i f o r m l y d i s t r i b u t e d p o r o s i t y i n Rod 1-Up. N o t e t h a t t h e p o r e s a r e m o s t l y s p h e r i c a l w i t h a f ew e l o n g a t e d . Hyd rogen c o n t e n t i s 0 .20 H 2 / 1 0 0 gm A l (25 X ) 65 2 3.10 Number o f p o r e s / c m and % a r e a p o r o s i t y i n Rod 1 - Down. The number o f p o r e s i s c h a r a c t e r i z e d by 2 s i z e s : p o r e s 1-5 ym i n d i a m e t e r and p o r e s > 30 ym i n d i a m e t e r 66 3.11 D i s t r i b u t i o n o f p o r e s i z e s a l o n g t h e l e n g t h o f Rod 1-Down. The f i r s t g r a p h i s f o r t h e i n i t i a l 2 cm o f t h e r o d , t h e s econd f o r t h e m i d d l e 2 cm, and t h e f i n a l i s f o r t h e l a s t 2 cm o f r o d t o s o l i d i f y ..' 67 2 3.12 Number o f p o r e s / c m and % a r e a p o r o s i t y i n Rod 2 - Up 69 3.13 D i s t r i b u t i o n o f p o r e s i z e s a l o n g t h e l e n g t h o f Rod 2-Up 70 2 3.14 Number o f p o r e s / c m and % a r e a p o r o s i t y i n Rod 3 - Up 71 3.15 D i s t r i b u t i o n o f p o r e s s i z e s a l o n g t h e l e n g t h o f Rod 3-Up 72 2 3.16 Number o f p o r e s / c m and % a r e a p o r o s i t y i n Rod 4 - Down . . . . 73 3.17 D i s t r i b u t i o n o f p o r e s s i z e s a l o n g t h e l e n g t h o f Rod 4-Down 74 3.18 M i c r o g r a p h o f Rod 4-Down i l l u s t r a t i n g t y p e B p o r o s i t y (25 X ) 75 3.19 E t c h e d m i c r o s t r u c t u r e o f d i r e c t i o n a l l y s o l i d i -f i e d h i g h p u r i t y a l uminum (A) Co lumnar g r a i n s o f Rod 4-Up (11 X ) , (B) S u b s t r u c t u r e o f Rod 2-Down (25 X) 77 v i i i F i g u r e No. Page 3 .20 P l o t o f c o n c e n t r a t i o n o f A g ^ ^ a l o n g t h e l e n g t h o f t h e r o d . T h e o r e t i c a l C S and CL, f o r d i f f u s i o n c o n t r o l l e d s e g r e g a t i o n o f t h e r o d s o l i d i f i e d i n t h e up d i r e c t i o n , a r e d o t t e d l i n e s 79 3.21 Ag^" "^ c o n c e n t r a t i o n p l o t t e d a s a f u n c t i o n o f d i s t a n c e a l o n g t h e r o d s o l i d i f i e d downward. T h e o r e t i c a l C S and f o r c o m p l e t e m i x i n g a r e p l o t t e d as d o t t e d l i n e s 80 3.22 D e n d r i t i c m i c r o s t r u c t u r e o f d i r e c t i o n a l l y s o l i d i f i e d A l + A g H O . The a r r o w s i n d i c a t e S i i n c l u s i o n (15 X) . . 82 3.23 S e g r e g a t i o n o f Ag i n t h e i n t e r d e n d r i t i c r e g i o n s . The numbers i n t h e m i c r o g r a p h c o r r e s p o n d s t o t h e peak s i n t h e i n t e n s i t y - d i s t a n c e c u r v e (50 X ) . . . . 85 3.24 P o r o s i t y p l o t t e d as a f u n c t i o n o f s u p e r c o o l i n g i n m e l t s 1 , 2 and 3. ( c a r b o n < 0 .02% and o x y gen ^ 250 ppm) 87 3.25 P o r e d i s t r i b u t i o n i n m e l t s 1, 2 and 3 88 3.26 M i c r o g r a p h s s how ing s p h e r i c a l p o r e s i n m e l t 3A i s a s - p o l i s h e d and B i s e t c h e d (350 X) 90 3.27 P o r e s i z e d i s t r i b u t i o n i n m e l t 4 . . 91 3.28 M i c r o g r a p h s s how ing p o r o s i t y i n m e l t 6. P h o t o A a s - p o l i s h e d and B as e t c h e d (350 X) 93 3.29 R e l a t i o n s h i p be tween p o r o s i t y and c a r b o n c o n t e n t i n s u p e r c o o l e d i r o n 94 3 .30 P o r e s i z e d i s t r i b u t i o n i n m e l t s 5 and 6 95 3.31 A s - p o l i s h e d (A) and e t c h e d (B) m i c r o g r a p h s o f m e l t 7 s how ing p o r o s i t y (350 X ) 97 3.32 P o r e s i z e d i s t r i b u t i o n i n m e l t s 7, 8 and 9 . . . . 98 i x F i g u r e No. Page 4 .1 S c h e m a t i c r e p r e s e n t a t i o n o f p o r o s i t y o b s e r v e d i n d i r e c t i o n a l l y s o l i d i f i e d h i g h p u r i t y alumimum (a) Low h y d r o g e n c o n c e n t r a t i o n s o l i d i f i e d i n b o t h up and down d i r e c t i o n (b) H i g h h y d r o g e n c o n t e n t s o l i d i f i e d downwards ( c ) H i g h h y d r o g e n c o n t e n t s o l i d i f i e d upwards . . . . 101 4 .2 R e l a t i o n be tween number o f p o r e s and c a r b o n c o n c e n t r a t i o n s f r o m Ohkubo e t a l ^ 9 ) n = number o f b l o w h o l e s p e r u n i t c r o s s - s e c t i o n a r e a 115 x ACKNOWLEDGEMENT The a u t h o r w o u l d l i k e t o e x p r e s s h e r t h a n k s t o h e r r e s e a r c h s u p e r v i s o r , D r . F r e d W e i n b e r g , f o r a l l h i s h e l p i n t h e c o m p l e t i o n o f t h i s t h e s i s . The a s s i s t a n c e o f t h e d e p a r t m e n t t e c h n i c i a n s and g r a d u a t e s t u d e n t s was g r e a t l y a p p r e c i a t e d as was t h e Depa r tmen t o f M e t a l l u r g y f o r f i n a n c i a l a s s i s t a n c e . The a u t h o r w o u l d a l s o l i k e t o a c k n o w l e d g e t h e E s c o F o u n d r i e s , W e s t e r n Canada S t e e l and A l c a n R e s e a r c h L a b s f o r c h e m i c a l a n a l y s e s . x i 1. INTRODUCTION The p r e s e n c e o f p o r e s , f o rmed d u r i n g s o l i d i f i c a t i o n , may s i g n i f i c a n t l y a l t e r t h e m e c h a n i c a l p r o p e r t i e s o f c a s t and w rough t p r o d u c t s . The p o r e s a r e e i t h e r k e p t t o a minimum p r o d u c i n g a sound c a s t i n g o r u s ed i n a c o n t r o l l e d f a s h i o n as i n r immed s t e e l . The s o u r c e s o f t h e p o r o s i t y a r e s o l i d i f i c a t i o n s h r i n k a g e and t h e d e c r e a s e i n s o l u b i l i t y o f a gas i n t h e m e l t w i t h d e c r e a s i n g t e m p e r a t u r e and s o l i d i f i c a t i o n . The d e c r e a s e i n s o l u b i l i t y w i t h d e c r e a s i n g t e m p e r a t u r e o f h y d r o g e n i n a l um inum i s shown i n F i g . 1 . 1 . The c o n d i t i o n s f o r p o r e f o r m a t i o n i n c a s t i n g s ha s b e e n d e t e r m i n e d i n t e rms o f m e l t c o m p o s i t i o n and p r o c e s s i n g v a r i a b l e s . The (3) f i n d i n g s o f s e v e r a l a u t h o r s f o l l o w . E v s t r a t o v and G a l k i n h a v e shown t h a t by a l t e r i n g d e o x i d a n t s and s l a g c o m p o s i t i o n s , p o r o s i t y c a n be v a r i e d i n e l e c t r i c f u r n a c e p r a c t i c e . The se f i n d i n g s w e r e b a s e d on s t a t i s t i c a l a n a l y s e s o f f a c t o r s common t o r e j e c t e d c a s t i n g s . T h e i r s t u d y showed t h a t p o r o s i t y i n c r e a s e s i n l ow c a r b o n s t e e l w i t h d e c r e a s i n g c a r b o n and s i l i c o n c o n t e n t s and i n c r e a s i n g c a s t i n g t e m p e r a t u r e . I n s t a i n l e s s s t e e l s , G a l k i n (4) e t a l h a v e shown t h a t t i t a n i u m n i t r i d e s decompose i n t o t i t a n i u m d i o x i d e and ga seous n i t r o g e n w h i c h f o rms s u b c r u s t p o r o s i t y . H i g h c a s t i n g t e m p -e r a t u r e s and h i g h t i t a n i u m c o n c e n t r a t i o n s w e r e f o u n d t o i n c r e a s e p o r o s i t y . T h i s i n f o r m a t i o n was u sed t o p r o d u c e b e t t e r q u a l i t y c a s t i n g s . F o r h y d r o g e n i n s t e e l , G u l y a e v and S o l n t s e v ^ " ^ show t h a t c o n t r o l o f m e l t t e m p e r a t u r e , s l a g c h e m i s t r y , p o u r i n g t e m p e r a t u r e , and d e o x i d i z i n g c o n d i t i o n s p l a y an i m p o r t a n t r o l e i n p o r o s i t y c o n t r o l . 1 2 Figure 1.1 Solubi l i ty of hydrogen in A l as a function of temperature from Ohno et a l (2) . 3 A n o t h e r deve l opmen t i n t h e u n d e r s t a n d i n g o f p o r o s i t y was t o a p p l y t he rmodynam i c s t o p o r e f o r m a t i o n . These c a l c u l a t i o n s we re ba sed on e q u i l i b r i u m s e g r e g a t i o n o f t h e gas and g a s - f o r m i n g s p e c i e s d u r i n g s o l i d i f i c a t i o n . E x p e r i m e n t s were done t o e s t a b l i s h t h e c r i t e r i a f o r p o r e f o r m a t i o n and u s e d t o d e t e r m i n e e m p i r i c a l p a r a m e t e r s . I n t h i s w a y , g u i d e -l i n e s f o r i n d u s t r i a l p r a c t i c e we re e s t a b l i s h e d . N i l l e s ^ ' ^ was among t h e f i r s t t o t a k e t h i s a p p r o a c h f o r c a l c u l a t i n g t h e p o r o s i t y i n r immed and s e m i - k i l l e d s t e e l s . M a t s u i e t a l ^ ' ^ a d a p t e d N i l l e s c a l c u l a t i o n s t o c o n d i t i o n s p e r t i n e n t t o t h e i r r immed s t e e l p r o d u c t i o n and added t h e f o l l o w i n g c r i t e r i o n f o r b l o w h o l e f o r m a t i o n : R - f <_ a (1) where R = g r owth r a t e o f b l o w h o l e s f = s o l i d i f i c a t i o n r a t e a = a c o n s t a n t r e p r e s e n t i n g t h e a b i l i t y o f a b u b b l e t o s e p a r a t e f r o m t h e s o l i d i f i c a t i o n i n t e r f a c e R i s c o n s i d e r e d p r o p o r t i o n a l t o t h e amount o f c a r b o n m o n o x i d e f o r m e d , f ' i s a measured q u a n t i t y , and a i s d e t e r m i n e d b y o b s e r v i n g t h e number o f b l o w h o l e s f ound i n c a s t i n g s made u n d e r v a r i o u s c o n d i t i o n s . R e l a t i o n s h i p s d e t e r m i n e d by M a t s u i and c o - w o r k e r s i n c l u d e i n t e n s i t y o f r i m m i n g a c t i o n , amount o f p o r o s i t y , and s h e l l t h i c k n e s s as a f u n c t i o n o f t e e m i n g r a t e and c o m p o s i t i o n o f t h e m e l t . The nucleation and growth of pores have been the subject of numerous studies. The growth of pores i s well understood and i s br i e f ly described in Section 1 . 1 . The nucleation of pores- the subject of this thesis- is described in Section 1 . 2 . 5 1.1 G rowth o f P o r e s * The g r o w t h o f p o r e s , once t h e y have r e a c h e d t h e c r i t i c a l n u c l e u s s i z e , c an be d e s c r i b e d i n t e rms o f s o l i d i f i c a t i o n r a t e and t h e d i f f u s i o n o f gas t o t h e p o r e ^ ' . A t s l o w s o l i d i f i c a t i o n r a t e s , t h e gas d i f f u s e t o t h e p o r e f o r m i n g a l a r g e p o r e ahead o f t h e s o l i d i f i c a t i o n f r o n t . When t h e s o l i d i f i c a t i o n r a t e i s h i g h , t h e p o r e becomes e n t r a p p e d b e h i n d t h e i n t e r f a c e and c a n n o t grow f u r t h e r . F i n a l l y a t i n t e r m e d i a t e g r o w t h r a t e s , t h e p o r e become e l o n g a t e d as t h e r e i s a b a l a n c e be tween t h e gas d i f f u s i n g t o t h e p o r e and t h e s o l i d i f i c a t i o n r a t e . The i n f l u e n c e o f s o l i d i f i c a t i o n r a t e on p o r e s s i z e and shape i s shown s c h e m a t i c a l l y i n F i g . 1.2 f o r s l o w , r a p i d , and i n t e r m e d i a t e r a t e s . I t s h o u l d be n o t e d t h a t p o r e g r o w t h i s s e l dom as s i m p l e as p r e s e n t e d a b o v e . L o c a l changes i n t h e s o l i d i f i c a t i o n r a t e o c c u r due t o f l u c t u a t i o n s i n t h e e x t r a c t i o n o f h e a t . T h i s i s more p r e v a l e n t i n m e t a l s s u c h as a luminum w h i c h have a h i g h t h e r m a l c o n d u c t i v i t y ( F o r A l s o l i d - 0.48 c a l / c m - s e c ° K ) . P o r e g r o w t h r e s u l t s m a i n l y by t h e d e c r e a s e i n s o l u b i l i t y o f t h e gas w i t h d e c r e a s i n g t e m p e r a t u r e . As shown i n F i g u r e 1 . 1 , t h e s o l u b i l i t y o f h y d r o g e n i n A l d e c r e a s e s s i g n i f i c a n t l y w i t h t e m p e r a t u r e ; h y d r o g e n and n i t r o g e n behave s i m i l a r l y i n i r o n a n d i t s a l l o y s . I n i r o n , t h e r e i s t h e a d d i t i o n a l f a c t o r o f c a r b o n monox ide f o r m a t i o n . The c a r b o n i n s o l u t i o n and oxygen i n s o l u t i o n combine a t a s u i t a b l e s i t e , e . g . a b u b b l e i n t h e m e l t , and f o r m CO g a s . S e g r e g a t i o n ahead o f t h e s o l i d -6 Figure 1.2 Relationship between pores and s o l i d -l iquid interface growth at (a) slow (b) fast , and (c) intermediate growth rates(10). 7 l i q u i d i n t e r f a c e p r omote s t h e g r o w t h o f p o r e s by s u p e r s a t u r a t i n g t h e m e l t . The c o m b i n a t i o n o f s e g r e g a t i o n e f f e c t s and d e c r e a s i n g s o l u b i l i t y y i e l d s a f a v o r a b l e e n v i r o n m e n t f o r p o r e g r o w t h . 1. 2 P o r e N u c l e a t i o n The f o r m a t i o n o f p o r e s i n c a s t i n g s c a n be d e s c r i b e d i n t e r m s o f homogeneous n u c l e a t i o n , h e t e r o g e n e o u s n u c l e a t i o n , o r n o n - n u c l e a t i n g phenomena. E a ch o f t h e s e mechan i sms i s d e s c r i b e d b e l o w . 1.2.1 Homogeneous N u c l e a t i o n o f P o r e s Homogeneous n u c l e a t i o n o f p o r e s o c c u r s when p o r e s f o r m i n t h e m e l t w i t h o u t b e i n g i n f l u e n c e d by i m p u r i t i e s o r e x t e r n a l s u r f a c e s . (13) F r e d r i k s s o n and S ven s s on g i v e e v i d e n c e t h a t p o r e s c a n f o r m by homogeneous n u c l e a t i o n i n t h e i n t e r d e n d r i t i c r e g i o n s o f a c a s t i n g . The a c t i v a t i o n ene r g y f o r n u c l e a t i o n o f a p o r e by homogeneous (14) n u c l e a t i o n i s g i v e n by t h e f o l l o w i n g f o r m u l a e a d a p t e d f o r m H i r t h e t a l AF* = (2) 3(AF r and V M 8 where F^ = H e l m h o l t z f r e e e n e r g y P = p r e s s u r e i n t h e b u b b l e r v P g = e q u i l i b r i u m p r e s s u r e o f gas i n t h e m e l t a = s u r f a c e t e n s i o n be tween gas and m e l t = p a r t i a l m o l a r v o l ume o f gas i n t h e m e l t T = t e m p e r a t u r e "K From T u r n b u l l e t a i ^ 1 5 ' 1 6 ) A F * = 6 0 1 ^ . E q u a t i o n s ( 2 ) and (3) c a n be comb ined g i v i n g a ? T / ? e r a t i o . A s s um ing S i e v a r t ' s l a w i s v a l i d , n a m e l y : (4) a = Kp 2 g g where a ^ = a c t i v i t y o f t h e gas i n s o l u t i o n K = e q u i l i b r i u m c o n s t a n t p = p a r t i a l p r e s s u r e o f t h e gas t h e c r i t i c a l gas c o n c e n t r a t i o n n e c e s s a r y f o r homogeneous n u c l e a t i o n t o o c c u r c an be c a l c u l a t e d . F r e d r i k s s o n e t a l u s e s t h i s a p p r o a c h f o r h y d r o g e n i n s t a i n l e s s s t e e l . The p r e s s u r e d r o p i n t h e i n t e r d e n d r i t i c r e g i o n as a f u n c t i o n o f f r a c t i o n s o l i d i f i e d i s g i v e n b y ^ " ^ ' 2 ^ : where (3 = s o l i d i f i c a t i o n s h r i n k a g e L = l e n g t h o f mushy zone V = s o l i d i f i c a t i o n r a t e 9 L-l = distance from so l id i f i ca t ion front X = primary dendrite spacing This change in pressure along with Equations(2), (3) and (4) determine the hydrogen concentration as a function of fraction so l id . Last ly , the segregation of hydrogen in the melt as a function of fraction sol id i s given by: c ° rL = H (6) Si [ l - f ( l -k ) ] where C° = or ig inal hydrogen content f = fraction sol id k = part i t ion coefficient between l iqu id and austenite Combining the above equations, the c r i t i c a l hydrogen concentration necessary for homogeneous nucleation decreases to half the value when the interdendrit ic pressure drop i s included. A plot of the hydrogen con-centration as a function of fraction so l id for three different conditions is shown in F i g . 