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Effect of temperature and interstitial composition on the yielding of niobium single crystals Harding, Hugh James 1961

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EFFECT OF TEMPERATURE AND INTERSTITIAL COMPOSITION ON THE YIELDING OF NIOBIUM SINGLE CRYSTALS HUGH JAMES HARDING A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE IN THE DEPARTMENT OF MINING AND METALLURGY We accept t h i s t h e s i s as conforming to the standard required from candidates f o r the degree of MASTER OF APPLIED SCIENCE Members of the Department of Mining and Metallurgy THE UNIVERSITY OF BRITISH COLUMBIA September 1961 In presenting t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree that permission f o r extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . I t i s understood that copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be allowed without my w r i t t e n permission. Department of Mining and Metallurgy, The U n i v e r s i t y of B r i t i s h Columbia, Vancouver 8, Canada. Date August 28th, 1961 ABSTRACT An i n v e s t i g a t i o n of the e f f e c t of temperature and i n t e r s t i t i a l composition on the y i e l d i n g c h a r a c t e r i s t i c s of high p u r i t y niobium sin g l e c r y s t a l s has been c a r r i e d out. Single c r y s t a l s of [liojj a x i a l o r i e n t a t i o n were p u r i f i e d and grown using an e l e c t r o n beam, floating-zone,zone-refiner. T e n s i l e specimens prepared from the single c r y s t a l s were tested i n tension at temperatures from +25° to -197°C. at a s t r a i n rate of 0.057 per minute. The v a r i a t i o n of y i e l d stress with t e s t i n g temperature was found to be discontinuous and consisted of two curves which in t e r s e c t e d at approx-imately -100°C. The t e n s i l e r e s u l t s can be c o r r e l a t e d to both Fisher's i n t e r p r e t a t i o n of the C o t t r e l l - B i l b y theory of y i e l d i n g and to the theory of Zener and Holloman, which involves a thermally activ a t e d process. The e f f e c t of additions of hydrogen, oxygen and nitrogen to niobium single c r y s t a l s over the temperature range +25° to -197°C was to increase the y i e l d s t r e s s . The unusual temperature dependence of y i e l d stress has been explained i n two possible ways: 1. Ordering or s u p e r l a t t i c e formation at the temperature at which the anomalous behaviour occurs. 2. The p r e f e r e n t i a l release of screw d i s l o c a t i o n s at: stress l e v e l s lower than that .required f o r edge d i s l o c a t i o n s . The r e s u l t i s a decrease i n the temperature dependence of y i e l d stress at the temperature when edge d i s l o c a t i o n s begin to take part i n y i e l d i n g . ACKNOWLEDGEMENT The author wishes to thank Dr. J . A. Lund and Professor W. M. Armstrong f o r t h e i r supervision and encouragement, and Mr. R. G. Butters f o r h i s t e c h n i c a l assistance during t h i s i n v e s t i g a t i o n . The author i s indebted to Dr. E. Teghtsoonian and fellow graduate students f o r many h e l p f u l discussions. The work was financed by the Defence Research Board, under grant No. 7510-36 and by the Steel Company of Canada Limited. TABLE OF CONTENTS Page I. INTRODUCTION . . . . . . . . . . . . . . . . . . . 1 I I . REVIEW OF PREVIOUS WORK 2 A. T h e o r e t i c a l Considerations . . . . . . . . . . . . . . 2 B. Previous Work on Vanadium . 3 C. Previous Work on Tantalum . . . . . . . . . . . . . . . 6 D. Previous Work on Niobium . . . . . . . . . . . . . . . 7 I I I . EXPERIMENTAL 8 A. Materials 8 1. High P u r i t y Niobium Rod . . . . . 8 2. Oxygen, Hydrogen and Nitrogen Gases . . . . . . 8 B. P u r i f i c a t i o n • • • • • 9 1. Zone-Refining . . . . . . . . . . . . . . . . . 9 2. I n t e r s t i t i a l Content V a r i a t i o n with Zone-Refining 11 C. Single C r y s t a l Growth and Orientation . . 13 D. I n t e r s t i t i a l Additions to Single Crystals . . . . . . lk 1. Doping Furnace lk 2. Gas P u r i f i c a t i o n . . . . . . . . . . . . . . . 