1.3. Curve a is for homogeneous nucleation, curve b is for homogeneous nucleation including the pressure drop, and c is the segregation curve for hydrogen. Point d indicates the fract ion so l id at which pore formation should occur. Pore formation therefore can occur in this system at fraction solids greater than 0.7 under the specified conditions. (12 17 18) Mori et a l * ' agree that the interdendrit ic pressure drop 10 60T F R A C T I O N S O L I D Figure 1.3 Segregation of hydrogen in steel as a function of fraction so l id i f i ed from Fredr icksson( i 3 ) . Curve a i s hydrogen so lub i l i ty presuming homogeneous nucleation, curve b is the so lubi l i ty for homogeneous nucleation which has been adjusted for the pressure drop according to so l id i f i ca t ion shrinkage, and curve c is the segregation of hydrogen in the l iqu id according to Equation (6). Point d marks the point where homogeneous nucleation theoretically takes place. The so l id i f i ca t ion rate i s 108 cm/min and the i n i t i a l hydrogen concentration i s 14 ppm. i s s u f f i c i e n t f o r homogeneous n u c l e a t i o n o f CO p o r e s i n i r o n t o o c c u r . Thus t h e y p o s t u l a t e t h a t e q u i l i b r i u m i s e s t a b l i s h e d be tween t h e CO p o r e s ( 1 9 ) and t h e i n t e r d e n d r i t i c l i q u i d . P i w o n k a and F l e m i n g s c o n c l u d e t h a t i n A l and A l + 4.5% Cu , t h e p o r o s i t y i s a r e s u l t o f an a d d i t i v e e f f e c t o f s o l i d i f i c a t i o n s h r i n k a g e ( d e c r e a s e i n i n t e r d e n d r i t i c p r e s s u r e ) and t h e d e c r e a s i n g s o l u b i l i t y o f d i s s o l v e d ga s . 1 .2 .2 H e t e r o g e n e o u s N u c l e a t i o n H e t e r o g e n e o u s n u c l e a t i o n o f p o r e s o c c u r s when t h e p o r e s n u c l e a t e on p a r t i c l e s p r e s e n t i n t h e s y s t e m o r on t h e s u r f a c e o f t h e c o n t a i n e r . (21) Tu r kdogan assumes t h a t h e t e r o g e n e o u s n u c l e a t i o n t a k e s p l a c e i n s t e e l and p r o c e e d s t o combine t he rmodynamic s and s e g r e g a t i o n d u r i n g s o l i d i f i c a t -i o n t o c a l c u l a t e d e o x i d a t i o n and i n t e r d e n d r i t i c p o r e f o r m a t i o n . I n (2 22) s u p p o r t o f T u r k d o g a n ' s a s s u m p t i o n s , Uda e t a l ' r e p o r t p o r o s i t y a s s o c i a t e d w i t h n u c l e a t i n g c a t a l y s t s , s u ch as n o n - m e t a l l i c i n c l u s i o n s . A b r i e f d e s c r i p t i o n o f t h e above s t u d i e s f o l l o w s . (21) T u r k d o g a n ' s e q u a t i o n s f o r t h e d e o x i d a t i o n r e a c t i o n s a r e d e r i v e d u s i n g e q u i l i b r i u m the rmodynamic s t o c o n s i d e r t h e e f f e c t s o f C, S i , Mn and S i - M n . T h i s a n a l y s i s d i f f e r s f r o m N i l l e s ^ ' ^ i n t h a t e q u i l i b r i u m s e g r e g a t i o n i s u s ed f o r s e g r e g a t i o n o f h y d r o g e n , n i t r o g e n 12 and carbon, whereas Ni l l es uses equilibrium segregation for a l l species. The equation for equilibrium segregation is based on the Lever Law: C 'L 1 - k g (7) o . where C = concentration in the l iqu id ^ C Q = i n i t i a l uniform concentration in the l iquid k Q = equilibrium distr ibut ion coefficient g = fraction so l id i f i ed For Mn and S i , complete mixing in the l iqu id is assumed enabling con-centration during so l id i f i ca t ion to be calculated using: C L = C Q ( 1 - g ) k ° - 1 (8) For oxygen, complete mixing with the l imit ing case of k Q 0 i s assumed applicable. The results show that blowhole formation is suppressed by increasing the Si and Mn contents while decreasing the carbon, oxygen, hydrogen, and nitrogen contents. The cr i ter ion used for pore formation is that the total pressure of gas in the melt is equal to or greater than 1 atm (101 KPa). (22) Uda et a l consider blowhole formation in the Fe-H system. With pure iron containing hydrogen, few blowholes were produced. When tungsten wire or aluminum f o i l were added to the molten iron and so l id i f i ed i n the same manner, many pores were formed. This suggests 13 t h a t h e t e r o g e n e o u s n u c l e i p romote b l o w h o l e f o r m a t i o n . Uda and Ohno (2) d e v e l o p a mechan i sm f o r p o r e f o r m a t i o n i n w e l d e d a luminum where t h e d e c r e a s i n g s o l u b i l i t y o f h y d r o g e n i n t h e c o o l i n g l i q u i d i s c o r r e l a t e d t o t h e d r i v i n g f o r c e f o r p o r e f o r m a t i o n . A t a t e m p e r a t u r e w e l l above t h e m e l t i n g p o i n t o f A l and one a tmo sphe re p r e s s u r e , no gas b u b b l e s a r e f o rmed i n t h e l i q u i d . As t h e l i q u i d c o o l s r a p i d l y , m i c r o - g a s b u b b l e s a r e n u c l e a t e d on n o n - m e t a l l i c i n c l u s i o n s s u c h as A ^ O ^ b e c a u s e o f t h e r e d u c e d s o l u b i l i t y o f h y d r o g e n . As t h e m e l t c o o l s t o t h e m e l t i n g p o i n t , c r y s t a l l i z a t i o n s t a r t s a t t h e mou ld w a l l s and gas b u b b l e s a r e e i t h e r ahead o f t h e i n t e r f a c e , f l o a t ou t o f t h e m e l t , o r become t r a p p e d i n t h e a d v a n c i n g i n t e r f a c e . Those t h a t a r e pushed ahead may c o a l e s c e . When s o l i d i f i c a t i o n i s c o m p l e t e t h e l a r g e b u b b l e s , a s w e l l as t h e r e m a i n i n g s m a l l p o r e s , may become t r a p p e d a t t h e c e n t e r l i n e . T h i s m o d e l o f p o r e f o r m a t i o n i s s c h e m a t i -c a l l y shown i n F i g . 1.4. The s equence i s shown f r o m t h e p o r e - f r e e m e l t a t h i g h t e m p e r a t u r e T^ t o t h e c o m p l e t e s o l i d a t t e m p e r a t u r e T^. 1 .2 .3 N o n - N u c l e a t i n g Mechan i sms o f P o r e F o r m a t i o n p o r o s i t y r e s u l t s f r o m gas b u b b l e s e n t r a i n e d d u r i n g s o l i d i f i c a t i o n , f r o m a i r p o c k e t s c o n t a i n e d i n p i e c e s o f r e f r a c t o r y i n t h e m e l t , o r f r o m o t h e r mechan i sms s u c h as r a d i a t i o n damage. I n t h e n o n - n u c l e a t i n g mechan i sms ( 1 0 , 1 1 , 2 3 , 3 0 ) o f p o r e f o r m a t i o n , Figure 1.4 Schematic diagram of model for pore formation in aluminum according to Uda and Ohno( 2). 15 27 30} C a l c u l a t i o n s by C a m p b e l l ' as o u t l i n e d b e l o w s u g g e s t s t h a t homogeneous n u c l e a t i o n i s i m p o s s i b l e and h e t e r o g e n e o u s n u c l e a t i o n p l a y s a m i n o r r o l e i n p o r e f o r m a t i o n . F o l l o w i n g C a m p b e l l , t h e c r i t e r i o n f o r n u c l e a t i o n o f p o r e s i s : P - P = P . (9) g e f where P = i n t e r n a l gas p r e s s u r e o f t h e b u b b l e g P^ = e x t e r n a l p r e s s u r e a c t i n g on t h e b u b b l e P^ = f r a c t u r e p r e s s u r e o f t h e l i q u i d m e t a l P g c an be a p p r o x i m a t e d .o by t h e s h r i n k a g e p r e s s u r e i n t h e i n t e r d e n d r i t i c r e g i o n , as t h e h y d r o s t a t i c p r e s s u r e i s n e g l i g i b l e . The (31) f r a c t u r e p r e s s u r e P^ o f a l i q u i d i s an e x t r e m e l y l a r g e n e g a t i v e number , f o r e x a m p l e , f o r l i q u i d i r o n P^ = - 70 ,000 atm (- 7.1 GPa) f r o m F i s h e r (29 30) C a m p b e l l ' c a l c u l a t e s t h e s h r i n k a g e p r e s s u r e r e s u l t i n g f r o m s h r i n k i n g o f t h e i n t e r d e n d r i t i c l i q u i d d u r i n g c o o l i n g and s o l i d i f -i c a t i o n . U s i n g a c y l i n d r i c a l m o d e l f o r t h e i n t e r d e n d r i t i c l i q u i d , he d e t e r m i n e s t h e s h r i n k a g e p r e s s u r e f o r b o t h c o n s t a n t and c h a n g i n g f r e e z i n g r a t e s . F o r t h e c o n s t a n t f r e e z i n g r a t e c a s e , t h e s o l i d i f i c a t i o n r a t e , a , i s g i v e n b y : a = 2.5 h C A T/bH (10) P 16 where C = heat capacity P h = thermal d i f fus iv i ty AT = temperature at surface of the melt - T mp b = radius of casting H = heat of fusion The d i f f erent ia l pressure along the length of a casting from Piwonka et a l ( 1 9 ) i s : P = 32.0 ByX2 L 2 / T 2 \ (11) a 4 ( l -B) y IT n b / where a = radius of l iqu id channel 3 = volume shrinkage y = viscosi ty of l iqu id metal L = length of casting n - reciprocal of X 2 X = dendrite spacing Assuming that deformation of a sol id metal near i t s melting point i s described by uniaxial creep, the s train rate t , as a function of stress o* is given by: e = K 1 ( sinh K 2 a ) m (12) (29) where K^, Yl , and m are constants given by Campbell . Combining equations (10), (11) and (12) yields an overal l pressure drop: 17 AP = m X _ a 2K 2.5 ah C Ax"1 1 / m K x bH 1 -i 2 / m 2/m - 1 / m (13) where b^ = b / n . This assumes that there is no flow of l iqu id metal in the interdendritic spaces. A plot of the pressure calculated from Equation (13) as a function of l iqu id core radius is shown in F i g . 1.5. The cr i ter ion for homogeneous nucleation of l iqu id iron i s shown as a horizontal l ine in F i g . 1.5. These results show that the shrinkage pressure is suff ic ient ly low for homogeneous nucleation only for core r a d i i in the 10 ^- 10 ^ cm range, which is too small to nucleate a pore. Campbell calculates core r a d i i for heterogeneous nucleation as well and concludes that the probability for this mechanism to operate is very small. Kaplan and Philbrook (25) consider the poss ib i l i ty of both homogeneous and heterogeneous nucleation of CO in l iquid iron. They (14) derive the rate of homogeneous nucleation, J , from Hirth et a l yielding the following expression: J = F exp(- AF /k^T) (14) where F = frequency function k = Boltzmann's constant i i * AF = the activation energy for nucleation, i s calculated in terms of a c r i t i c a l radius. * For heterogeneous nucleation, a value of AF is determined assuming CO bubbles nucleate at the l iqu id iron - l iqu id iron oxide interface. The 18 E *-o UJ RADIUS O F LIQUID C O R E ( c m ) Figure 1.5 Pressure in the residual l iqu id of a 1 cm radius cylinder of iron. The fracture pressure necessary for homogeneous nucleation intersects the calculated pressure l ine at core r a d i i between 10 -^ - 1 0 - 7 cm. From C a m p b e l l ( 3 ° ) . 19 r e s u l t s o f t h e i r c a l c u l a t i o n show t h a t t h e r a t e o f homogeneous n u c l e a t i o n 21 ft a t 1900°K i s J = 10 / e x p ( 3 . 8 x 10 ) b u b b l e s p e r s econd and f o r h e t e r o -21 6 geneous n u c l e a t i o n a t 1900°K , J = 10 / e x p ( 1 . 4 x 10 ) b u b b l e s / s e c o n d . I n e f f e c t , J i s n e g l i g i b l y s m a l l i n b o t h c a s e s . These r e s u l t s w o u l d i n d i c a t e t h a t b o t h homogeneous n u c l e a t i o n and h e t e r o g e n e o u s n u c l e a t i o n a t a l i q u i d i r o n - l i q u i d i r o n o x i d e i n t e r f a c e i s u n l i k e l y and t h e r e f o r e s u p p o r t s t h e n o n - n u c l e a t i n g mechan i sms p r o p o s e d f o r p o r e f o r m a t i o n . B u r n s and B e e c h ^ ' ' " ' 2 " ^ and H a r k n e s s e t a l ^ 2 ( ^ a p p l y (21) T u r k d o g a n ' s mode l o f p o r e f o r m a t i o n t o a w i d e r a n g e o f s t e e l s . The o n l y d i f f e r e n c e i n t h e c a l c u l a t i o n s i s t h e c h o i c e o f t he rmodynamic d a t a . Bu rn s and B e e c h ^ " ^ r u l e o u t b o t h homogeneous n u c l e a t i o n o f t h e p o r e s and t h e r e q u i r e m e n t o f a c r i t i c a l gas p r e s s u r e b e c a u s e t h e p o r e d i s -t r i b u t i o n i s n o t u n i f o r m t h r o u g h o u t t h e i n g o t s . They a l s o c o n c l u d e t h a t h e t e r o g e n e o u s n u c l e a t i o n i s n o t s i g n i f i c a n t b e c a u s e t h e o n l y i n c l u s i o n s t o f o r m on s o l i d i f i c a t i o n w o u l d be i r o n o x i d e w h i c h w o u l d a l s o b e u n i f o r m l y d i s t r i b u t e d . These c o n c l u s i o n s a r e b a s e d on t h e d i s t r i b u t i o n o f p o r e s o b s e r v e d e x p e r i m e n t a l l y i n s t e e l c a s t i n t o t i l t e d m o u l d s . They f o u n d t h a t p o r o s i t y p r e d o m i n a t e d on t h e u p - t i l t f a c e . The movement o f e n t r a i n e d b u b b l e s i n t h e l i q u i d i n t h e i r e x p e r i m e n t s i s shown s c h e m a t i c a l l y i n F i g . 1.6. The b u b b l e s i m p i n g i n g on t h e u p - t i l t f a c e a r e b e l i e v e d t o b e c a u g h t by t h e p r o j e c t i n g d e n d r i t e s a t t h e s o l i d - l i q u i d i n t e r f a c e and e i t h e r a r e pushed ahead o f t h e i n t e r f a c e o r become t r a p p e d i n t h e i n t e r d e n d r i t i c r e g i o n s . I n b o t h c a s e s t h e p o r e s w o u l d p r e d o m i n a t e i n t h e uppe r h a l f o f t h e i n g o t , as was o b s e r v e d . Figure 1 . 6 Assumed motion of entrained bubbles in .a t i l t e d mould from Burns and Beech (23). 21 1.3 Objective of Present Work From the previous sections i t is apparent that the nucleating mechanism for pore formation in metals has not c learly been established. Models assuming homogeneous, heterogeneous, and non-nucleating mechanisms for nucleation are s t i l l being proposed. The purpose of this present investigation is to attempt to resolve some of the discrepancies between the proposed mechanisms. Two aspects of the problem are considered: (1) In some mechanisms i t is assumed that the gas concen-tration in the l iqu id ahead of the advancing so l id - l iqu id interface increases during so l id i f i ca t ion . Gas bubbles are generated when a c r i t i c a l concentrations is reached and the bubbles grow or move in the l i q u i d depending on the so l id i f i ca t ion conditions. These assumptions, as they relate to real casting conditions, are examined in A l and A l alloys containing measured amounts of hydrogen. (2) The question of heterogeneous v . s . homogeneous nucleation of gas bubbles i s examined in pure iron containing CO. Small iron samples are supercooled extensively to establish that no so l id i f i ca t ion nuclei are present and then the samples are examined for porosity. If the so l id i f i ca t ion nuclei are the same nuclei on which pores form, then the results should indicate whether homogeneous or heterogeneous nucleation is the operative mechanism. 2. EXPERIMENTAL PROCEDURE 2.1 A luminum 2 . 1 . 1 P r e p a r a t i o n o f A l , A l + C u , and A l + T i B ^ I n g o t s C o n t a i n i n g Hyd r ogen H e a t s c o n t a i n i n g a p p r o x i m a t e l y 1 Kg o f a l uminum w e r e m e l t e d i n a g r a p h i t e c r u c i b l e u s i n g a l o w f r e q u e n c y i n d u c t i o n f u r n a c e . The a luminum was o f s u p e r - p u r e g r a d e (99.99%) p r o v i d e d b y A l c a n . H y d r o g e n was added by i n j e c t i n g s team i n t o t h e l i q u i d m e t a l u s i n g t h e c o n f i g u r a t -i o n shown s c h e m a t i c a l l y i n R i g . 2 . 1 . The p r o c e d u r e u sed was t o i n i t i a l l y p a s s s t eam t h r o u g h t h e g r a p h i t e r o d u n t i l i t r e a c h e d t h e r m a l e q u i l i b r i u m w i t h t h e s t e a m , i . e . no l i q u i d w a t e r f o rmed on t h e g r a p h i t e . Upon r e a c h i n g e q u i l i b r i u m , t h e i n j e c t i o n r o d was t h e n submerged f o r a s h o r t p e r i o d i n t h e m e l t . No e x p l o s i v e r e a c t i o n o c c u r r e d a t any t i m e . M e l t t e m p e r a t u r e d u r i n g s team i n j e c t i o n and p r i o r t o p o u r i n g w e r e measu red w i t h b a r e j u n c t i o n C h r o m e l - A l u m e l t h e r m o c o u p l e s . A f t e r i n j e c t i o n t h e p u r e a l um inum m e l t s w e r e c a s t i n t o a s e r i e s o f mou ld s unde r c o n t r o l l e d c o n d i t i o n s t o r e l a t e t h e c a s t i n g c o n d i t i o n s t o p o r e f o r m a t i o n . F o r t h e A l + Cu and A l c o n t a i n i n g T i ^ , t h e r e q u i r e d amounts o f Cu (99 .98%) o r T i l i ^ h a r d e n e r w e r e added t o m e l t s o f t h e p u r e a luminum w i t h h y d r o g e n . The a l l o y s we re t h e n c a s t a s d e s c r i b e d i n s e c t i o n 2 . 1 . 2 . 23 i Steam S air set Alumina Tube o o o o pi o o Grophite Rod |-I4cm OD x20cm 0 - 6 4 c m ID 0-125 cm oles Aluminum Graphite Cruicible o o Induction Coil o o Figure 2.1 Schematic representation of apparatus for injecting steam into A l bath. 25 2 . 1 . 2 C a s t i n g C o n d i t i o n s A l , A l + C u , and A l + T i l s ^ we re c a s t i n t o c y l i n d r i c a l g r a p h i t e mou ld s 36 mm ID by 15 .24 cm l o n g . The mou l d s we re u s e d e i t h e r a t room t e m p e r a t u r e , o r p r e h e a t e d t o 625°C i n a m u f f l e f u r n a c e t o p r o m o t e l a r g e g r a i n s i z e and s l o w c o o l i n g r a t e s . The c a s t s made a r e l i s t e d i n T a b l e 2.1 and T a b l e 2 . 2 . D i r e c t i o n a l l y s o l i d i f i e d i n g o t s w e r e c a s t i n a f i b e r f a x t u b e a s shown s c h e m a t i c a l l y i n F i g u r e 2 . 2 . The c h i l l shown i n F i g u r e 2.2 was e i t h e r s t a i n l e s s s t e e l , c o p p e r , o r r e f r a c t o r y b r i c k ; d i f f e r e n t m a t e r i a l s b e i n g u sed t o change t h e r a t e o f h e a t r e m o v a l f r o m t h e m e t a l and t h e r e f o r e t h e s o l i d i f i c a t i o n r a t e . Maximum g r o w t h r a t e s a r e o b t a i n e d 2 w i t h t h e c o p p e r c h i l l w h i c h has a t h e r m a l c o n d u c t i v i t y o f 0.9 c a l / s e c - c m and i n t e r m e d i a t e g r o w t h r a t e s w i t h t h e s t a i n l e s s s t e e l c h i l l h a v i n g a 2 t h e r m a l c o n d u c t i v i t y o f 0 .04 c a l / s e c - c m . The p o l i s h e d t o p s u r f a c e s o f t h e c h i l l s w e r e c o a t e d w i t h a t h i n l a y e r o f c o l l o i d a l g r a p h i t e t o e n s u r e t h a t t h e m o l t e n m e t a l d i d n o t r e a c t w i t h t h e c h i l l . The c o a t i n g d i d n o t a p p e a r t o a f f e c t t h e t h e r m a l c h a r a c t e r i s t i c s o f t h e c h i l l . The i n g o t s w e r e s e c t i o n e d and p o l i s h e d t o r e v e a l t h e p o r o s i t y . The number and s i z e o f t h e p o r e s i n t h e c a s t i n g s we re d e t e r m i n e d b y e x a m i n i n g 5 a r e a s on t h e s e c t i o n e d f a c e a t 25X. The c a s t i n g s whe re t h e n e t c h e d t o r e v e a l t h e g r a i n b o u n d a r i e s . 26 T a b l e 2.1 A l AND A l ALLOYS CAST INTO GRAPHITE MOULDS • Heat # M e l t A l l o y M i n u t e s o f Steam I n j e c t i o n a t 850°C C a s t i n g T e m p e r a t u r e °C Mou ld T e m p e r a t u r e °C 4 - 2 6 - 1 A l 0 850 25 4 - 2 6 - 2 A l 2 .0 850 25 4 - 2 6 - 3 A l 3 .0 850 25 4 - 2 9 - 1 A l 1.0 840 25 4 - 2 9 - 2 A l 2 .0 840 25 4 - 2 9 - 3 A l 2 .0 855 25 4 - 2 9 - 4 A l 3.0 855 25 4 - 2 9 - 5 A l + 2 . 5 % T i B 2 3.0 865 25 5 -6 -3 Al+3%Cu 1.0 850 25 5 -26 -2 A l+3%Cu 2 .0 850 625 6 -10 -1 A l 1.5 850 25 6 - 10 -2 A l 1.5 850 625 6 - 10 -3 A l + l % C u 1.5 865 25 6 -10 -4 A l + l % C u 1.5 . 865 625 6 - 10 - 5 Al+2%Cu 1.5 865 25 6 - 10 -6 Al+2%Cu 1.5 865 625 6 -11 -7 99.999A1 1.5 865 25 6 - 11 - 8 99.999A1 1.5 865 625 6 -11 -9 A l + 0 . 2 5 % T i B 2 1.5 865 25 6 - 11 -10 A l + 0 . 2 5 % T i B 2 1.5 865 625 6 -17 -11 A l 1.5 860 625 6 - 17 -12 A l+0 .5%Cu 1.5 860 25 6 -17 -13 A l+0 .5%Cu 1.5 860 625 27 T a b l e 2.2 DIRECTIONAL CASTS IN F IBERFAX TUBES Hea t # M e l t A l l o y M i n u t e s o f Steam I n j e c t i o n a t 850°C C a s t i n g T e m p e r a t u r e °C - 1 C h i l l 5 -4 -1 A l 1.0 850 S t a i n l e s s S t e e l 5 - 4 -2 A l 1.5 860 Copper 5 -4 -3 A l 1.5 860 R e f r a c t o r y B r i c k 5 -6 -1 A l+3%Cu 1.0 850 Copper 5 - 6 -2 Al+3%Cu 1.0 850 S t a i n l e s s S t e e l 5 - 6 - 3 A l+3%Cu 2 .0 850 R e f r a c t o r y B r i c k 28 WATER INLET L" 6MM ID / / / / / / / / / / / / / / / / A \ \ \ \ \ 22CM big / / I I. A VYCOR T U B E 46 MM ID FIBER FAX TUBE 52 MM 10 .CHILL I3CM WATER OUTLET Figure 2.2 Schematic representation of the casting arrangement for direct ional ly cast ingots. 29 2 . 1 . 3 D e n s i t y Measu rement s S i n c e t h e h y d r o g e n c o n t e n t o f t h e c a s t a luminum and i t s a l l o y s c o u l d n o t be measu red d i r e c t l y i n t h e Depa r tmen t o f M e t a l l u r g y , UBC, an a t t e m p t was made t o d e t e r m i n e h y d r o g e n c o n t e n t f r o m t h e d e n s i t y o f t h e c a s t m e t a l . D e n s i t y measurement s w e r e made by c o m p a r i n g t h e w e i g h t o f an i n g o t s e c t i o n i n a i r w i t h t h e w e i g h t o f t h e s e c t i o n when immersed i n w a t e r o r m e r c u r y u s i n g t h e f o l l o w i n g r e l a t i o n s h i p : W P s = (M - M) X P (15) s whe re W = w e i g h t o f s e c t i o n i n a i r s ° M = w e i g h t o f g i v e n vo l ume o f l i q u i d M g = w e i g h t o f s e c t i o n i n g i v e n v o l ume o f l i q u i d p = d e n s i t y o f l i q u i d P = d e n s i t y o f s e c t i o n s Two e q u i v a l e n t t e s t s e c t i o n s w e r e c u t f r o m one h a l f o f e a c h i n g o t and t h e d e n s i t y measu red u s i n g b o t h m e r c u r y and w a t e r . The f o l l o w i n g p r e c a u t i o n s we re t a k e n t o r e d u c e o r e l i m i n a t e e r r o r s i n mea su remen t s : i . The t e m p e r a t u r e o f t h e w a t e r and m e r c u r y b a t h s we re h e l d c o n s t a n t , i i . Rough edges o f t h e i n g o t s e c t i o n s w e r e smoothed t o p r e v e n t a i r b u b b l e a t t a c h m e n t . 30 i i i . S e c t i o n s we re a g i t a t e d i n t h e b a t h t o remove any a i r b u b b l e s a t t a c h e d t o t h e s u r f a c e o f t h e s e c t i o n s , i v . The p r e s e n c e o f t h e i m m e r s i n g f i x t u r e ( s t a i n l e s s s t e e l b a s k e t i n w a t e r and s t a i n l e s s s t e e l p l u n g e r i n m e r c u r y ) we re a l l o w e d f o r , i n c a l c u l a t i n g t h e d e n s i t i e s . The r e s u l t a n t d e n s i t y measurement s f o r a s e r i e s o f c a s t i n g i n w h i c h s t eam had b e e n i n j e c t e d f o r v a r y i n g t i m e s i s l i s t e d i n T a b l e 2 . 3 . The d e n s i t i e s do n o t show any c o r r e l a t i o n w i t h t h e l e n g t h o f t i m e s team was i n j e c t e d i n t o t h e m e t a l . S i n c e t h e p o r o s i t y w o u l d i n c r e a s e w i t h i n c r e a s i n g s team i n j e c t i o n , i t was c o n c l u d e d t h a t t h i s t e c h n i q u e was n o t s a t i s f a c t o r y . A l s o , i t i s n o t e d t h a t t h e r e i s no s i m i l a r i t y b e t w e e n t h e d e n s i t i e s measu red by i m m e r s i o n i n m e r c u r y and w a t e r . 2 . 1 . 4 Hyd r ogen A n a l y s i s To d e t e r m i n e t h e h y d r o g e n c o n t e n t o f t h e c a s t a l uminum and a luminum a l l o y s , s amp le s w e r e s e n t t o t h e A l c a n R e s e a r c h L a b s , K i n g s t o n , O n t a r i o f o r a n a l y s e s . The s amp le s we re p r e p a r e d by c a s t i n g t h e m e l t s c o n t a i n i n g h y d r o g e n i n t o c o p p e r mou l d s h a v i n g t h e d i m e n s i o n s shown i n F i g , 2 . 3 . I m m e d i a t e l y a f t e r t h e s amp le s had s o l i d i f i e d , t h e y we re quenched i n i c e w a t e r . 31 T a b l e 2.3 DENSITY MEASUREMENTS OF A l IN WATER AND MERCURY N o t e : P u r e A l d e n s i t y i s 2 .70 gm/cc a t 20°C I n g o t M i n u t e s o f Steam A d d i t i o n a t 850°C C a s t i n g T e m p e r a t u r e °C D e n s i t y U s i n g H (gm/cc) D e n s i t y U s i n g H 2 0 ( g m / c c ) 4 - 2 6 - 1 A 0 850 2 . 5 8 1 no d a t a 4 - 2 6 - 1 B 0 850 2 . 6 0 6 no d a t a 4 - 2 6 - 2A 2 .0 850 2.65^^ no d a t a 4 - 2 6 - 2B 2 .0 850 2 . 6 5 6 no d a t a 4 - 2 6 - 3A 3.0 850 2 . 6 0 5 no d a t a 4 - 2 6 - 3B 3.0 850 2 . 6 0 5 no d a t a 4 - 2 9 - 1 A 1.0 840 2 . 6 9 8 2 . 6 9 9 0 4 - 29 - 1B 1.0 840 2 . 6 6 , 6 2 . 7 2 8 4 4 - 29 - 2A 2 .0 840 2 . 4 3 2 2 . 707 . 4 4 - 29 - 2B 2 .0 840 2 . 6 6 9 2 . 7 1 1 , 4 4 - 2 9 - 3 A 2 .0 855 2 . 5 2 . no d a t a 4 - 2 9 - 3B 2 .0 855 2 . 6 4 5 no d a t a 4 - 2 9 - 4A 3.0 855 2 . 6 4 8 2 . 6 9 3 ? 4 - 2 9 - 4 B 3.0 855 2 . 6 5 , 6 2 . 6 8 6 £ o < 3-85cm > I 1<- 2-54 cm -> I 1 0-2 cm Figure 2.3 Schematic representation of cy l indr ica l copper mould for hydrogen analysis samples. 33 The h y d r o g e n c o n t e n t o f t h e m e l t s a t a g i v e n s team i n j e c t i o n t i m e , s t eam i n j e c t i o n t e m p e r a t u r e , and p o u r i n g t e m p e r a t u r e a r e l i s t e d i n T a b l e 2 . 4 . The t o t a l p e r i o d o f s t eam i n j e c t i o n t i m e l i s t e d i n t h e t a b l e i s a c u m u l a t i v e t i m e . F o r e x a m p l e , h e a t # 7-5 c a s t i n g 2A was c a s t w i t h o u t s t eam i n j e c t i o n a t 830°C . The m e l t was c o o l e d t o 7 25°C , s team i n j e c t e d f o r 0 .75 m i n u t e s , t h e b a t h r e h e a t e d t o 790°C and c a s t i n g 2B p o u r e d . The m e l t was t h e n c o o l e d t o 7 25°C , s t eam i n j e c t e d f o r 0 .75 m i n ( g i v i n g a t o t a l i n j e c t i o n t i m e o f 1.5 m in ) h e a t e d t o 9 00°C , and c a s t i n g 2C p o u r e d . The same g e n e r a l p r o c e d u r e was u s ed f o r a l l t h e c a s t i n g s l i s t e d i n T a b l e 2 . 4 . The h y d r o g e n c o n t e n t o f t h e c a s t i n g was f o u n d t o i n c r e a s e p r o g r e s s i v e l y w i t h i n c r e a s i n g i n j e c t i o n t i m e f o r m e l t t e m p e r a t u r e s o f 735°C and 8 4 5 ° C , as shown i n F i g . 2 . 4 , r e a c h i n g a maximum i n a p p r o x i m a t e l y 2.5 m i n u t e s . I n c r e a s i n g t h e i n j e c t i o n m e l t t e m p e r a t u r e i n c r e a s e d t h e h y d r o g e n c o n t e n t a s shown i n F i g . 2 . 4 . The dependence o f t h e h y d r o g e n c o n t e n t on t h e p o u r i n g t e m p e r a t u r e f o r A l and two A l+Cu a l l o y s i s shown i n F i g . 2 . 5 . N o t e t h a t t h e amount o f i n j e c t e d h y d r o g e n i s d i f f e r e n t f o r t h e t h r e e c a s e s r e s u l t i n g f r o m t h e d i f f e r e n t i n j e c t i o n t i m e s and i n j e c t i o n m e l t t e m p e r a t u r e s u s e d . The r e s u l t s show t h a t t h e h y d r o g e n c o n t e n t d e c r e a s e s w i t h d e c r e a s i n g p o u r i n g t e m p e r a t u r e . T h i s d e c r e a s e i s m i n i m a l f o r t h e A l + l % C u a l l o y w h i c h c o n t a i n s t h e l a r g e s t amount o f h y d r o g e n and becomes s i g n i f i c a n t a s t h e h y d r o g e n c o n t e n t i s d e c r e a s e d . 34 T a b l e 2 .4 A l AND A l+Cu CAST FOR HYDROGEN ANALYSIS Hea t C a s t M e l t A l l o y T o t a l Steam I n j e c t i o n T ime M i n u t e s T e m p e r a t u r e o f Steam I n j e c t i o n °C P o u r i n g T e m p e r a t u r e °C Hyd rogen C o n t e n t cc/lOOgm 7-5 1A A l 1.0 840 865 0.77 IB 2 .0 840 820 1.24 7 -5 2A A l 0 - 830 0 .25 2B 0 .75 725 790 0 .93 2C 1.50 725 900 0 .94 2D 2 .50 840 820 1.43 7 -5 3A A l 3 .0 840 820 1.43 3B 4 . 0 865 855 1.27 10 -20 6A A l 0 • - 745 0 .26 6B 1.0 755 735 0 .63 6C 2 .0 735 735 0 .89 6D 2 .0 - 915 0 .65 6E 2 .0 - 820 0.47 6F 2.5 890 890 1.05 11 -13 1 A l 0 - 880 0 .20 11 -13 2 A l 2 .0 790 830 0 .94 11 -13 3 A l 2 .0 865 880 1.23 11 -13 4 A l 1.0 • 760 750 0 .65 7-7 4A A l + l % C u 1.5 835 805 0.86 4B 3 .0 890 865 1.02 4C 3 .0 - 745 0.97 7-7 5A A l+3%Cu 1.5 840 820 0.76 5B 1.5 - 915 0.96 5C 3 .0 820 790 1.06 I M M E R S I O N T E M P » 8 4 0 ° C P O U R I N G T E M P - B 8 2 0 ° C Figure 2.4 Hydrogen content as a function of immersion time for two immersion-pouring temperature. 3 6 2 © o o z U l h-z o o z U J IT o >• X 1-2 1 I of 0-8 1 0-6 I 0 - 4 4 -|-A1.+ I%CU,3MIN- STEAM INJECTED AT 8 9 0 ° C 2 - AL+3%CU, I-5M IN-STE A M INJECTED A T 8 4 0 ° C 3- A L , I MIN- S T E A M INJECTED AT 7 3 0 ° C 0.2 + 7 0 0 80 0 900 POURING TEMPERATURE ( ° C ) Figure 2.5 Relationship between hydrogen content and pouring temperature. 37 From t h e above d a t a on h y d r o g e n c o n t e n t s a s a f u n c t i o n o f i n j e c t i o n t i m e , i n j e c t i o n m e l t t e m p e r a t u r e , and p o u r i n g t e m p e r a t u r e , t h e h y d r o g e n c o n t e n t o f t h e c a s t i n g s made u n d e r c o n t r o l l e d h y d r o g e n i n j e c t i o n and s o l i d i f i c a t i o n c o n d i t i o n s was e s t a b l i s h e d . 2 . 1 . 5 D i r e c t i o n a l S o l i d i f i c a t i o n o f H i g h P u r i t y A luminum A s e r i e s o f 99 .999% A l r o d s c o n t a i n i n g h y d r o g e n w e r e d i r e c t i o n a l -l y s o l i d i f i e d a t a c o n s t a n t s o l i d i f i c a t i o n r a t e and w i t h a c o n s t a n t t emp -e r a t u r e g r a d i e n t . The r e s u l t a n t d i s t r i b u t i o n o f p o r o s i t y i n t h e r o d s was i n v e s t i g a t e d . The r o d s w e r e made by a d d i n g h y d r o g e n t o m o l t e n a l u m i n u m , a s d e s c r i b e d i n S e c t i o n 2 . 1 . 1 and c a s t i n g t h e m e l t i n t o u n h e a t e d s p l i t g r a p h i t e m o u l d s . The r e s u l t a n t c a s t i n g s w e r e 6.0 mm i n d i a m e t e r by 10 cm l o n g . Samples f o r h y d r o g e n a n a l y s i s , a s d e s c r i b e d i n S e c t i o n 2 . 1 . 4 , w e r e a l s o c a s t f r o m t h e same m e l t . The r o d s w e r e t h e n e n c a p s u l a t e d i n V y c o r t u b e s , r e m e l t e d , and d i r e c t i o n a l l y s o l i d i f i e d u s i n g t h e a p p a r a t u s shown i n F i g u r e 2 . 6 . To e n s u r e t h a t t h e m o l t e n a luminum d i d n o t come i n t o c o n t a c t w i t h t h e 7 mm ID V y c o r t u b e , t h e i n n e r s u r f a c e o f t h e t u b e was c o a t e d w i t h a l a y e r o f Aquadag . The c a s t r o d was p u t i n t o t h e t u b e when t h e Aquadag was p a r t i a l l y d r i e d , t h e n t h e g r a p h i t e was f u l l y d r i e d w i t h a bun sen b u r n e r . The t u b e Figure 2.6 Schematic representation of apparatus used in direct ional so l id i f i ca t ion of high purity aluminum. 39 was t h e n e v a c u a t e d t o 0.05 t o r r ( 6 . 7 P a ) w i t h a m e c h a n i c a l pump and s e a l e d unde r vacuum. Two o r t h r e e r o d s we re d i r e c t i o n a l l y s o l i d i f i e d a t one t i m e . The r o d s w e r e h e l d i n t h e h o t zone f o r 1 h ou r and t h e n s l o w l y moved o u t o f t h e f u r n a c e . The r o d s w e r e s o l i d i f i e d e i t h e r by l o w e r i n g them t h r o u g h t h e f u r n a c e w h i c h r e s u l t e d i n s o l i d i f i c a t i o n o c c u r r i n g i n a v e r t i c a l upwards d i r e c t i o n ( r e f e r r e d t o as up s o l i d i f i c a t i o n ) o r b y r a i s i n g them o u t o f t h e f u r n a c e w h i c h r e s u l t e d i n s o l i d i f i c a t i o n i n t h e down d i r e c t i o n . The g r o w t h r a t e was m a i n t a i n e d c o n s t a n t a t 5.1 cm/hr f o r a l l t h e s a m p l e s . The t e m p e r a t u r e was measu red d u r i n g s o l i d i f i c a t i o n b y a c h r o m e l - a l u m e l t h e r m o c o u p l e p l a c e d i n t h e V y c o r h o l d e r w i t h t h e s a m p l e s . The 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 s amp le s f o r b o t h t h e upward and downward g r o w t h d i r e c t i o n s a r e p l o t t e d as a f u n c t i o n o f f u r n a c e p o s i t i o n i n F i g . 2.1. The same t e m p e r a t u r e g r a d i e n t was m a i n t a i n e d t h r o u g h o u t a l l o f t h e e x p e r i m e n t s . A f t e r s o l i d i f i c a t i o n t h e r o d s we re s e c t i o n e d l o n g i t u d i n a l l y and p o l i s h e d . The Q u a n t i m e t was u sed t o measure t h e p o r o s i t y a t a m a g n i f i c a t i o n o f 290 X o v e r an a r e a 500 ym s q u a r e . The s e c t i o n s we re t h e n e t c h e d u s i n g a m o d i f i e d K e l l e r ' s e t c h t o r e v e a l t h e m i c r o s t r u c t u r e . 2.1.6 T r a c e r S t u d i e s An a l l o y o f 99.999% a l uminum c o n t a i n i n g a p p r o x i m a t e l y 0.5% Ag was d i r e c t i o n a l l y s o l i d i f i e d i n t h e manner d e s c r i b e d i n S e c t i o n 2.1.5. 40 t 1 r 0 10 20 30 DISTANCE (cm) Figure 2.7 Temperature prof i le of ver t i ca l furnace used in direct ional so l id i f i ca t ion experiments. The sol id l ine is for the furnace without samples and the dotted l ines show the temperature of the samples so l id i f i ed in the up and down direcitons. 41 T h i s was done t o e s t a b l i s h d i r e c t l y t h e s o l u t e s e g r e g a t i o n w h i c h o c c u r s unde r t h e s o l i d i f i c a t i o n c o n d i t i o n s u sed i n t h e d i r e c t i o n a l s o l i d i f i c a t i o n o f S e c t i o n 2 . 1 . 5 . F o r t h e s e t e s t s t h e samp le p r e p a r a t i o n and e x a m i n a t i o n p r o c e d u r e d e s c r i b e d i n S e c t i o n 2 . 1 . 