15 3- Doping Treatments 15 k. Hardness Measurements . . . . . . . . . . . . . 16 E. T e n s i l e Specimen Preparation 17 1. Machining . . . . . . . . . . . . . . 17 2. Surface Preparation . . . . . . . . . 17 3. X-ray . 18 F. Tensile T e s t i n g 18 1. T e n s i l e Apparatus 18 2. Temperature Measurement and Control 18 3. Y i e l d Stress Measurements . . . . 22 i v TABLE OF CONTENTS CONTINUED Page IV. EXPERIMENTAL OBSERVATIONS AND RESULTS 23 A. High Pu r i t y Single Crystals 23 1. Y i e l d Stress versus Temperature . 23 2. Elongation versus Temperature . . . 27 3- Reduction i n Area 27 k. Mechanisms of Deformation 27 B. Single Crystals Doped with Hydrogen . . . 31 1. E f f e c t of Hydrogen on Y i e l d Stress 31 2. E f f e c t of Hydrogen on D u c t i l i t y 3k 3. E f f e c t of Hydrogen on the Deformation Mechanism . 37 C. Single Crystals Doped with Oxygen kl 1. E f f e c t of Oxygen on Y i e l d Stress kl 2. E f f e c t of Oxygen on D u c t i l i t y kk 3. E f f e c t of Oxygen on the Deformation Mechanism . . kj D. Single C r y s t a l Doped with Nitrogen 1+8 1. E f f e c t of Nitrogen on Y i e l d Stress kQ 2. E f f e c t of Nitrogen on D u c t i l i t y 52 3- E f f e c t of Nitrogen on the Deformation Mechanism . 52 E. Dependence Of Y i e l d Stress on the Rate of Change of Bath Temperature ' . . . . 5^ V. DISCUSSION 57 A. Temperature Dependence of Y i e l d Stress . 57 B. E f f e c t of I n t e r s t i t i a l Content on the Temperature Dependence of Y i e l d Stress 62 C. E f f e c t of I n t e r s t i t i a l Content on D u c t i l i t y 6k D. Deformation Mechanisms 65 E. Load-Elongation Curves . . . . . . . . . . 66 V TABLE OF CONTENTS CONTINUED Page F. Explanation of Anomalous Behaviour of the Temperature Dependence of Y i e l d Stress . . . . . . . . . . . . oo 1. I n t e r s t i t i a l Ordering 66 2. Dislocation-Solute Interaction . . . 69 VI. CONCLUSIONS . 73 VII. RECOMMENDATIONS FOR FUTURE WORK 75 VIII. BIBLIOGRAPHY . 76 IX. APPENDICES • 78 vi FIGURES No. Page 1 . Log P l o t of Y i e l d Stress versus Reciprocal Temperature f o r Single and P o l y c r y s t a l s of Vanadium . . . . . . 5 2 . The El e c t r o n Bombardment Furnace of the Zone-Refiner . . 9 3 . I n t e r s t i t i a l Removal as a Function of Number of Zone Refining Passes . . . . . . . 1 2 k. T y p i c a l Single Crystals Produced During Zone-Refining . 1 3 5 . A T y p i c a l X-ray Photograph of a Single C r y s t a l . . . . . . 1 ^ 6 . Split-Jaw Gripping of Tensile Specimen . . . . . . . . . 1 9 7. Specimen and Universal Heads i n P o s i t i o n f o r Tens i l e 8 . Relationship Between Internal and External Specimen Temperature as a Function of Time. Ambient Temperature Change, + 3 8 C 2 1 9 . Y i e l d Stress versus Temperature f o r High Pu r i t y Single C r y s t a l s 2.k 1 0 . Y i e l d Stress versus the Reciprocal Temperature f o r High P u r i t y Single Crystals 2 5 1 1 . Log Y i e l d Stress versus Reciprocal Temperature f o r High Pu r i t y Single Crystals . 2 6 1 2 . Load Elongation Curves f o r High Pu r i t y Single C r y s t a l s , Temperature i n Degrees Centigrade . . 2 8 1 3 - Percent Elongation versus Temperature f o r High Pu r i t y Single C r y s t a l s 2 9 lk-. a). Double Necking i n High Pu r i t y Niobium Single C r y s t a l s " b) Uniform Elongation at 2 5 ° C c) Localized Necking at - 1 9 7 ° C 3 0 1 5 . Load-Elongation Curves f o r Single Crystals Doped with Hydrogen . . . . . . . . . 3 2 1 6 . Y i e l d Stress versus Temperature f o r Single Crystals Doped with Hydrogen . . . . . . . 3 3 1 7 . Log Y i e l d Stress versus Reciprocal Temperature f o r Single Crystals Doped with Hydrogen . . . 3 5 1 8 . Percent Elongation versus Temperature f o r Single Crystals Doped with Hydrogen . . . . 3 6 v i i FIGURES CONTINUED No. Page 19• Percent Reduction i n Area versus Temperature f o r * Single Crystals Doped with Hydrogen . . . . . . . . 38 20. Temperature versus D u c t i l i t y of Vanadium at D i f f e r e n t , Hydrogen Concentration. A f t e r Eustice and Carlson . 