5 was m o d i f i e d i n t h e f o l l o w i n g way s : i . No s team was i n j e c t e d i n t o t h e m e l t , i i . No s amp le s we re c a s t f o r h y d r o g e n a n a l y s i s , i i i . A g " ^ ^ was added t o t h e m e l t . P r i o r t o d i r e c t i o n a l s o l i d i f i c a t i o n t h e as c a s t r o d s we re s canned w i t h a G e i g e r C o u n t e r t o e n s u r e t h a t t h e i n i t i a l d i s t r i b u t i o n o f A g ^ " ^ was u n i f o r m . The r o d s we re d i r e c t i o n a l l y s o l i d i f i e d i n e i t h e r t h e up o r down d i r e c t i o n s f o r h a l f t h e i r l e n g t h , and t h e n quenched t o room t e m p e r a t u r e by r e m o v i n g them f r o m t h e f u r n a c e . The q u e n c h i n g was c a r r i e d ou t t o examine t h e s o l u t e c o n c e n t r a t i o n i n t h e l i q u i d ahead o f t h e a d v a n c i n g i n t e r f a c e . The s e g r e g a t i o n o f t h e Ag " * " ^ i n t h e r o d s was d e t e r m i n e d by s e c t i o n i n g t h e r o d s and m e a s u r i n g t h e a c t i v i t y o f e a c h s e c t i o n . The r o d s w e r e c u t w i t h a j e w e l l e r y saw p e r p e n d i c u l a r t o t h e s o l i d i f i c a t i o n d i r e c t i o n i n t o segments 0.5 and 0.25 cm l o n g . These segments we re w e i g h e d and p l a c e d i n t o 3 dram s t o p p e r e d v i a l s . The a c t i v i t y o f t h e segment i n e ach v i a l was measu red w i t h a P i c k e r N u c l e a r T r a n s c a l e r I I A u t o m a t i c S c i n t i l l a t i o n C o u n t e r . The c o u n t e r was c a l i b r a t e d w i t h one 42 gram o f A l + 0 . 5% A g ^ " ^ p r i o r t o c o u n t i n g . The e n e r g y r a n g e o f gamma a c t i v i t y d e t e c t e d by t h e s c a l e r was 800 KeV - 1 MeV, ( t h e e n e r g y o f Ag"*""^ i s 890 KeV ) 20 ,000 c o u n t s f o r a p e r i o d o f up t o two m i n u t e s was made on each segment w h i c h k e p t t h e c o u n t i n g e r r o r t o l e s s t h a n 0 . 5 % . The b a c k g r o u n d c o u n t was d e t e r m i n e d m a n u a l l y b e f o r e t h e s amp le a c t i v i t y was c o u n t e d and a u t o m a t i c a l l y s u b t r a c t e d f r o m t h e t o t a l . S e v e r a l segments i n t h e r e g i o n o f t h e quenched i n t e r f a c e we re p o l i s h e d and e t c h e d t o compare t h e a s - g r o w n and quenched m i c r o s t r u c t u r e s . The JEOL M o d e l JXA3A E l e c t r o n P r o b e m i c r o a n a l y z e r was u s e d t o examine s h o r t r a n g e v a r i a t i o n s i n s i l v e r c o m p o s i t i o n due t o m i c r o s e g r e g a t i o n i n t h e s a m p l e s . 2 . 1 . 7 P o l i s h i n g and E t c h i n g o f A l and A l A l l o y s S i n c e p u r e a luminum s c r a t c h e s e a s i l y , i t i s d i f f i c u l t t o p o l i s h . The a u t h o r f ound t h a t A l c a n be s u c c e s s f u l l y p o l i s h e d u s i n g t h e f o l l o w i n g t e c h n i q u e : _ P o l i s h on 180 g r i t p a p e r , r i n s e s amp le t h o r o u g h l y w i t h t a p w a t e r . - P o l i s h on 320 g r i t p a p e r , r i n s e s amp le t h o r o u g h l y w i t h t a p w a t e r P o l i s h on 600 g r i t p a p e r u n c o n t a m i n a t e d * , wash hands and s amp le w i t h soap and t a p w a t e r , t h e n p o l i s h * * u s i n g 5 ym a l u m i n a and an u n c o n t a m i n a t e d * p o l i s h i n g c l o t h w i t h d i s t i l l e d w a t e r , wash hands and samp le g e n t l y w i t h soap and d i s t i l l e d w a t e r , t h e n c l e a n u l t r a s o n i c a l l y ; p o l i s h u s i n g 0.6 ym C e r i u m O x i d e o r M a g n e s i a on u n c o n t a m i n a t e d * c l o t h w i t h d i s t i l l e d w a t e r . Wash g e n t l y w i t h soap and d i s t i l l e d w a t e r , r i n s e w i t h e t h n o l and b l o w d r y ; c a n be u l t r a s o n i c a l l y c l e a n e d a g a i n i f d e s i r e d . The e t c h u s e d f o r A l , A l + R^, and A l + T i B ^ was a two s t a g e e t c h : K e l l e r ' s e t c h (12 HC ' l : 6 H N 0 3 : 1 HP: 1 H^O) f o l l o w e d by n i t r i c a c i d . I t was n o t e d t h a t a " m o d i f i e d " K e l l e r ' s (3 H C l : 5 H N 0 3 : 2 HF: 2 H 2 0 ) e t c h e d 99 .999% A l b e t t e r t h a n t h e n o r m a l K e l l e r ' s g i v e n a b o v e . T u c k e r ' s e t c h (45 H C l : 15 H N O ^ 15 H F ( 4 8 % ) : 25 H 2 0 ) was u s e d on A l - C u a l l o y s . * U n c o n t a m i n a t e d means p r e v i o u s l y u s ed o n l y on A l o r o t h e r s o f t m e t a l s . * * T h i s s t e p may be o m i t t e d . 44 2.2 S u p e r c o o l i n g o f I r o n A s e r i e s o f e x p e r i m e n t s w e r e c a r r i e d ou t i n w h i c h i r o n r o d s w e r e m e l t e d and s o l i d i f i e d i n a manner w h i c h i n h i b i t e d n u c l e a t i o n and p r o d u c e d e x t e n s i v e s u p e r c o o l i n g b e f o r e s o l i d i f i c a t i o n o c c u r r e d . 2 .2 .1 M a t e r i a l s and A p p a r a t u s S h o r t r o d s ( 1 . 3 cm i n d i a m e t e r by 2 cm l o n g ) o f Armco I r o n w e r e m e l t e d u s i n g a l o w impedance i n d u c t i o n u n i t i n t h e a p p a r a t u s s c h e m a t i c a l l y shown i n F i g u r e 2 . 8 . The c o m p o s i t i o n i n w t . % o f t h e Armco Fe i s : C - 0 .02 Mn - 0 .15 S i - 0 .09 N i - 0 .04 Cu - 0 .125 C r - 0 . 05 p - 0 .085 N 2 - 0 .01 o2 - 0 .010 To a v o i d n u c l e a t i o n on t h e c r u c i b l e w a l l s , t h e i r o n r o d s w e r e p l a c e d i n a V y c o r s l e e v e 16 mm ID i n a bed o f S i 0 2 f l o u r . A 20 c c a l u m i n a c r u c i b l e c o n t a i n e d t h e f l o u r , s l e e v e and i r o n . 2 . 2 . 2 T e m p e r a t u r e Measurement The t e m p e r a t u r e was measu red u s i n g a P t / P t - 1 0 % Rh t h e r m o c o u p l e . T h i s t h e r m o c o u p l e was p l a c e d I n a 0 .6 cm 0D a l u m i n a s h e a t h wrapped w i t h ARGON INLET 45 ALUMINA TUBE I CM OD MOLYBDENUM SUSCEPTOR PT/PT-lO%RH "THERMOCOUPLE IN ALUMINA SHEATH-6MM0D ALUMINA PASTE SILICA FLOUR -VYCOR SLEEVE-16 MM ID 20 CC ALUMLNA CRUCIBLE INDU CTION COIL ALUMINA PEDESTAL 35 MM OD QUARTZ TUBE ure 2.8 Schematic representation of supercooling apparatus. 46 a molybdenum s h e e t t h a t was a s u s c e p t o r f o r i n d u c e d c u r r e n t s . The s h e a t h and s u s c e p t o r we re p u t i n s i d e a 1 cm OD a l u m i n a t u b e . A l u m i n a p a s t e was p u t a r ound t h e b o t t o m o f t h e t u b e e x p o s i n g o n l y t h e t i p o f t h e s h e a t h . The t i p was p l a c e d on t h e s u r f a c e o f t h e m e l t and t h e t e m p e r a t u r e r e c o r d e d d u r i n g c o o l i n g . 2 . 2 . 3 P r o c e d u r e A f t e r t h e a p p a r a t u s had been f l u s h e d w i t h a r g o n , t h e i r o n was m e l t e d i n t h e a r g o n a t m o s p h e r e . The s h e a t h e d t h e r m o c o u p l e was t h e n l o w e r e d t o t o u c h t h e m e l t s u r f a c e . When t h e t e m p e r a t u r e had s t a b i l i z e d , power t o t h e i n d u c t i o n c o i l was s w i t c h e d o f f , and t h e s amp le a l l o w e d t o c o o l r a p i d l y . T h i s p r o c e s s was r e p e a t e d s e v e r a l t i m e s ( u s u a l l y 4) b e f o r e a s amp le w o u l d s u p e r c o o l . F i g u r e 2.9 shows a t y p i c a l t e m p e r a t u r e -t i m e p l o t f o r t h r e e c y c l e s s how ing p r o g r e s s i v e l y more s u p e r c o o l i n g . The a r g o n f l o w r a t e , g e n e r a l l y 0 .25 - 0 .30 m / h r was a d j u s t e d w i t h each c y c l e i n o r d e r t o g e t a c o o l i n g r a t e o f a p p r o x i m a t e l y 600°C/min . w h i c h i s t h e (32) opt imum r a t e f o r s u p e r c o o l i n g r e p o r t e d by F l e m i n g s and f r o m e x p e r i e n c e i n t h e p r e s e n t e x p e r i m e n t s . A f t e r s u p e r c o o l i n g , t h e i r o n was a l l o w e d t o c o o l u n d e r a r g o n and t h e n removed f r o m t h e a p p a r a t u s . I n t h e p r e s e n t e x p e r i m e n t s i t was f o u n d t h a t o n l y one samp le i n s e v e n s u p e r c o o l e d . The s amp le s w h i c h d i d n o t s u p e r c o o l w e r e m e l t e d and c o o l e d a t l e a s t s i x t i m e s b e f o r e b e i n g r e j e c t e d . 47 Figure 2.9 Temperature-time record showing supercooling of sample #2 for cycles 4,5 and 6. 48 2 . 2 . 4 A d d i t i o n o f C a r b o n and Oxygen To d e t e r m i n e t h e e f f e c t s o f C and 0 c o n t e n t , and t h u s CO, on p o r e f o r m a t i o n i n s u p e r c o o l e d i r o n , C and 0 we re added t o t h e i r o n r o d s . I n i t i a l l y , l a m p b l a c k and o x i d i z e d Armco i r o n (made by h e a t i n g c h i p s i n a m u f f l e f u r n a c e f o r 3 h o u r s a t 1200°C) we re p l a c e d i n t o d r i l l e d h o l e s i n t h e r o d s . An a t t e m p t was t h e n made t o s u p e r c o o l t h e s e r o d s , b u t was w i t h o u t s u c c e s s . C and 0 we re t h e n added i n t h e same manner t o s e v e r a l r o d s t h a t had b e e n p r e v i o u s l y s u p e r c o o l e d . T h i s method o f a d d i n g t h e C and 0 p r o v e d s u c c e s s f u l i n t h a t s u p e r c o o l i n g was o b t a i n e d i n t h r e e c a s e s . 2 . 2 . 5 E x a m i n a t i o n and A n a l y s i s The t e s t s amp le s w h i c h had s u p e r c o o l e d w e r e s e c t i o n e d l o n g i t u -d i n a l l y ; one h a l f s e c t i o n examined m e t a l l o g r a p h i c a l l y and t h e o t h e r h a l f u s ed f o r c a r b o n and oxygen a n a l y s i s . * F o r gas a n a l y s i s , t h e m a t e r i a l was c u t i n t o 1 gram p i e c e s , t h e n p l a c e d i n t o h o t s u l p h u r i c a c i d (40°C) f o r a few m i n u t e s t o remove o x i d e s c a l e . The c a r b o n a n a l y s i s was c a r r i e d o u t a t W e s t e r n Canada S t e e l . Oxygen a n a l y s i s was done on t h e L e c o A n a l y z e r i n t h i s d e p a r t m e n t . S e v e r a l s amp le s w e r e a n a l y z e d f o r n i t r o g e n b y E s c o F o u n d r i e s , C o q u i t l a m , B.C. * P o r o s i t y was q u a n t i t a t i v e l y c h a r a c t e r i z e d u s i n g t h e Q u a n t i m e n t a t 290 X . 3. RESULTS 3.1 E f f e c t o f C a s t i n g C o n d i t i o n s on P o r o s i t y i n A l and A l A l l o y s P o r o s i t y was examined i n t h e f o l l o w i n g t h r e e g r oup s o f c a s t i n g s : i . P u r e a luminum c a s t i n t o g r a p h i t e mou ld s h e l d a t room t e m p e r a t u r e , v a r y i n g t h e h y d r o g e n c o n t e n t and p o u r i n g t e m p e r a t u r e , i i . A l uminum and a l um inum a l l o y s c a s t i n t o h e a t e d g r a p h i t e m o u l d s , i i i . U n i d i r e c t i o n a l l y c a s t A l and A l + 3 w t . % C u . 3 .1 .1 P o r o s i t y i n P u r e A luminum C a s t i n g s The number , s i z e , s h a p e , and d i s t r i b u t i o n o f t h e p o r e s o b s e r v e d i n t h e a luminum c a s t i n g s a r e g i v e n i n T a b l e 3 . 1 . The a p p e a r a n c e o f an a luminum c a s t i n g a f t e r i t was s e c t i o n e d v e r t i c a l l y , p o l i s h e d , and e t c h e d , i s shown i n F i g u r e 3 . 1 . I n g e n e r a l , i t was o b s e r v e d t h a t p o r o s i t y t e n d e d t o be c o n c e n t r a t e d t o w a r d s t h e o u t s i d e s u r f a c e o f t h e c a s t i n g i n a manner s i m i l a r t o t h a t o b s e r v e d i n a r immed s t e e l . F i g u r e 3.2 shows t h i s r immed p o r e d i s t r i b u t i o n a c r o s s t h e r e g i o n s marked A and B on F i g u r e 3 . 1 . The p o r e s c o v e r a r a n g e o f s i z e s a s i n d i c a t e d i n T a b l e 3 . 1 . 2 F o r e x a m p l e , f o r c a s t 4 - 2 9 - 1 (shown i n F i g . 3 . 1 ) , t h e r e a r e 180 p o r e s / c m , 49 Table 3.1 POROSITY IN A l CASTINGS Casting Hydrogen Content cc/lOOgm Minutes of Steam Addition at 850°C Pouring Temperature °C Porosity Number Small Hole Max. Size mm Large Hole Min. Size mm Pores/cm^ Shape & Distribution 4-26-1 0.25 0 850 0.03 - 1000 Spherical, rimmed 4-26-2 1.3 2.0 850 - 0.1 50 Spherical, rimmed 4-26-3 1.4 3.0 850 0.08 - 180 Spherical, rimmed 4-29-1 0.75 1.0 840 0.02 - 1000 Spherical, rimmed - - - - 0.06 180 -4-29-4 1.4 3.0 855 0.04 - 225 Spherical, rimmed - - - - 0.2 25 -4-29-5 1.5+2.5%T±B2 3.0 865 0.16 — 200 Interdendritic U l o Figure 3.1 Casting 4-29-1 showing rimmed porosity and the grain structure. Pore dis tr ibut ion for bands A and B are shown in Figure 3.2. (4X) 52 DISTANCE ACROSS INGOT (cm) F i g u r e 3.2 P o r e d i s t r i b u t i o n a c r o s s c a s t i n g 4 - 2 9 - 1 d e m o n s t r a t i n g r immed p o r o s i t y . A c o r r e s p o n d s t o Band A and B t o Band B i n F i g . 3 . 1 . P o r e s c o u n t e d were t h o s e v i s i b l e a t 5 X m a g n i f i c a t i o n . 53 0.06 mm i n d i a m e t e r o r l a r g e r and 1000 p o r e s / c m z l e s s t h a n 0 .02 mm i n d i a m e t e r w i t h f ew p o r e s be tween 0 .02 and 0 .06 mm i n d i a m e t e r . O v e r a l l , t h e p o r e s a r e s p h e r i c a l , 0.1 mm i n d i a m e t e r o r s m a l l e r , and a r e n o t g e n e r a l l y a s s o c i a t e d w i t h g r a i n b o u n d a r i e s . Whenever p o r e s w e r e on g r a i n b o u n d a r i e s , t h e b o u n d a r i e s w e r e k i n k e d , i n d i c a t i n g t h a t t h e p o r e had p r e v e n t e d t h e g r a i n b o u n d a r y f r o m m o v i n g . An examp le o f a k i n k e d g r a i n b o u n d a r y i s shown i n F i g . 3 . 3 . I n c r e a s i n g t h e h y d r o g e n c o n t e n t f r o m 0.25 - 1.5 c c/100 gm c a u s e d f e w e r b u t l a r g e r p o r e s t o f o r m . The s m a l l changes i n p o u r i n g t e m p e r a t u r e , 840° - 8 6 5 ° C , had no a p p a r e n t e f f e c t on t h e p o r o s i t y . I n one c a s t i n g , 4 - 2 9 - 5 , 2 .5% T i B 2 was added t o t h e m e l t w h i c h p r o d u c e d a f i n e e q u i a x e d g r a i n s t r u c t u r e a p p r o x i m a t e l y 0.3 mm i n d i a m e t e r . The p o r e s i n t h i s c a s t i n g w e r e s l i g h t l y l a r g e r t h a n t h e o t h e r c a s t i n g s and w e r e u n i f o r m 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 i n g o t . 3 . 1 . 2 E f f e c t o f Mou ld T e m p e r a t u r e on P o r o s i t y T a b l e 3.2 l i s t s t h e o b s e r v a t i o n s o f t h e p o r o s i t y i n A l and A l a l l o y c a s t i n g s , p o u r e d i n t o g r a p h i t e mou ld s h e a t e d t o 625°C as w e l l as e q u i v a l e n t c a s t i n g s made i n mou ld s a t 25°C f o r c o m p a r i s o n . The h y d r o g e n c o n t e n t was k e p t n e a r l y u n i f o r m by i n j e c t i n g s t eam f o r 1.5 m i n u t e s a t 850°C i n t o a l l t h e m e l t s . 54 F i g u r e 3.3 K i n k e d g r a i n b o u n d a r y i n a l uminum c a s t i n g 4 - 2 9 - 1 i n d i c a t e d by a r r o w . The k i n k s a r e a s s o c i a t e d w i t h p o r e s (11 X ) . F i g u r e 3.4 C a s t s t r u c t u r e o f A l + 0 .25 w/o T i B ^ c a s t i n t o 625°C m o u l d . N o t e p o r o s i t y i n r immed c o n f i g u r a t -i o n . ( 2 .1 X ) . T a b l e 3.2 OBSERVATIONS OF POROSITY IN A l AND ITS ALLOYS CAST INTO HEATED GRAPHITE MOULDS C a s t i n g Number M a t e r i a l Mou ld Tempe ra tu re °C Hydrogen C o n t e n t cc/lOOgm P o r o s i t y S m a l l H o l e Max. S i z e mm L a r g e H o l e M i n . S i z e mm P o r e s / c m Shape & D i s t r i b u t i o n 6 -11 -7 99.999%A1 25 1.0 0.02 600 S p h e r i c a l & Rimmed 0.28 0.2 6 -11 -8 99.999A1 625 1.0 0.04 250 S p h e r i c a l & Rimmed 0.4 0.27 6 - 10 -1 99.99A1 25 1.0 0.08 2000 S p h e r i c a l & Rimmed 0.4 0 .15 6 -17 -11 99.99A1 625 1.0 0.04 150 S p h e r i c a l & Rimmed 0 .28 15 6 -11 -9 A l + 0 . 2 5 T i B 2 25 1.0 0.02 525 I n t e r d e n d r i t i c 0.08 60 6 - 11 -10 A l + 0 . 2 5 T i B 2 625 1.0 0.08 1000 I n t e r d e n d r i t i c 0.36 60 6 -17 -12 A l+0 .5%Cu 25 0 .80 0.04 120 I n t e r d e n d r i t i c 0 .12 10 6 -17 -13 Al+5%Cu 625 0 .80 0.36 40 I n t e r d e n d r i t i c 6 - 10 -3 A l + l % C u 25 0.86 0.02 870 I n t e r d e n d r i t i c 6 - 10 - 4 A l + l % C u 625 0.86 0.1 800 I n t e r d e n d r i t i c 0.4 40 6 - 10 -5 Al+2%Cu 25 0.94 0.06 200 I n t e r d e n d r i t i c 6 - 10 -6 Al+2%Cu 625 0.94 0.02 100 I n t e r d e n d r i t i c 0.3 10 5 -6 -3 Al+3%Cu 25 0.83 0.08 135 I n t e r d e n d r i t i c * 5 -6 -2 Al+3%Cu 625 0.90 0 .12 20 I n t e r d e n d r i t i c * U l 56 The effect of mould temperature is i l lus trated by comparing the porosity in the f i r s t two castings l i s ted in the table for high purity aluminum. With the mould temperature increased to 625°C from 25°C, the number of pores is reduced and the size increased. The dis tr ibut ion of pores is not affected by the mould temperature, as in both cases the pores are in a rimmed configuration. Also, the pores are not associated with grain boundaries in the castings. The casting 6-10-1 of 99.99% A l in a 25°C mould appears to have a random distr ibut ion of many small spherical pores and a few larger pores as l i s ted i n Table 3.2. This random distr ibut ion of pores dif fers from the corresponding casting 4-29-1 l i s ted in Table 3.1 in which there was a rimmed dis tr ibut ion . Since the other 99.99% A l castings l i s t ed in Table 3.1 exhibit a rimmed dis tr ibut ion , i t is presumed that some unknown factor has caused the random distr ibut ion in 6-10-1. The second casting of 99.99% Al with a 625°C mould has over 100/cm small and some large spherical pores distributed in a rimmed configuration. Comparing the normal results of a 25°C mould casting (4-29-1) with the high temperature mould castings (6-17-11), the effect of high mould temperature is similar to that for the 99.999% A l , i . e . the pores decrease in number and increase in size with the heated mould. A small amount of T i B 2 was added to 99.99% A l to reduce the grain size. The increase in mould temperature only affected the pore size which was increased. Figure 3.4 shows casting 6-11-10 with a grain 57 s i z e o f a p p r o x i m a t e l y 2 .0 mm and t h e r immed d i s t r i b u t i o n o f p o r e s . F o u r A l + Cu a l l o y s w e r e examined u s i n g mou ld t e m p e r a t u r e s o f 25°C and 625°C . The c a s t i n g s a t b o t h m o u l d t e m p e r a t u r e s e x h i b i t e d no s i g n i f i c a n t changes i n p o r o s i t y w i t h i n c r e a s i n g c o p p e r c o n t e n t . T h e r e w e r e many s m a l l p o r e s f o rmed i n t h e 25°C mou ld s and a f ew l a r g e p o r e s f o rmed i n t h e h e a t e d m o u l d s . The p o r e s we re b o t h s p h e r i c a l and e l o n g a t e d d e p e n d i n g on t h e g eomet r y o f t h e i n t e r d e n d r i t i c r e g i o n s i n w h i c h t h e y we re l o c a t e d . F i g u r e 3.5 shows an e l o n g a t e d i n t e r d e n d r i t i c p o r e i n t h e A l + 1% Cu a l l o y c a s t i n t o a 625°C m o u l d . I n t h e A l + 3% Cu c a s t i n g , t h e p o r e s w e r e f ound i n t h e i n t e r d e n d r i t i c s econd phase.. F i g . 3.6 shows t h e s e p o r e s i n t h e i n t e r d e n d r i t i c r e g i o n s a s s o c i a t e d w i t h t h e s e cond p h a s e . 3 . 1 . 3 D i r e c t i o n a l l y C a s t A l and A l + 3% Cu The p o r o s i t y o b s e r v e d i n t h e d i r e c t i o n a l c a s t i n g s i s l i s t e d i n T a b l e 3 . 3 . P o r o s i t y i n u n i d i r e c t i o n a l l y c a s t A l c o n s i s t s o f many s m a l l s p h e r i c a l p o r e s , l e s s t h a n 0.5 mm i n d i a m e t e r , r a n d o m 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 a s t i n g . The p o r e s a r e n o t r e l a t e d i n any a p p a r e n t way w i t h t h e g r a i n b o u n d a r i e s ; h o w e v e r , when a p o r e i s a t a g r a i n b o u n d a r y , t h e b o u n d a r y i s k i n k e d s i m i l a r t o t h a t d e s c r i b e d i n S e c t i o n 3 . 1 . 1 . The p o r o s i t y i n c a s t i n g 5 . 4 . 2 i s shown i n F i g u r e 3 . 7 . No i n c r e a s e i n t h e number o r s i z e o f p o r e s w i t h i n c r e a s i n g d i s t a n c e f r o m t h e c h i l l f a c e i s o b s e r v e d . T h i s i s shown i n F i g . 3.8 f o r two b a n d s , one n e a r t h e c h i l l and t h e o t h e r r emo te f r o m t h e c h i l l a s Figure 3.6 Porosity in cast 5-26-2, A l + 3% Cu, showing pores in the interdendritic second phase (8 X) T a b l e 3.3 POROSITY IN DIRECTIONAL CASTINGS C a s t i n g Hyd rogen C o n t e n t cc/lOOgm M i n u t e s o f Steam A d d i t i o n a t 850°C C h i l l P o r o s i t y Number S m a l l H o l e Max. S i z e mm L a r g e H o l e M i n . S i z e mm Po re s/cm2 Shape & D i s t r i b u t i o n 5 -4 -1 A l 0.76 S t a i n l e s s S t e e l 0.12 80 S p h e r i c a l & Random 5 -4 -2 A l 1.0 Copper 0.1 90 S p h e r i c a l & Random 5 -4 -3 A l 1.0 R e f r a c t o r y B r i c k 0.2 0 .48 1.3 140 S p h e r i c a l & Random 5-6-1 A l+3%Cu 0.83 Copper 0.08 100 I n t e r d e n d r i t i c 5 - 6 -2 Al+3%Cu 0.83 S t a i n l e s s S t e e l 0.12 60 I n t e r d e n d r i t i c 5 -26 -1 Al+3%Cu 0.90 R e f r a c t o r y B r i c k 0.18 80 I n t e r d e n d r i t i c 60 F i g u r e 3.8 P o r e d i s t r i b u t i o n a c r o s s F i g . 3 .7 . A i s p o r e coun t f o r Band A and B f o r Band B. The p o r e coun t i s f o r v i s i b l e p o r e s a t 5 X m a g n i f i c a t i o n . 62 i n d i c a t e d on F i g . 3 . 7 . The p o r o s i t y i s e s s e n t i a l l y t h e same i n b o t h bands w i t h a r e l a t i v e l y c o n s t a n t number o f p o r e s t h r o u g h o u t t h e c e n t r e o f t h e c a s t i n g and d e c r e a s i n g number n e a r t h e s u r f a c e . When t h e c h i l l m a t e r i a l i s changed f r o m c o p p e r t o s t a i n l e s s s t e e l , w h i c h m a r k e d l y r e d u c e s t h e f r e e z i n g r a t e , t h e p o r o s i t y does n o t change . The r e f r a c t o r y b r i c k c h i l l p r o d u c e s a l a r g e e q u i a x e d g r a i n s t r u c t u r e , and i n c r e a s e s t h e number and s i z e o f t h e p o r e s . The p o r o s i t y i n t h e d i r e c t i o n a l l y c a s t A l + 3% Cu a l l o y s i s i n t e r d e n d r i t i c and a s s o c i a t e d w i t h t h e second p h a s e . The p o r e s a r e b o t h s p h e r i c a l and e l o n g a t e d , d e p e n d i n g on t h e geomet r y o f t h e s econd pha se component w i t h w h i c h t h e y a r e a s s o c i a t e d . S i m i l a r t o t h e A l c a s t i n g s , t h e d i s t r i b u t i o n o f p o r e s i s u n i f o r m t h r o u g h o u t t h e c a s t i n g , t h e number and s i z e b e i n g i n d e p e n d e n t o f d i s t a n c e f r o m t h e c h i l l and e s s e n t i a l l y i n d e p e n d e n t o f f r e e z i n g r a t e . T h e r e i s no e v i d e n c e o f p o r e s k i n k i n g t h e g r a i n b o u n d a r i e s , b u t t h e l a r g e r p o r e s , 0.1 mm i n w i d t h o r l a r g e r , a r e p r e f e r e n t i a l l y s i t u a t e d on g r a i n b o u n d a r i e s . 3.2 D i r e c t i o n a l S o l i d i f i c a t i o n o f H i g h P u r i t y A luminum The p o r o s i t y o b s e r v e d i n d i r e c t i o n a l l y s o l i d i f i e d h i g h p u r i t y a l uminum i s l i s t e d i n T a b l e 3 .4 . P o r o s i t y i n t h e s o l i d i f i e d r o d s c a n be p l a c e d i n t o t h e f o l l o w i n g c a t e g o r i e s - A : u n i f o r m l y d i s t r i b u t e d s m a l l s p h e r i c a l p o r e s ; B: i n i t i a l l y a f ew l a r g e s p h e r i c a l p o r e s c h a n g i n g 63 T a b l e 3.4 POROSITY IN DIRECTIONALLY SOLID IF IED HIGH PURITY ALUMINUM Rod # Hyd rogen C o n t e n t (cc /100 gm) S o l i d i f i c a t i o n D i r e c t i o n P o r o s i t y Type D i a m e t e r o f L a r g e s t Pore(mm) 1 0.20 Up A 0.05 Down A 0.07 2 0.60 Up A 0.05 Down B 0.15 3 0.94 Up C 0.053 Down B 0.70 4 1.24 Up C 0.046 Down B 1.5 A - U n i f o r m d i s t r i b u t i o n o f s m a l l s p h e r i c a l p o r e s w i t h a few e l o n g a t e d p o r e s . B - I n i t i a l l y a few l a r g e s p h e r i c a l p o r e s c h a n g i n g t o e l o n g a t e d p o r e s t h e n t o many s m a l l s p h e r i c a l p o r e s i n t h e l a s t m e t a l t o s o l i d i f y . C - I n i t i a l l y many l a r g e and s m a l l s p h e r i c a l p o r e s , p r o g r e s s i v e l y d e c r e a s i n g i n number t o a few p o r e s i n t h e l a s t m e t a l t o s o l i d i f y . t o e l o n g a t e d p o r e s and t h e n t o many s m a l l s p h e r i c a l p o r e s i n t h e l a s t m e t a l t o s o l i d i f y ; C: i n i t i a l l y many l a r g e and s m a l l s p h e r i c a l p o r e s p r o g r e s s i v e l y d e c r e a s i n g i n numbers t o a few p o r e s i n t h e l a s t m e t a l t o s o l i d i f y . v I n Rod 1, t o w h i c h no h y d r o g e n was a d d e d , t h e r e i s no s i g n i f i c a n t d i f f e r e n c e i n t h e p o r e s i z e and d i s t r i b u t i o n f o r r o d s s o l i d i f i e d i n e i t h e r t h e up o r down d i r e c t i o n s . The p o r e s a r e u n i f o r m l y d i s t r i b u t e d , s m a l l , and u s u a l l y s p h e r i c a l . F i g . 3.9 shows t h e p o r o s i t y i n Rod 1 s o l i d i f i e d upwa rd s . The number o f p o r e s and t h e % a r e a p o r o s i t y f o r Rod 1 - Down as a f u n c t i o n o f r o d p o s i t i o n a r e shown i n F i g u r e 3 . 10 . The number o f p o r e s i n c h a r a c t e r i z e d b y p l o t t i n g two c u r v e s one f o r s m a l l p o r e s , 1-5 um i n d i a m e t e r and one f o r p o r e s l a r g e r t h a n 30 um i n d i a m e t e r . A s m a l l i n c r e a s e i n t h e number o f l a r g e r p o r e s and an i n c r e a s e i n % a r e a f r o m t h e i n i t i a l t o f i n a l p a r t t o s o l i d i f y i s s e e n i n F i g . 3 . 10 . Howeve r , t h e s c a t t e r o f t h e d a t a i s s u ch t h a t t h e p o r o s i t y c a n be c o n s i d e r e d as e f f e c t i v e l y u n i f o r m . F i g . 3.11 shows t h e p o r e s i z e d i s t r i b u t i o n a t 3 p o s i t i o n s a l o n g t h e r o d . The m a j o r i t y o f t h e p o r e s a r e s m a l l and t h e r e i s a s l i g h t d e c r e a s e i n t h e o v e r a l l number. S o l i d i f y i n g Rod 2 i n t h e up s o l i d i f i c a t i o n d i r e c t i o n r e s u l t s i n t y p e A p o r o s i t y - a u n i f o r m d i s t r i b u t i o n o f s m a l l p o r e s . I n t h e down s o l i d i f i c a t i o n d i r e c t i o n , t h e p o r o s i t y changes t o t y p e B as I n i t i a l Middle F ina l Figure 3.9 Uniformly distributed porosity in Rod 1-Up. Note that the pores are mostly spherical with a few elongated. Hydrogen content i s 0.20 ^ / l O O gm A l (25 X) Figure 3.10 Number of pores/cm and % area porosity in Rod 1-Down. The number of pores is characterized by 2 sizes: pores l-5vim in diameter and pores > 30pm in diameter. 67 CM o (VI o \ CO U J or o Cu T J / /: / > Z Z 3 304 20 to 0 F I N A L 1-5 5-10 10-15 15-20 20-30 > 30 PORE SIZES ( /JLm) Figure 3.11 Distribution of pore sizes along the length of Rod 1-Down. The f i r s t graph is for the i n i t i a l 2 cm of the rod, the second for the middle 2 cm, and the f ina l is for the last 2 cm of rod to so l id i fy . 68 d e s c r i b e d a b o v e . F i g . 3 .12 shows t h e number o f p o r e s and t h e a r e a o f p o r o s i t y i n Rod 2 - Up . B o t h t h e number and % a r e a a r e e f f e c t i v e l y u n i f o r m a l o n g t h e r o d . The c o r r e s p o n d i n g p o r e d i s t r i b u t i o n f o r p o s i t i o n s a l o n g t h e r o d i s g i v e n i n F i g u r e 3 . 1 3 . The d i s t r i b u t i o n o f p o r e s i s a l s o s e e n t o r e m a i n e f f e c t i v e l y c o n s t a n t a l o n g t h e r o d . Rod 3 s o l i d i f i e d i n t h e up d i r e c t i o n e x h i b i t s t y p e C p o r o s i t y c h a r a c t e r i z e d by a d e c r e a s e i n t h e number o f p o r e s a s t h e r o d i s s o l i d i f i e d . F i g . 3.14 shows t h e d e c r e a s e i n s i z e and % a r e a as s o l i d i f i c a t i o n p r o c e e d s . F i g . 3.15 shows t h a t t h e r e i s r e l a t i v e l y l i t t l e change i n p o r e d i s t r i b u t i o n b u t t h e r e i s an o v e r a l l d e c r e a s e i n t h e number o f p o r e s . Rod 3 - Down e x h i b i t s t y p e B b e h a v i o u r c h a r a c t e r i z e d by a g r a d u a l change f r o m l a r g e s p h e r i c a l p o r e s t o s m a l l s p h e r i c a l p o r e s . D a t a f o r p o r e s i z e , % a r e a , and d i s t r i b u t i o n a l o n g Rod 4 - Down a r e g i v e n i n F i g s . 3.16 and 3 . 17 . The % a r e a p o r o s i t y i n F i g . 3 .16 i s o b s e r v e d t o be l a r g e i n t h e i n i t i a l l y s o l i d i f i e d m e t a l . T h i s i s a s c r i b e d t o t h e number o f l a r g e p o r e s o v e r 1 mm i n d i a m e t e r . The r e l a t i v e d i s -t r i b u t i o n o f p o r e s c hange s f r o m a f ew s m a l l p o r e s and s e v e r a l l a r g e r p o r e s n e a r t h e b e g i n n i n g o f s o l i d i f i c a t i o n t o many s m a l l p o r e s o f a l l s i z e s i n t h e f i n a l s o l i d i f i e d m e t a l a s shown i n F i g . 3 . 17 . The a p p e a r a n c e o f t h e p o r e s a l o n g t h e r o d i s shown i n F i g . 3 . 1 8 , i l l u s t r a t i n g t h e change d e s c r i b e d a b o v e . 69 >-CO O or o o l i 1 j 1 I 0 2 4 6 8 10 INITIAL FINAL DISTANCE (cm) Figure 3.12 Number of pores/cm and % area porosity in Rod 2-Up. PORE S I Z E S ( ^ m ) Figure 3.13 Distribution of pore sizes along the length of Rod 2-Up. 71 F i g u r e 3.14 Number o f p o r e s / c m 2 and % a r e a p o r o s i t y i n Rod 3 -Up. 72 73 > 0 2 4 6 8 10 INITIAL FINAL DISTANCE (cm) Figure 3.16 Number of pores/cm and % area porosity in Rod 4-Down. 4 0 1 P O R E S I Z E S 1/i.m) Figure 3.17 Distribution of pores sizes along the length of Rod 4-Down. Figure 3.18 Micrograph of Rod 4-Down i l l u s t r a t i n g type B porosity. (25 X) . 76 I n g e n e r a l , an i n c r e a s e i n h y d r o g e n c o n t e n t i n c r e a s e s t h e s i z e , number , and % a r e a o f t h e p o r e s . S i n c e t h e s o l i d i f i c a t i o n d i r e c t i o n a f f e c t s t h e t y p e o f p o r o s i t y f o r m e d , t h e e f f e c t o f c h a n g i n g h y d r o g e n on p o r o s i t y i s c o n s i d e r e d s e p a r a t e l y f o r s o l i d i f i c a t i o n i n t h e up and down d i r e c t i o n s . A s t h e h y d r o g e n c o n t e n t i n c r e a s e s , t h e % a r e a p o r o s i t y i s i n c r e a s e d i n t h e up d i r e c t i o n as e v i d e n t by com-p a r i n g F i g . 3.12 and 3 .14 . Type C p o r o s i t y i n t h e up d i r e c t i o n changes o n l y i n i n c r e a s i n g t h e number o f p o r e s w i t h i n c r e a s i n g h y d r o g e n c o n t e n t . I n t h e down s o l i d i f i c a t i o n d i r e c t i o n , w i t h t y p e B p o r o s i t y , t h e s i z e o f t h e l a r g e p o r e i n c r e a s e s w i t h i n c r e a s i n g h y d r o g e n c o n t e n t . L a u e X - r a y d i f f r a c t i o n p a t t e r n s on t h e r o d s show t h a t t h e r o d s a r e n o t s i n g l e c r y s t a l s . The l o n g c o l u m n a r g r a i n s a r e shown i n F i g . 3 . 1 9 A - a s l i g h t l y e t c h e d s e c t i o n o f Rod 4 - Up. A f u l l y e t c h e d s u r f a c e o f a v e r t i c a l s e c t i o n o f Rod 2 - Down i s shown i n F i g . 3 .19B. A c o l u m n a r s u b s t r u c t u r e i s o b s e r v e d t o b e p r e s e n t i n t h e r o d . The p o r e s a r e n o t r e l a t e d t o e i t h e r t h e g r a i n b o u n d a r i e s o r t h e s u b s t r u c t u r e b o u n d a r i e s . 3.3 D i r e c t i o n a l S o l i d i f i c a t i o n o f A l + Ag " * " ^ Two r o d s o f A l + 0 .5% Ag w e r e s o l i d i f i e d d i r e c t i o n a l l y . These r o d s w e r e s o l i d i f i e d i n e i t h e r t h e up o r down d i r e c t i o n s f o r p a r t F i g u r e 3.19 E t c h e d m i c r o - s t r u c t u r e o f d i r e c t i o n a l l y s o l i d i f i e d h i g h p u r i t y a l um inum (A) Co lumnar g r a i n s o f Rod 4-Up (11 X ) (B) S u b s t r u c t u r e o f Rod 2-Down (25 X ) . 78 of their length then quenched to room temperature. The observed con-centration of Ag"*""^  along the rods is shown in Figs . 3.20 (up) and 3.21 (down) in which act iv i ty is plotted as a function of distance along the rod. In F i g . 