39 21. Specimen Doped with 300 ppm Hydrogen -a) Reduction i n Area at 25°0. b) Reduction i n Area and Fracture at -25°C. c) Cleavage F a i l u r e at -197°C. . kO 22. Twin Lines Observed on Specimen Surface (-197°C.) . . . . kO 23. Load-Elongation Curves f o r Single Crystals Doped with Hydrogen . . . . . . . k2 2k. Y i e l d Stress versus Temperature f o r Single C r y s t a l s Doped with Oxygen . . . . . . . . . . . . . . . . . 3^ 25. Log Y i e l d Stress versus Reciprocal Temperatures f o r Single Crystals Doped with Oxygen k$ 26. Percent Reduction i n Area versus Temperature f o r Single Crystals Doped with Oxygen . . . . 46 27. a) P l a s t i c Deformation and, b) Cleavage of Single C r y s t a l s Doped with 2000 ppm Oxygen kk 28. Load-Elongation Curves f o r Single Crystals Doped with Nitrogen . . . . . . . . k9 29. Y i e l d Stress versus Temperature f o r Single Crystals Doped with Nitrogen 50 30. Log Y i e l d Stress versus Reciprocal Temperature f o r Single Crystals Doped with Nitrogen . . . 51 31. Percent Elongation versus Temperature f o r Single C r y s t a l s Doped with Nitrogen . . . . . . . . . . . . 53 32. Y i e l d Strength as a Function of Elapsed Time to Observance of the Y i e l d Point During a Rapid Change i n Testing Temperatures f o r High Purity Single Crystals . . . 55 33. Y i e l d Strength as a Function of Warming Rate f o r High Pur i t y Single C r y s t a l s 56 34. Log Y i e l d Stress versus Reciprocal Temperature f o r Niobium, Vanadium and Tantalum . 60 35* Hypothetical Curve of Stress versus Reciprocal Absolute Temperature . . . . . . . . . . . . . . . 67 36. Temperature V a r i a t i o n of the Temperature-Dependent Part of the L a t t i c e F r i c t i o n Stress. A f t e r Heslop and Petch 2 ^ . 67 v i i i TABLES No. Page I. Analysis of As-Received Niobium . . . . . . . . . . . . 8 I I . I n t e r s t i t i a l Content of Zone-Refined Niobium . . . . . 11 I I I . Analysis of Metal Used by Investigators shown i n Figure 3 4 . . . . . . . . . . . . . . . . . . . . . 6 l A l . Appendix I. The Results of Tensile Tests 79 A. High Pu r i t y Single Crystals 79 B. Single Crystals Doped with Hydrogen . . . . . . 80 C. Single Crystals Doped with Oxygen . . . . . . . 8 l D. Single Crystals Doped with Nitrogen . . . . . . 82 E. Single Crystals Subjected to A Rapid Change of Testing Temperature . . . . . . . . . . . . . 83 •A2. Appendix I I . A. Example Calculations . . . . . . . . . . . . . . 85 B. C a l c u l a t i o n of Probable Error . 86 A3. Appendix I I I . Doping Treatments 88 EFFECT OF TEMPERATURE AND INTERSTITIAL COMPOSITION ON THE YIELDING OF NIOBIUM SINGLE CRYSTALS I. INTRODUCTION The desirable mechanical properties of the body-centred cubic r e f r a c t o r y metals have recently a t t r a c t e d the i n t e r e s t of many investigators» S p e c i f i c a t t e n t i o n has been d i r e c t e d towards the properties of these metals at low temperatures where they e x h i b i t r a p i d l y increasing y i e l d strengths as absolute zero i s approached. The r e l a t i o n s h i p between upper y i e l d stress and temperature has been t h e o r e t i c a l l y explained i n several d i f f e r e n t ways, any one of which does not f i t a l l experimental r e s u l t s . I t has been suggested that there are s p e c i f i c temperature ranges within which each of the several proposed theories are s e l f - c o n s i s t e n t . The increase i n upper y i e l d stress i s accompanied by a d i s t i n c t decrease i n d u c t i l i t y . The degree of reduction i n d u c t i l i t y appears to depend on the metal i n question. The temperature at which the reduction occurs i s dependent on composition, p a r t i c u l a r l y the content of i n t e r s t i t i a l solute elements, grain s i z e and s t r a i n r a t e . In a few cases a p a r t i a l recovery of d u c t i l i t y at extremely low temperatures has been detected. Cleavage f r a c t u r e of the r e f r a c t o r y metals occurs i n conjunction with the d u c t i l i t y t r a n s i t i o n when