3.20, the act iv i ty of Ag1"*"0 decreases s l ight ly along the length of the rod. I n i t i a l l y , the concentration of A g 1 1 ^ is greater than the average concentration (activity = 6.94 x 10^ counts/ gm - min) and in the f i n a l metal s o l i d i f i e d , the concentration drops below the average. If macro-segregation occurs in the upward growth direction i t w i l l l i k e l y follow a diffusion controlled process with no convective flow in the l i q u i d . This is because the cool , so lute-r ich l iqu id remains at the so l id i f i ca t ion front while the less dense, hotter, si lver-poor l iqu id i s stable at the top of the tube. In l ight of this reasoning, the theoretical diffusion controlled segregation for the i n i t i a l transient C g and the solute contour i n the l i q u i d ahead of the quenched interface C are plotted in Figure 3.20, using: c = c s o { ( 1 " V Q " e x P < k o f X>] + k o j ( 1 6 ) and 1 - k CT = C L o 1 + —r-Z exp( - f x ) (17) where C g = concentration of solute in the sol id C o = a v e r a S e concentration of solute 94-> 7 9 o c E i cn +— c •3 o o h-< 3+' The ore tic a I Experimental Quench f . 0 2 INITIAL Figure 3.20 •4 4 6 8 10 DISTANCE (cm) F I N A L Plot of concentration of Ag"*"^ along the length of the rod. Theoretical C S and CL» for diffusion controlled segregation of the rod so l id i f i ed in the up direct ion, are dotted l ines . 10 * o 8 £ E 6 o» CO c o o 4 + < 2 f 0 | f NITIAL O G Theoretical Exp erimental Quench , 4 • 8 -I 9 10 FINAL DISTANCE (cm) Figure 3.21 Ag concentration plotted as a function of distance along the rod so l id i f i ed downward. Theoretical C G and CL for complete mixing are plotted as dotted l ines . C = c o n c e n t r a t i o n o f s o l u t e i n t h e l i q u i d k = 0 . 2 , d i s t r i b u t i o n c o e f f i c i e n t o _3 R = 1.42 x 10 cm/sec , g r owth r a t e - 5 2 D = 5 x 10 cm / s e c , d i f f u s i v i t y X = l e n g t h o f s o l i d p h a s e The i n i t i a l t r a n s i e n t i n t h e s o l i d and t h e c o n c e n t r a t i o n i n t h e l i q u i d we re n o t d e t e c t e d i n t h e measurements shown i n F i g . 3 . 20 . T h i s c o u l d be a r e s u l t o f t h e r e l a t i v e l y n a r r o w w i d t h o f t h e p e a k s , o r t h a t t h e peak s w e r e n o t as l a r g e as t h a t t h e o r e t i c a l l y p r e d i c t e d . S e c t i o n s o f t h e r o d we re p o l i s h e d and e t c h e d t o d e t e r m i n e t h e c a s t s t r u c t u r e . The s t r u c t u r e t h r o u g h o u t t h e r o d i s d e n d r i t i c a s shown i n F i g . 3 .22 w i t h no s i g n i f i c a n t d i f f e r e n c e b e t w e e n t h e d i r e c t i o n a l l y s o l i d i f i e d and quenched m a t e r i a l . The b l a c k d o t s i n d i c a t e d by a r r o w s i n t h e m i c r o g r a p h a r e S i i n c l u s i o n s c au sed b y some d i s s o l u t i o n o f t h e V y c o r c a p s u l e c o n t a i n i n g t h e m o l t e n m e t a l . S i n c e t h e S i i n c l u s i o n s a r e l o c a t e d b o t h w i t h i n t h e d e n d r i t e s and i n t h e i n t e r d e n d r i t i c r e g i o n s , t h e i r e f f e c t on s e g r e g a t i o n o f Ag^" "^ i s c o n s i d e r e d n e g l i g i b l e . I n t h e down d i r e c t i o n , t h e r e s h o u l d b e c o m p l e t e m i x i n g i n t h e l i q u i d ahead o f t h e i n t e r f a c e due t o c o n v e c t i v e c u r r e n t s c a u s e d by t h e dense c o o l l i q u i d c o n s t a n t l y f l o w i n g t o t h e b o t t o m o f t h e h o t 82 Figure 3.22 Dendritic microstructure of direct ional ly so l id i f i ed A l + AgHO. The arrows indicate Si inclusion (15 X). 83 m e l t . The c o n c e n t r a t i o n o f Ag"1"1"'"' shown i n F i g . 3 . 2 1 , i s i n i t i a l l y l a r g e , t h e n d e c r e a s e s w i t h i n t h e f i r s t 2.5 cm t o a s t a b l e l e v e l b e l o w t h e a v e r a g e 4 c o n c e n t r a t i o n : 7.33 x 10 c o u n t s / g m - m i n . The t h e o r e t i c a l e q u a t i o n s f o r C and C T a s s um ing c o m p l e t e m i x i n g i n t h e l i q u i d a r e : C = k C (1 - g ) k o " 1 (17) s o o s and C T = C (1 - g ) k ° " 1 (18) L o s whe re g g = f r a c t i o n s o l i d i f i e d C and C a r e p l o t t e d i n F i g . 3.21 as d o t t e d l i n e s . The t h e o r e t i c a l s i~> s o l u t e c o n c e n t r a t i o n C g f o r a p r o g r e s s i v e l y s o l i d i f i e d r o d i s much l o w e r t h a n t h e o b s e r v e d v a l u e s . T h e r e a r e s e v e r a l p o s s i b l e e x p l a n a t i o n s f o r t h e i n c r e a s e d l e v e l o f s o l u t e i n t h e e x p e r i m e n t a l c u r v e s . N u c l e i o f s o l i d p a r t i c l e s w h i c h a r e s o l u t e - p o o r may f o r m t h e n s i n k t o t h e b o t t o m o f t h e l i q u i d , l e a v i n g t h e m e t a l s o l u t e - r i c h i n t h e i n i t i a l l y s o l i d i f i e d s e c t i o n . T h e r e a l s o may be some s u p e r c o o l i n g a few c e n t i m e t e r s f r o m t h e t o p o f t h e r o d , p r o d u c i n g v e r y f i n e d e n d r i t e s w h i c h w i l l t r a p t h e s i l v e r . A t h i r d p o s s i b i l i t y i s t h a t n u c l e a t i o n may n o t o c c u r a t t h e t o p o f t h e r o d b u t a s m a l l d i s t a n c e away, e i t h e r t r a p p i n g s i l v e r above t h e s o l i d band o r p e r m i t t i n g d i f f u s i o n c o n t r o l l e d s e g r e g a t i o n t o o c c u r f o r a s h o r t d i s t a n c e b e l o w t h e r o d . The m i c r o s t r u c t u r e o f t h e r o d w h i c h was s o l i d i f i e d i n t h e down d i r e c t i o n i s t h e same as t h e d e n d r i t i c s t r u c t u r e i n t h e r o d s o l i d i f i e d upward s . 84 M i c r o s e g r e g a t i o n was o b s e r v e d t o be p r e s e n t i n t h e r o d s as shown i n F i g . 3 . 23 . The e l e c t r o n p r o b e was u sed t o p l o t X - r a y i n t e n s i t y c o r r e s p o n d i n g t o s i l v e r as a f u n c t i o n o f d i s t a n c e a c r o s s a segment o f t h e r o d . F i g . 3 . 2 3 , c l e a r l y shows t h a t t h e i n t e r d e n d r i t i c r e g i o n s a r e r i c h i n Ag 3.4 P o r o s i t y i n S u p e r c o o l e d I r o n A s e r i e s o f s h o r t i r o n r o d s c o n t a i n i n g v a r i o u s amounts o f c a r b o n and oxygen w e r e s o l i d i f i e d a f t e r s u p e r c o o l i n g and examined f o r p o r o s i t y . The r e s u l t s a r e g i v e n i n T a b l e 3 . 5 . Samples 1 ,7 ,8 and 9 l i s t e d i n t h e t a b l e w i t h 0°C u n d e r c o o l i n g were s u p e r c o o l e d b e f o r e t h e a d d i t i o n o f C a n d / o r 0. A f t e r t h e a d d i t i o n s , t h e s e m e l t s w o u l d n o t s u p e r c o o l . I n a l l c a s e s t h e p o r e s w e r e s p h e r i c a l o r e l o n g a t e d w i t h smooth w a l l s and n o t g e n e r a l l y a s s o c i a t e d w i t h t h e g r a i n b o u n d a r i e s . I n m e l t s 1-3 w i t h l ow c a r b o n c o n t e n t and a r e l a t i v e l y c o n s t a n t o x ygen c o n t e n t , t h e e f f e c t o f i n c r e a s e d s u p e r c o o l i n g o f t h e m e l t i s an i n c r e a s e i n t h e % a r e a o f t h e p o r o s i t y and a s h a r p i n c r e a s e i n t h e p o r e d e n s i t y i n t h e f i r s t s u p e r c o o l i n g i n t e r v a l . T h i s r e l a t i o n s h i p i s shown i n F i g . 3 . 24 . The p o r e s i z e d i s t r i b u t i o n s f o r m e l t s 1-3 a r e g i v e n i n F i g . 3 . 2 5 . I n c o n f o r m i t y w i t h T a b l e 3 . 5 , t h e t o t a l number o f p o r e s i n c r e a s e s s h a r p l y be tween m e l t s 1 and 2 and r e m a i n e s s e n t i a l l y t h e same f o r m e l t s 2 and 3. C o n s i d e r i n g t h e p o r e d i s t r i b u t i o n , t h e number o f DISTANCE Figure 3.23 Segreation of Ag in the interdendrit ic regions. The numbers in the micrograph corresponds to the peaks in the intensity-distance curve. (50 X). T a b l e 3.5 POROSITY IN SUPERCOOLED IRON M e l t Number M e l t A d d i t i o n Ca rbon % Oxygen ppm S u p e r c o o l i n g °C % A r e a P o r o s i t y T o t a l No. P o r e s / c m ^ x l O 2 P o r e Shape 1 - < 0.02 288 0 0 .334 36 .0 S + E 2 - < 0.02 258 116 0.478 62 .0 S 3 - < 0.02 245 213 0.842 65.6 S 4 Oxygen < 0.02 672 47 0 .334 143 .2 S + E 5 Ca rbon 0.03 257 83 0.394 69 .2 S 6 Ca rbon 0.055 265 213 0 .392 88 . 0 S + E 7 Ca rbon + Oxygen 0.16 256 0 0 .363 52 . 0 S 8 Ca rbon 0.375 156 0 0 .382 110 . 0 S + E 9 Ca rbon 0.189 92 0 0 .336 5 6 . 0 S S = S p h e r i c a l E = E l o n g a t e d 00 ON 87 Figure 3.24 Porosity plotted as a function of supercooling in melts 1, 2 and 3. (carbon < 0.02% and oxygen ^ 250 ppm). MELT I <l 1-5 5- 10 10-15 15-20 >20 P O R E SIZES (^m) Figure 3.25 Pore distr ibution in melts 1 , 2 and 3. 89 s m a l l e s t p o r e s (< lym) i n c r e a s e s w i t h i n c r e a s e d s u p e r c o o l i n g . The l - 5 y m p o r e s and p o r e s > 20ym i n c r e a s e i n number be tween m e l t s 1 and 2 b u t a r e n e a r l y c o n s t a n t be tween 2 and 3. The r e m a i n i n g p o r e s i z e s a r e c o n s t a n t o r f l u c t u a t e i n a random manner . The l a r g e i n c r e a s e i n % a r e a p o r o s i t y w i t h o u t a s i g n i f i c a n t change i n t h e p o r e d i s t r i b u t -i o n o r i n c r e a s e i n number o f p o r e s be tween m e l t s 2 and 3 i s due t o t h e number o f t h e p o r e s > 30ym. I n m e l t 3 , t h e l a r g e s t p o r e s w e r e a p p r o x i m a t e l y 40 ym i n d i a m e t e r , w h i l e i n m e l t 2 t h e r e w e r e 1 o r 2 p o r e s 30 ym i n d i a m e t e r and t h e r e s t a p p r o x i m a t e l y 20 ym i n d i a m e t e r . F i g . 3.26 shows t h e p o r o s i t y i n m e l t 3. The p o r e s a r e s p h e r i c a l and i n c o m p a r i n g p h o t o A ( u n e t c h e d ) w i t h B ( e t c h e d ) , i t c a n be s e e n t h a t t h e p o r e s a r e n o t g e n e r a l l y a s s o c i a t e d w i t h t h e g r a i n b o u n d a r i e s . The e f f e c t o f i n c r e a s e d oxygen c o n t e n t on t h e s u p e r c o o l i n g was examined i n m e l t 4 . W i t h t h e c a r b o n l e v e l f i x e d , t h e o xygen was i n c r e a s e d - f r o m abou t 260 ppm t o 672 ppm. A r e l a t i v e l y s m a l l s u p e r c o o l i n g was o b t a i n e d f o r t h i s m e l t . M e l t 4 was f o u n d t o h a v e a % a r e a p o r o s i t y e q u i v a l e n t t o t h a t o f m e l t 1 b u t a p o r e d e n s i t y s i g n i f i c a n t l y g r e a t e r t h a n m e l t 1. T h i s c an be a c c o u n t e d f o r by c o m p a r i n g t h e p o r e d i s t r i b u t i o n o f m e l t 4 i n F i g . 3.27 w i t h t h a t o f m e l t 1, F i g . 3 . 25 . I n m e l t 4 t h e r e a r e many more p o r e s l e s s t h a n 10 ym i n d i a m e t e r and v e r y few l a r g e p o r e s , w h i l e i n m e l t 1, t h e r e a r e few s m a l l p o r e s and s e v e r a l p o r e s > lOym. These p o r e d i s t r i b u t i o n s c a u s e t h e % a r e a p o r o s i t y t o be t h e same f o r b o t h m e l t s . The i n c r e a s e Figure 3.26 Micrographs showing spherical pores i n melt 3A is as-polished and B is etched (350 X). 91 CVJ O P O R E S I Z E S (yLcm) Figure 3.27 Pore size distr ibution in melt 4. 92 i n p o r e s < lOum w i t h i n c r e a s e d o xygen c o n t e n t i n d i c a t e s t h a t t h e n u c l e a t i n g r a t e o f p o r e s i n c r e a s e s s i g n i f i c a n t l y . The n u c l e a t i o n o f s o l i d s i s a l s o a f f e c t e d by t h e i n c r e a s e d oxygen c o n t e n t a s i n -d i c a t e d b y t h e d e c r e a s e i n s u p e r c o o l i n g o b s e r v e d i n t h e m e l t . I n m e l t s 5 and 6, t h e c a r b o n l e v e l was i n c r e a s e d a s m a l l amount w i t h t h e oxygen c o n t e n t m a i n t a i n e d a t t h e r e f e r e n c e l e v e l o f m e l t s 1 - 3 . The s u p e r c o o l i n g o b t a i n e d t e n d e d t o r e m a i n l a r g e r w i t h t h e h i g h e r c a r b o n c o n t e n t ; t h e r e l a t i v e l y l o w u n d e r c o o l i n g o f 83°C f o r m e l t 5 i s n o t c o n s i d e r e d s i g n i f i c a n t . The a p p e a r a n c e o f p o r e s i n m e l t s 5 and 6 w e r e s i m i l a r t o t h e o t h e r m e l t s , i . e . t h e p o r e s a r e s p h e r i c a l o r e l o n g a t e d and n o t r e l a t e d i n any o b v i o u s way t o t h e c a s t s t r u c t u r e o r g r a i n b o u n d a r i e s . An example o f t h e p o r e s i n an a s - p o l i s h e d and e t c h e d s e c t i o n o f m e l t 6 i s shown i n F i g . 3 . 28 . The e f f e c t o f i n c r e a s e d c a r b o n c o n t e n t on % a r e a p o r o s i t y and p o r e d e n s i t y i s shown i n F i g . 3 . 29 . M e l t 2 i s u sed f o r t h e 0.02% c a r -bon d a t a p o i n t . T h e p o r e d e n s i t y i n c r e a s e s a l i t t l e w i t h h i g h e r c a r b o n c o n t e n t w h i l e t h e a r e a p o r o s i t y d e c r e a s e s a s l i g h t amount. M e l t s 5 and 6 h a ve r o u g h l y t h e same % a r e a o f p o r e s a s m e l t 1. The p o r e s i z e d i s -t r i b u t i o n f o r m e l t s 5 and 6 a r e shown i n F i g . 3 . 30 . T h e r e i s a h i g h e r d e n s i t y o f s m a l l p o r e s (< lOym) and l a r g e p o r e s (> 20um) i n m e l t 6 as compared t o m e l t 5. T h e r e a r e , h o w e v e r , more p o r e s i n t h e 15-20ym s i z e o f m e l t 5 t h a n i n 6. I n c r e a s i n g t h e c a r b o n c o n t e n t does n o t s i g -n i f i c a n t l y a f f e c t t h e a b i l i t y t o s u p e r c o o l t h e l i q u i d , i n d i c a t i n g F i g u r e 3.28 M i c r o g r a p h s show ing p o r o s i t y i n m e l t 6. P h o t o A a s - p o l i s h e d and B a s e t c h e d . (350 X ) . 94 Figure 3.29 Relationship between porosity and carbon content in supercooled iron. 95 M E L T 5 PORE SIZES (^m) Figure 3.30 Pore size dis tr ibut ion in melts 5 and 6. t h a t n u c l e a t i o n o f s o l i d i s u n a f f e c t e d by c a r b o n a d d i t i o n . The n u c l e a t i o n o f p o r e s i s a l s o n o t s i g n i f i c a n t l y a f f e c t e d b y c a r b o n a d d i t i o n . F o r e x a m p l e , i n m e l t s 3 and 6 w h i c h have t h e same s u p e r -c o o l i n g , t h e p o r e d e n s i t y i s r o u g h l y t h e same. However , t h e % a r e a p o r o s i t y and t h e p o r e s i z e d i s t r i b u t i o n a r e q u i t e d i f f e r e n t f o r t h e two m e l t s due t o t h e p r e s e n c e o f l a r g e p o r e s i n m e l t 3. A p p r o x i m a t e l y 0 .5% c a r b o n was added t o m e l t s 7 ,8 and 9 . The c a r b o n i n t h e s e m e l t s r e a c t e d w i t h t h e oxygen r e s u l t i n g i n l o w e r oxygen c o n c e n t r a t i o n s (N .B . a d d i t i o n a l o xygen was added t o m e l t 7 ) . The c a r b o n was added a f t e r t h e i n i t i a l m e l t e x h i b i t e d s u p e r c o o l i n g up t o 300°C . Howeve r , a f t e r t h e c a r b o n was a d d e d , t h e m e l t s no l o n g e r e x h i b i t e d s u p e r c o o l i n g a s i n d i c a t e d on T a b l e 3 . 5 . I n c r e a s i n g t h e amount o f c a r b o n i n t h e m e l t d i d n o t s i g n i f i c a n t l y change t h e a p p e a r a n c e o f t h e p o r e s as compared t o t h e l o w c a r b o n m e l t s . An e xamp le o f t h e p o r e s i n m e l t 7 i s shown i n F i g . 3 . 3 1 . I t i s n o t e d i n F i g . 3 .31B, t h e e t c h e d s e c t i o n , t h a t t h e p o r e s a r e n o t g e n e r a l l y a s s o c i a t e d w i t h e i t h e r g r a i n b o u n d a r i e s o r a p a r t i c u l a r p h a s e . The % a r e a p o r o s i t y i n a l l t h e s e m e l t s was e s s e n t i a l l y e q u i v a l e n t t o t h e l o w c a r b o n m e l t s 1,5 and 6. The d e n s i t y o f p o r e s f o r 7 and 9 i s c o m p a r a b l e t o m e l t s 2 ,3 and 5, w h i l e m e l t 8 ha s abou t d o u b l e t h e number o f p o r e s . The p o r e s i z e d i s t r i b u t i o n f o r m e l t s 7-9 a r e shown i n F i g . 3 . 32 . I n a l l t h r e e m e l t s , t h e p o r e s a r e c o n c e n t r a t e d i n t h e l - 1 5um 97 Figure 3.31 As-polished (A) and etched (B) micrographs of melt 7 showing porosity (350 X ) . 20 PORE S I Z E S ( fjLm) Figure 3.32 Pore size distr ibut ion in melts 7, 8 and 9. size range. From the above observations i t can be concluded that pore nucleation is not s ignif icantly affected by carbon content of less than0.02%C. There may be some effect for carbon concentrations greater than this amount as shown by the higher pore density of melt 8 (0.38%C). The fact that melts 7, 8 and 9 did not supercool may be due to the introduction of nucleating impurities with the carbon addition to the melt. 4. DISCUSSION The d i s c u s s i o n i s d i v i d e d i n t o t h r e e s e c t i o n s . The f i r s t d e a l s w i t h d i r e c t i o n a l l y s o l i d i f i e d a luminum and A l + A g ^ ^ . The n e x t s e c t i o n d i s c u s s e s p o r o s i t y i n A l and A l a l l o y s c a s t u n d e r d i f f e r e n t c o n d i t i o n s and t h e t h i r d s e c t i o n i s c o n c e r n e d w i t h p o r o s i t y i n s u p e r c o o l e d i r o n . 4 .1 P o r o s i t y i n D i r e c t i o n a l l y S o l i d i f i e d A l and A l + Ag^" "^ 4 . 1 . 1 P o r o s i t y i n H i g h P u r i t y A luminum The s i z e , d i s t r i b u t i o n , and shape o f t h e p o r e s o b s e r v e d i n d i r e c t i o n a l l y s o l i d i f i e d h i g h p u r i t y a l uminum r o d s i s i l l u s t r a t e d s c h e m a t i c a l l y i n F i g . 4 . 1 . As shown i n F i g . 4 . 1 ( a ) , t h e p o r e s a r e s m a l l and u n i f o r m l y d i s t r i b u t e d i n t h e l o w h y d r o g e n r o d s s o l i d i f i e d e i t h e r upwards o r downwards. W i t h a h i g h e r h y d r o g e n c o n c e n t r a t i o n i n t h e l i q u i d , t h e p o r e s a p p e a r as shown s c h e m a t i c a l l y i n F i g . 4 .1 (b) and (c ) f o r f r e e z i n g downward and upward r e s p e c t i v e l y . F r e e z i n g downward, l a r g e s p h e r i c a l p o r e s a r e f o rmed i n i t i a l l y c h a n g i n g t o e l o n g a t e d p o r e s and t h e n t o s m a l l s p h e r i c a l p o r e s i n t h e f i n a l l i q u i d t o s o l i d i f y . I n t h e upward s o l i d i f i c a t i o n d i r e c t i o n b o t h l a r g e and s m a l l p o r e s a r e f o rmed d e c r e a s i n g i n number as s o l i d i f i c a t i o n p r o c e e d s . The s i z e and shape o f t h e p o r e s i s a f u n c t i o n o f t h e h y d r o g e n c o n t e n t o f t h e l i q u i d m e t a l , t h e mo rpho l o g y o f t h e s o l i d - l i q u i d 100 o o o o o o o o o o o o o o o o o o o o o o o o o o o o (a) INITIAL F I N A L OO o o o 0 0 (TO o o o o o o o o (b) F I N A L INITIAL Figure 4.1 Schematic representation of porosity observed in directionally so l id i f ied high purity aluminum. (a) Low hydrogen concentration so l id i f i ed i n both up and down direction. (b) High hydrogen content so l id i f i ed downwards. (c) High hydrogen content so l id i f i ed upwards. 102 i n t e r f a c e , and t h e r a t e o f advance o f t h e i n t e r f a c e ; a l l a r e i n t e r r e l a t e d i n a comp lex f a s h i o n . F o r e x a m p l e , t h e p r e s e n c e o f a p o r e a t t h e i n t e r -f a c e can r e d u c e t h e l o c a l h e a t f l o w i n t o t h e s o l i d , t h e r e b y s i g n i f i c a n t l y c h a n g i n g b o t h t h e l o c a l m o r p h o l o g y and t h e f r e e z i n g r a t e o f t h e i n t e r f a c e . I n t u r n , t h e changes i n t h e i n t e r f a c e i n f l u e n c e t h e p o r e s i z e and s h a p e . The g r o w t h d i r e c t i o n may a l s o s i g n i f i c a n t l y i n f l u e n c e t h e p o r o s i t y . F r e e z i n g downward t e n d s t o t r a p a t t h e i n t e r f a c e any b u b b l e s w h i c h a r e l a r g e enough t o b e a b l e t o r i s e i n t h e l i q u i d due t o b u o y a n c y . I n downward s o l i d i f i c a t i o n , t h e l i q u i d ahead o f t h e i n t e r f a c e i s w e l l m i x e d due t o c o n v e c t i o n r e s u l t i n g f r o m h i g h d e n s i t y c o l d l i q u i d b e i n g above t h e l o w e r d e n s i t y warmer l i q u i d . A c c o r d i n g l y , any p o r e s f o r m i n g ahead o f t h e i n t e r f a c e w i l l t e n d t o be moved away f r o m t h e i n t e r f a c e i n t h e f l o w i n g l i q u i d . F r e e z i n g upwards a l l o w s p o r e s w h i c h f o r m and grow t o f l o a t t o t h e t o p o f t h e l i q u i d . I n t h e upward d i r e c t i o n , t h e r e i s l i t t l e c o n v e c t i v e f l o w i n t h e l i q u i d due t o t h e d e n s i t y g r a d i e n t s . C o n s i d e r i n g t h e p r e s e n t o b s e r v a t i o n s , t h e u n i f o r m d i s t r i b u t i o n o f s m a l l p o r e s i n t h e l o w l e v e l h y d r o g e n c a s e ( F i g . 4 . 1a ) i n d i c a t e s t h a t t h e p o r e s n u c l e a t e r e a d i l y b u t do n o t grow s i g n i f i c a n t l y due t o t h e l o w l e v e l o f h y d r o g e n i n t h e v i n c i n i t y o f t h e p o r e . The p o r e s a r e s u f f i c i e n t l y s m a l l t h a t t h e y w o u l d n o t r i s e i n t h e l i q u i d a t a r a t e f a s t e r t h a n t h e a d v a n c i n g i n t e r f a c e , and a r e l i k e l y n u c l e a t e d i n t h e 103 i n t e r d e n d r i t i c l i q u i d w i t h i n t h e s o l i d - l i q u i d i n t e r f a c e . I t f o l l o w s t h a t t h e p o r e s wou l d r e m a i n a t t h e i r n u c l e a t i o n s i t e f o r b o t h upward and downward s o l i d i f i c a t i o n and w o u l d n o t s i g n i f i c a n t l y a f f e c t t h e l o c a l h e a t f l o w . The o b s e r v e d u n i f o r m d i s t r i b u t i o n o f s m a l l p o r e s t h r o u g h o u t t h e s amp le t h e r e f o r e a p p e a r s r e a s o n a b l e . A t h i g h e r h y d r o g e n l e v e l s , t h e p o r e s v a r y i n s i z e , s h a p e , and d e n s i t y a l o n g t h e s o l i d i f i e d r o d and a r e i n f l u e n c e d by b o t h f r e e z i n g d i r e c t i o n and s o l i d i f i c a t i o n r a t e . The s o l i d i f i c a t i o n r a t e i s n o t c o n s t a n t o v e r t h e w h o l e l e n g t h o f t h e r o d due t o end e f f e c t s . The f i r s t s e c t i o n o f l i q u i d o u t o f t h e f u r n a c e s o l i d i f i e s r a p i d l y and n o n - d i r e c t i o n a l l y . When s t e a d y s t a t e i s r e a c h e d s o l i d i f i c a t i o n p r o c e e d s d i r e c t i o n a l l y and a t a s l o w e r , u n i -f o r m r a t e a l o n g most o f t h e r o d . I n t h e f i n a l s e c t i o n o f t h e r o d , t h e s o l i d i f i c a t i o n r a t e a g a i n i n c r e a s e d and t h e s o l i d i f i c a t i o n i s n o t d i r e c t i o n a l s i n c e t h e r o d i s n e a r l y o u t o f t h e f u r n a c e and t h e h e a t i n p u t d r o p s a p p r e c i a b l y . F o r l i q u i d c o n t a i n i n g h i g h h y d r o g e n c o n c e n t r a t i o n s and f r e e z -i n g downward, t h e p o r e s a r e o b s e r v e d t o n u c l e a t e r e a d i l y and grow t o a l a r g e s i z e as i l l u s t r a t e d i n F i g . 4 . 1 b . The l a r g e s i z e o f t h e p o r e s i n t h e i n i t i a l f r e e z i n g zone i s a r e s u l t o f t h e l a r g e amount o f h y d r o g e n i n t h e s u r r o u n d i n g a r e a w h i c h e a s i l y moves t o t h e p o r e s i n t h e w e l l m i x e d l i q u i d . The s p h e r i c a l shape o f t h e p o r e s a r e due t o t h e g r o w t h o f s o l i d 104 on e a c h s i d e o f t h e p o r e w h i c h p r e v e n t t h e p o r e f r o m e l o n g a t i n g a s t h e i n t e r f a c e a d v a n c e s . F u r t h e r a l o n g t h e r o d , a f t e r t h e s o l i d f i c a t i o n r a t e has s t a b i l i z e d , t h e p o r e s become e l o n g a t e d . He re t h e p o r e s grow a t abou t t h e same r a t e as t h e s o l i d i f i c a t i o n f r o n t a d v a n c e s . I n t h e f i n a l m e t a l t o s o l i d i f y , t h e p o r e s w h i c h n u c l e a t e i n t h e s o l i d - l i q u i d i n t e r -f a c e , a r e t r a p p e d , and c a n n o t grow as s o l i d i f i c a t i o n r a p i d l y o c c u r s . T h u s , t h e s i z e and shape o f p o r e s f o rmed d u r i n g downward f r e e z i n g w i t h h i g h h y d r o g e n l e v e l s c a n be a c c o u n t e d f o r b y t h e h y d r o g e n c o n c e n t r a t i o n and l o c a l s o l i d i f i c a t i o n c o n d i t i o n s . The p o r o s i t y a t h i g h h y d r o g e n l e v e l s s o l i d i f i e d i n t h e up d i r e c t i o n i s shown s c h e m a t i c a l l y i n F i g . 4 . 1 c . These p o r e s e a s i l y n u c l e a t e i n t h e s o l i d - l i q u i d i n t e r f a c e . I f enough h y d r o g e n i s p r e s e n t t h e p o r e s grow and i f t h e y grow l a r g e enough, c a n b r e a k away f r o m t h e i n t e r f a c e and f l o a t t o t h e t o p o f t h e l i q u i d m e t a l . S i n c e t h e o b s e r v e d l a r g e p o r e s i n t h e down d i r e c t i o n a r e t h r e e t i m e s t h e s i z e o f t h o s e i n t h e up d i r e c t i o n ( & 5 0 ym) , t h e f l o a t a t i o n o f p o r e s i s a l i k e l y o c c u r r e n c e . As s o l i d i f i c a t i o n p r o g r e s s e s t o t h e f i n a l m e t a l t o s o l i d i f y , t h e number o f p o r e s d e c r e a s e . T h i s i s p o s s i b l y due t o t h e p o r e s w h i c h f l o a t t o t h e t o p o f t h e l i q u i d d e c r e a s i n g t h e h y d r o g e n c o n t e n t i n t h e m e l t and c r e a t i n g a h i g h p a r t i a l p r e s s u r e o f h y d r o g e n above t h e m e l t w h i c h u n d e r t h e e x i s t i n g c o o l i n g c o n d i t i o n s c a n n o t r e a c h e q u i l i b r i u m . From t h e above d i s c u s s i o n t h e f o l l o w i n g c o n c l u s xons a r e 105 r e a c h e d : 1. P o r e s n u c l e a t e e a s i l y . 2 . I n t h e m i d - r e g i o n o f t h e r o d s w h i c h s o l i d i f i e d u n i f o r m l y , t h e p o r e s t r u c t u r e r e m a i n s c o n s t a n t . T h i s i n d i c a t e s t h a t t h e r e i s no i n c r e a s e d c o n c e n t r a t i o n o f h y d r o g e n ahead o f t h e a d v a n c i n g s o l i d - l i q u i d i n t e r f a c e w i t h p r o g r e s s i v e s o l i d i f i c a t i o n ; t h e r e f o r e , s e g r e g a t i o n i s e n t i r e l y i n t e r d e n d r i t i c . 3 . The s i z e , s h a p e , and d i s t r i b u t i o n o f p o r e s i s m a i n l y a f u n c t i o n o f h y d r o g e n c o n t e n t and l o c a l f r e e z i n g c o n d i t i o n s . 4 . 1 . 2 D i r e c t i o n a l S o l i d i f i c a t i o n o f A l + Ag"^"*"^ The e x p e r i m e n t s on d i r e c t i o n a l l y s o l i d i f i e d a l um inum c o n -t a i n i n g a s m a l l amount o f A g ^ ^ ^ d e m o n s t r a t e d t h a t t h e r e was no e f f e c t i v e c o n c e n t r a t i o n o f s o l u t e ahead o f t h e s o l i d - l i q u i d i n t e r -f a c e , and t h e r e f o r e , no s i g n i f i c a n t m a c r o s e g r e g a t i o n a l o n g t h e r o d a s s o c i a t e d w i t h p r o g r e s s i v e s o l i d i f i c a t i o n . T h i s i s t r u e f o r b o t h e x t e n s i v e m i x i n g i n t h e l i q u i d ( f r e e z i n g downward) and no m i x i n g ( f r e e z i n g u p w a r d ) . S i n c e t h e d i s t r i b u t i o n c o e f f i c i e n t f o r Ag i n A l i s a p p r o x i m a t e l y + 0 .2 and t h e s o l u b i l i t y o f h y d r o g e n d e c r e a s e s r a p i d l y w i t h t e m p e r a t u r e ( k Q c an be assumed a p p r o x i m a t e l y 0 ) , i t i s r e a s o n a b l e t h a t h y d r o g e n w o u l d behave i n a manner s i m i l a r t o t h a t o f s i l v e r . The r e a s o n f o r t h e a b s e n c e o f m a c r o s e g r e g a t i o n i s t h e m o r p h o l o g y o f t h e s o l i d - l i q u i d i n t e r f a c e . S o l i d i f i c a t i o n p r o c e e d s by d e n d r i t i c g r o w t h i n t h e s o l i d - l i q u i d r e g i o n p r o d u c i n g m i c r o -s e g r e g a t i o n be tween t h e d e n d r i t e a rms . The mode l s f o r s e g r e g a t i o n assume a p l a n a r i n t e r f a c e , w h i c h i s n e i t h e r a p p l i c a b l e i n t h e p r e s e n t e x p e r i m e n t s n o r i n n e a r l y a l l c a s t i n g and s o l i d i f i c a t i o n s i t u a t i o n s . T h u s , any mechan i sm o f p o r e f o r m a t i o n w h i c h r e q u i r e s t h e c o n c e n t r a t i o n o f a d i s s o l v e d gas t o b u i l d up ahead o f a p l a n e i n t e r f a c e o v e r macro d i s t a n c e s i n o r d e r t o r e a c h c r i t i c a l c o n c e n t r a t -i o n s i s o f l i m i t e d a p p l i c a b i l i t y i n r e a l c a s t i n g s i t u a t i o n s . 4 . 2 The E f f e c t o f C a s t i n g C o n d i t i o n s on P o r o s i t y The p o r o s i t y f o u n d i n A l and A l a l l o y s c a s t u n d e r v a r i o u s c o n d i t i o n s c an be s ummar i zed as f o l l o w s : 1. P o r o s i t y was p r e s e n t i n a l l o f t h e c a s t i n g s examined i n t h i s i n v e s t i g a t i o n . 2 . I n p u r e a l um inum c a s t i n g s , p o r e s a r e s p h e r i c a l , h a v e a ' r i m m e d ' d i s t r i b u t i o n , and a r e n o t r e l a t e d t o t h e g r a i n b o u n d a r y c o n f i g u r a t i o n i n t h e c a s t i n g . 3 . I n t h e A l+Cu a l l o y s t h e p o r e s a r e s p h e r i c a l and a r e s i t u a t e d i n i n t e r d e n d r i t i c r e g i o n s , o f t e n a s s o c i a t e d w i t h s e c o n d phase m a t e r i a l i f p r e s e n t . I n g e n e r a l , t h e p o r e s a r e n o t l o c a t e d a t g r a i n b o u n d a r i e s . 4. Increasing the hydrogen content of the melt results in the formation of fewer but larger pores. 5. Casting into heated moulds, as compared to moulds at room temperature, results in a decrease in the number of pores and an increase i n pore size. 6. The distr ibut ion of pores in unidirect ional ly cast pure A l and Al+Cu i s uniform throughout the casting. 7. Castings with a fine grain structure resulting from the addition of T i l ^ to the melt, contain the same size and dis tr ibut ion of pores as observed i n castings without the hardener added. 8. When individual pores are located at grain boundaries, the boundaries are often 'kinked'. 4.2.1. Porosity and Segregation The presence of pores in both the high purity aluminum and •the Al-Cu alloys for a l l casting conditions suggest that pores nucleate readily in aluminum. The porosity appears to be associated with microsegregation of hydrogen, since the pores are located in the interdendrit ic regions and are associated with second phase material. The uniformity of the distr ibut ion of pores in the direct ional ly cast material indicates that macrosegregation of hydrogen does not occur. Significant macrosegregation would lead to high hydrogen concentrations in the f i n a l part of the casting to so l id i fy and enhanced porosity, which is not observed. The observation that pores were not generally associated with grain boundaries i s consistent with the above statement. A l l the castings so l id i f i ed dendri t ica l ly and a l l would have micro-segregation of hydrogen occurring in the interdendrit ic spaces. These would generally not be associated with grain boundaries and would be on a smaller scale than grain diameters in the casting. 4.2.2 Influence of Hydrogen Content on Porosity The hydrogen content of the melt influences the number and size of pores generated during so l id i f i ca t ion . If a small pore i s present in the l i q u i d , the pore w i l l grow by diffusion of hydrogen from the surrounding l iquid to the pore. If the hydrogen level in the surrounding l iquid i s high the pore correspondingly can grow larger as is observed and possibly coalesce. Since movement of hydrogen in the l iqu id is by a diffusion process, which is time dependent, the ultimate size of the pores w i l l strongly depend on the loca l freezing rate of the casting. Thus, even with high hydrogen concentrations in the melt, small pores w i l l be obtained i f the freezing rate is rapid. In addition,high freezing rates w i l l inh ib i t small pores from coalescing to produce larger pores. 109 The experiments in which castings were made in heated moulds demonstrate the effect of freezing rate on pore size and density for a given hydrogen content. Slower cooling rates associated with heated moulds result in fewer and larger pores similar in effect to increasing the hydrogen content. 4.2.3 The Effect of Grain Size and Second Phase Material Reducing the grain size in the castings by the addition of T i B 2 had no significant effect on the size and distr ibut ion of pores in the casting. This is consistent with the postulate that the pores nucleate in the interdendritic region. The dendrite arm spacing i s primarily a function of the loca l rate of so l id i f i ca t ion of the metal and is not related to the grain size. Since both the pure A l and the A l + T i B 2 were cast under similar thermal conditions the dendrite spacing should be similar and therefore the pore size and dis tr ibut ion should be similar as i s observed. It was observed that pores tend to be associated with second phase material when i t is present. It is a poss ib i l i ty that the second phase i s a nucleating agent for the pores, since the second phase occurs in the interdendritic regions as do the pores. Small volumes of second phase might well be present i n the Cu alloys which should 1 1 0 not have second phase material according to the phase diagram, due to nonequilibrium segregation. However, the present results c lear ly indicate that the second phase material is not a necessary nucleating agent, since pores nucleate readily in pure material and have roughly the same density as in the Al+Cu al loys . 4 . 2 . 4 The Rimmed Distribution of Pores The rimmed distr ibut ion of pores observed in sections of A l castings is believed to form by the following mechanism. Immediately after pouring,a shel l of sol id rapidly forms on the mould wal l . Any small pores formed i n this shel l are trapped in the closely spaced interdendrit ic regions. With further so l id i f i ca t ion , the freezing rate drops due to progressive heating of the mould and the pores become larger. By the time 1 cm of sol id has formed, the freezing rate i s re lat ive ly slow and the pores are large. The pores are s t i l l enveloped in the dendritic skelton. Subsequent to th is , with the lower freezing rates and coarse dendritic structure, the pores which form have time to grow, coalesce, and float to the top of the central l iqu id pool. This would account for the decrease in the number and size of pores in the centre of the casting, producing the rimmed dis tr ibut ion observed. I l l 4 . 2 . 5 U n i f o r m D i s t r i b u t i o n o f P o r e s I n A l + Cu c a s t i n g s , t h e r e a r e few p o r e s a l o n g t h e mou ld w a l l s whe re t h e i n i t i a l s o l i d s h e l l i s f o r m e d ; h o w e v e r , t h e r e s t o f t h e c a s t i n g has a u n i f o r m p o r e d e n s i t y . Few p o r e s a r e f o rmed i n t h e s o l i d s h e l l due t o t h e r a p i d f r e e z i n g r a t e o f l i q u i d m e t a l n e x t t o t he m o u l d . The u n i f o r m d i s t r i b u t i o n o f p o r e s i n t h e c e n t r e o f t h e c a s t i n g i s a r e s u l t o f t h e l o w e r f r e e z i n g r a t e and l a r g e mushy zone f o rmed i n A l + Cu a l l o y s . S e g r e g a t i o n o f Cu i n t h e i n t e r d e n d r i t i c s p a c e s o f t h e s o l i d - l i q u i d i n t e r f a c e fo rms t h e e u t e c t i c pha se w h i c h s o l i d i f i e s a t a much l o w e r t e m p e r a t u r e , t h u s , t h e l i q u i d a l l o y w i l l s o l i d i f y o v e r a w i d e r a n g e o f t e m p e r a t u r e s a l t e r i n g t h e m o r p h o l o g y o f t h e mushy z o n e . I n t h i s e n v i r o n m e n t , p o r e s a r e t r a p p e d i n t h e i n t e r d e n d r i t i c r e g i o n s and c a n n o t c o a l e s c e o r b r e a k away f r o m t h e a d v a n c i n g i n t e r f a c e c a u s i n g a u n i f o r m d i s t r i b u t i o n o f i n t e r d e n d r i t i c p o r e s . 4 . 2 . 6 K i n k i n g a t G r a i n B o u n d a r i e s The k i n k s i n g r a i n b o u n d a r i e s o f t e n o b s e r v e d a t p o r e s s i t u a t e d on b o u n d a r i e s i s b e l i e v e d t o r e s u l t f r o m p o r e s i n h i b i t i n g g r a i n b o u n d a r y m i g r a t i o n . The f o r c e s c a u s i n g t h e g r a i n bounda r y t o m i g r a t e w o u l d c a u s e t h e b o u n d a r y t o bow out on e i t h e r s i d e o f t h e p o r e . M i g r a t i o n c o u l d o n l y c o n t i n u e i f t h e b o u n d a r y bowed a r o u n d t h e p o r e and r e f o r m e d on t h e o t h e r s i d e . The l o c k i n g e f f e c t o f t h e p o r e s on t h e g r a i n b o u n d a r i e s i s c l e a r e v i d e n c e t h a t t h e g r a i n b o u n d a r y c o n f i g u r a t i o n o b s e r v e d i n t h e c a s t i n g s i s e s s e n t i a l l y t h e same as t h a t p r e s e n t a t t h e end o f s o l i d -i f i c a t i o n . A c c o r d i n g l y , t h e p o s s i b i l i t y t h a t p o r e s n u c l e a t e d a t g r a i n b o u n d a r i e s and t h a t t h e g r a i n b o u n d a r i e s s u b s e q u e n t l y move away f r o m t h e p o r e s p r i o r t o o b s e r v a t i o n i s u n l i k e l y . 4 . 3 N u c l e a t i o n o f P o r e s i n S u p e r c o o l e d I r o n 3 . 4 : The f o l l o w i n g i s a summary o f r e s u l t s p r e s e n t e d i n S e c t i on 1 . P o r e s a r e p r e s e n t i n i r o n w h i c h s u p e r c o o l e d e x t e n s i v e l y b e f o r e s o l i d i f i c a t i o n . 2. The number o f p o r e s i n c r e a s e s as t h e s u p e r c o o l i n g i n c r e a s e s . 3 . The p o r e s a r e g e n e r a l l y s p h e r i c a l w i t h smooth s u r f a c e i n t e r i o r s . 4 . I n c r e a s i n g t h e oxygen c o n t e n t r e s u l t s i n a d e c r e a s e i n t h e s u p e r c o o l i n g o f t h e m e l t and an i n c r e a s e i n p o r e d e n s i t y . 5. I n c r e a s i n g t h e c a r b o n c o n t e n t r e s u l t s i n a s l i g h t i n c r e a s e i n p o r e d e n s i t y . These r e s u l t s a r e d i s c u s s e d i n t e rms o f n o n - m e t a l l i c i n c l u s i o n s , c a r b o n and oxygen c o n t e n t , and s u p e r c o o l i n g . 113 A.3.1 Non-Metallic Inclusions The a b i l i t y to strongly supercool l iqu id metal prior to so l id i f i ca t ion is direct evidence that non-metallic inclusions, acting as nucleating s i tes , are not present in the melt. The observation that these melts, after so l id i f icat ion,contain extensive porosity and that the porosity increases with increased supercooling clearly indicates that the nucleating mechanism for pore formation is not heterogeneous nucleation on non-metallic part ic les . This is further supported by the observation that no oxides or other non-metallic Inclusions were found on the in ter ior walls of the pores in materials which had strongly supercooled prior to s o l i d -(2 10 22) i f i c a t i o n . In other investigations ' ' non-metallic inclusions have been observed on the inter ior walls of pores. This would indicate that pores do nucleate on non-metallic inclusions. The present results show that this is not a necessary condition for pore formation. 4.3.2 Effect of Oxygen and Carbon Content Increasing the concentration of oxygen in the melt causes a decrease in supercooling and an increase in porosity. In the case with high oxygen levels , i t is possible that oxides formed during so l id i f i ca t ion may act as nucleating sites for both pores and so l id i f i ca t ion . Ohashi (3 3) et a l measured the c r i t i c a l supercooling of e lec tro lyt ic iron to 114 w h i c h o x i d e p a r t i c l e s we re added . F o r MnO a d d i t i o n s , t h e y o b s e r v e d a c r i t i c a l s u p e r c o o l i n g o f 52 .7°C and f o r S i C ^ a s u p e r c o o l i n g o f 29 .5°C w h i c h a r e s i m i l a r t o t h e p r e s e n t v a l u e s f o r t h e m e l t s t o w h i c h o xygen was added . These o x i d e s added t o t h e m e l t , w h i c h m a r k e d l y r e d u c e t h e amount o f s u p e r c o o l i n g , c l e a r l y a c t as n u c l e a t i n g s i t e s f o r s o l i d i f i c a t i o n . The p r e s e n t o b s e r v a t i o n t h a t i n c r e a s i n g o xygen l e v e l s i n t h e m e l t w h i c h l e a d t o t h e f o r m a t i o n o f o x i d e s , i n c r e a s e s t h e p o r o s i t y i n d i c a t e s t h a t t h e o x i d e s c a n a c t as h e t e r o g e n e o u s n u c l e a t i n g s i t e s f o r p o r e s . I n c r e a s i n g t h e c a r b o n c o n t e n t i n t h e p r e s e n t e x p e r i m e n t s p r o d u c e d a s l i g h t i n c r e a s e i n p o r o s i t y i n t h e i r o n . The r e l a t i o n s h i p be tween (9) p o r o s i t y and c a r b o n c o n t e n t has been i n v e s t i g a t e d by Ohkubo e t a l g i v i n g t h e r e s u l t s shown i n F i g . 4 . 2 . An i n c r e a s e i n p o r o s i t y w i t h c a r b o n c o n t e n t i n t h e p r e s e n t r e s u l t s f a l l w i t h i n t h e e x p e r i m e n t a l e r r o r s c a t t e r band o f F i g . 4 . 2 . 4 . 3 . 3 S u p e r c o o l i n g (13) A c c o r d i n g t o F r e d r i k s s o n , m i c r o p o r e s c an be n u c l e a t e d homogeneous l y i n t h e i n t e r d e n d r i t i c r e g i o n s due t o t h e p r e s s u r e d r o p c a u s e d by s o l i d i f i c a t i o n s h r i n k a g e . The p r o b a b i l i t y t h a t homogeneous n u c l e a t i o n w i l l o c c u r i s i n c r e a s e d w i t h i n c r e a s e d s o l i d i f i c a t i o n r a t e and gas c o n t e n t . 115 ' 20.0, C (%) Figure 4.2 Relation between number of pores and carbon concentrations from Ohkubo et al^ 9 ) ii = number of blowholes per unit cross-section area. 1 1 6 I n t h e p r e s e n t r e s u l t s i t i s o b s e r v e d t h a t p o r o s i t y i n c r e a s e s w i t h i n c r e a s e d s u p e r c o o l i n g b e f o r e s o l i d i f i c a t i o n . T h i s i n d i c a t e s t h e s p h e r i c a l p o r e s , m o s t l y 20 um o r l e s s i n d i a m e t e r , c o u l d b e homogeneous l y n u c l e a t e d . I t i s b e l i e v e d t h a t t h e s e p o r e s a r e i n t e r d e n d r i t i c . The a c t u a l s i z e o f t h e i n t e r d e n d r i t i c r e g i o n i s n o t known s i n c e t h e d e n d r i t e s c o u l d n o t d e l i n e a t e d by e t c h i n g . The maximum s i z e o f p o r e s o b s e r v e d i s a p p r o x i m a t e l y 40 ym i n d i a m e t e r . T h e s e f ew l a r g e p o r e s a r e c l e a r l y i n t e r d e n d r i t i c due t o t h e i r e l o n g a t e d s h a p e . The m i c r o p o r e s f o rmed a r e n o t due t o s o l i d i f i c a t i o n s h r i n k a g e b e c a u s e i n e a c h o f t h e s u p e r c o o l e d m e l t s a l a r g e s h r i n k a g e c a v i t y (22 10% a r e a p o r o s i t y ) was f o r m e d . I n s u p e r c o o l e d m e l t s , p a r t o f t h e l i q u i d s o l i d i f i e s r a p i d l y u n t i l t h e t e m p e r a t u r e o f t h e m e l t r e a c h e s t h e l i q u i d u s . F o l l o w i n g t h i s , s o l i d i f i c a t i o n i s much s l o w e r and dependen t on t h e r a t e o f h e a t e x t r a c t i o n f r o m t h e s y s t e m . The p o r o s i t y i s a s s o c i a t e d w i t h t h e r e m a i n i n g l i q u i d w h i c h s o l i d i f i e s f r o m t h e l i q u i d u s t e m p e r a t u r e . 5. CONCLUSIONS 1. Homogeneous n u c l e a t i o n o f p o r e s i n c a s t i n g s i s p o s s i b l e i n t h e i n t e r d e n d r i t i c r e g i o n s . 2 . H e t e r o g e n e o u s n u c l e a t i o n and n o n - n u c l e a t i n g mechan i sms o f p o r e f o r m a t i o n a r e s u f f i c i e n t b u t n o t n e c e s s a r y . 3. P o r e s i z e , shape and d i s t r i b u t i o n a r e dependent on t h e l o c a l s o l i d i f i c a t i o n r a t e s and t h e gas c o n t e n t i n t h e l i q u i d m e t a l . 4 . S e g r e g a t i o n o f h y d r o g e n i n a l um inum i s i n t e r d e n d r i t i c . T h e r e i s no m a c r o s e g r e g a t i o n w i t h d e n d r i t i c g r o w t h . 5. I n A l + Cu , t h e d e n s i t y o f p o r e s r e m a i n s c o n s t a n t w i t h Cu a d d i t i o n s up t o 3%. 117 6. SUGGESTIONS FOR FUTURE WORK 1. Repea t s u p e r c o o l i n g e x p e r i m e n t s w i t h a n a l l o y s u c h as Fe+25Ni t o show t h a t t h e p o r e s f o rmed a r e i n t e r d e n d r i t i c . U se h y d r o g e n and n i t r o g e n gas a d d i t i o n s a s w e l l a s CO. 2 . S o l i d i f y A l w i t h added h y d r o g e n a t s e v e r a l s o l i d i f i c a t i o n r a t e s and d e t e r m i n e i f F r e d r i k s s o n ' s m o d e l f o r p o r e g r o w t h a p p l i e s . 3. A c c u r a t e l y c h a r a c t e r i z e s e g r e g a t i o n o f Ag^" "^ i n A l ( w i t h o u t S i i n c l u s i o n s ) . 4 . D e v e l o p a mode l o f p o r e f o r m a t i o n n o t ba sed upon m a c r o s e g r e g a t i o n . 118 119 REFERENCES - BIBLIOGRAPHY 1. P i w o n k a , T . S . and F l e m i n g s , M . C . : T r a n s A IME, 236 Aug . 1966 p p . 1157. 2. Uda , M. and Ohno, S.: T r a n s . N a t i o n a l R e s e a r c h I n s t i t u t e o f M e t a l s , 16 #2, 1974, p p . 6 7 . 3. G u l y a e v , B . B . : E d i t o r , Gases i n C a s t M e t a l s , C o n s u l t a n t s B u r e a u , New Y o r k , 1965 , p p . 5 5 . 4 . G u l y a e v , B .B . : i b i d , p p . 6 4 . 5. G u l y a e v , B . B . : i b i d , p p . 1 . 6. N i l l e s , P . : I r o n & S t e e l m a k i n g I n s t i t u t e J o u r n a l I S I J (London) 202 ( 1 9 6 7 ) , p p . 6 0 1 . 7. S c i m a r , R. and N i l l e s , P . : CNRM M e t a l l u r g i c a l R e p o r t s No . 11 (1967) , p p . 1 7 . 8. M a s u i , A . , S a t o , H. , Ohkubo, M. and M i y o s h i , S . : T r a n s I S I J 8_ (1968) , p p . 1 9 5 . 9. Ohkubo, M . , M a s u i , A . , S a t o , H . , W a k a b a y a s h i , S. and M i y o s h i , S . : T r a n s I S I J 10 ( 1 9 7 0 ) , p p . 2 8 4 . 10 . B e e c h , J . : The M e t a l l u r g i s t & M a t e r i a l s T e c h n o l o g i s t , A p r . 1974 , V o l . 6, #3, p p . 1 2 9 . 1 1 . B u r n s , D. and B e e c h , J . : I r o n m a k i n g & S t e e l m a k i n g Q u a r t e r l y , 1974, //4, p p . 2 3 9 . 12 . M o r i , K., K a m i m o r i , A . , D e g u c h i , M. and Sh imoda , T . , T e t s u - t o - H a g n e 59 (1973) p p . 8 8 7 . 13 . F r e d r i k s s o n H. and S v e n s s o n , I .S.: M e t . T r a n s . 7B_, Dec . 1976 , p p . 5 9 9 . 14. H i r t h , J . P . , P o u n d , G.M. and S t . P i e r r e , G.R. , M e t . T r a n s . 1» A p r . 1 970 , p p . 9 3 9 . 1 5 . F i s h e r , J . C . , H o l l o m a n , J . H . , and T u r n b u l l , D.: J . A p p l i e d P h y s i c s , 1948, 1 9 , p p . 7 7 5 . 16. T u r n b u l l D. and F i s h e r , J . C . : J . Chem. Phys . 17 (1949) ,pp .71 . 17. M o r i , K . , H i r a i w a , T . , and Nomura, H . : Trans 1S1J , 16 (1976) pp .36 . 18. M o r i , K . , Kamimor i , A . , Deguch i , M. and Shimoda, T . : T e t s u - t o -Hagne, 59 (1973), pp .874. 19. P iwonka , T . S . and F l emings , M . C . : Trans AIME, 236, Aug. 1966, pp. 1157. 20. C o b l e , R . L . and F lemings , M . C : Met . Trans 2 (1971) pp .409 . 21 . Turkdogan, E . T . : Trans AIME, 233, Dec. 1965, pp .2100. 22. Uda, M . , Dan, T. and Ohno, S . : Trans 1S1J , 1_6 (1976), pp .664. 23. Burns , D. and Beech, J . : I n t e r n a t i o n a l Symposium on M e t a l l u r g i c a l Chemis t ry , U . o f S h e f f i e l d , 1971, pp .229 . 24. Campbe l l , J . : Conference on S o l i d i f i c a t i o n of M e t a l s , B r i g h t o n , L S I , P110, 1967, p p . 1 8 . 25. K a p l a n , R . S . and P h i l b r o o k , W.O. : Met Trans _3» Feb. 1972, pp .483 . 26. Harkness , B . , N i c h o l s o n , A . and Murray , J . D . : I S I J o u r n a l (London) Sept . 1971 pp .692 . 27. Campbe l l , J . : Trans AIME 239, Feb. 1967, pp .138. 28. Campbe l l , J . : Trans AIME 245, Oc t . 1969, pp .2325. 29. Campbe l l , J . : Trans AIME 242, Feb. 1968, pp .264 . 30. Campbe l l , J . : i b i d , pp .268 . 31 . F i s h e r , J . : J . A p p l i e d P h y s i c s , 19, Nov. 1948, pp .1062. K a t a m i s , T . F l e m i n g s , M . C . : Trans AIME, 236 (1966) , pp .1523. 32 33 Ohashi, T . , Hiromoto, T . , F u j i i , H . , N u r i , Y. and Asano, K. Tetso-to-Hagne, 62(1972) pp .614. 

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