the y i e l d strength exceeds the cleavage fr a c t u r e strength. Several i n v e s t i g a t i o n s i n d i c a t e that measurable amounts of p l a s t i c s t r a i n precede cleavage f r a c t u r e . The purpose of t h i s i n v e s t i g a t i o n was to determine the r e l a t i o n s h i p between y i e l d strength and temperature f o r pure niobium sin g l e c r y s t a l s over the temperature range of +25° to -197°C, and to cor r e l a t e the r e s u l t s with previous work. The e f f e c t of i n t e r s t i t i a l composition on the y i e l d stress-temperature r e l a t i o n s h i p and associated d u c t i l i t y t r a n s i t i o n was also to be investigated. _ 2 -I I . REVIEW OF PREVIOUS WORK A. Th e o r e t i c a l Considerations An explanation of the marked dependence of y i e l d stress on temperature of the body-centred cubic t r a n s i t i o n metals has been developed by C o t t r e l l and B i l b y '• The C o t t r e l l - B i l b y theory suggests that d i s -l o c a t i o n s are anchored by a surrounding atmosphere of solute atoms and that an applied stress, with the a i d of thermal f l u c t u a t i o n s , supplies the energy required f o r the d i s l o c a t i o n loops to break away from t h e i r surrounding atmospheres. I t i s the d i s l o c a t i o n break-away that i s associated with the y i e l d point. As the temperature i s reduced, the degree of thermal f l u c t u a t i o n i s also reduced, and a higher stress l e v e l i s required-to free the d i s l o c a t i o n loops. F i s h e r ' s ^ i n t e r p r e t a t i o n of the C o t t r e l l - B i l b y theory of the y i e l d point requires that the product of the stress and absolute temperature be constant. Hence, i f t h i s theory i s v a l i d , a p l o t of y i e l d stress versus the r e c i p r o c a l of the absolute temperature must y i e l d a s t r a i g h t l i n e . The theory of abrupt or catastrophic y i e l d i n g developed by k 6 C o t t r e l l and l a t e r modified by Wessel states that d i s l o c a t i o n s p i l e d up at grain boundaries or other b a r r i e r s cause an increase i n stress concent-r a t i o n i n the region of the pile-up. The abrupt y i e l d becomes associated with the r e l a x a t i o n of the l o c a l i z e d stresses due to the a c t i v a t i o n of new d i s l o c a t i o n sources. The r e l a x a t i o n occurs through the break-away of the p i l e d up d i s l o c a t i o n groups. Once t h i s break-away has occurred i n a few l o c a l i z e d regions, the r e l a x a t i o n i n the v i c i n i t y of the break-away causes higher microstresses i n neighbouring p i l e d up groups, i n i t i a t i n g a general cataclysmic y i e l d i n g and an associated drop i n load. The modification that Wessel applied to C o t t r e l l ' s 4 theory was that the degree of pre-- 3 -y i e l d p l a s t i c s t r a i n was several orders of magnitude greater than that which C o t t r e l l ^ suggested. Wessel^ has, i n a reasonable manner, calculated the required movement of d i s l o c a t i o n s to b a r r i e r s necessary to agree with experimental i n d i c a t i o n s of p r e y i e l d p l a s t i c s t r a i n . B r i t t l e f r a c t u r e that accompanies the d u c t i l i t y t r a n s i t i o n has 6 7 been described by Wessel and C o t t r e l l . At lower temperatures, the l o c a l i z e d stresses generated by p i l e d up d i s l o c a t i o n s exceed the cohesive strength of the metal, r e s u l t i n g i n microcracking. P r i o r to and simulta-neously with the formation of these microcracks, many of the p i l e d up d i s l o c a t i o n groups w i l l i n i t i a t e p l a s t i c flow i n more favourably oriented ma t e r i a l . The i n i t i a t i o n of one or more microcracks together with some p l a s t i c flow i s b e l i e v e d to cause the abrupt y i e l d i n g . Eventually a stable crack i s formed and b r i t t l e f r a c t u r e occurs. Q Zener and HollomSn have developed a theory i n contrast to the 5 one proposed by Fisher 1.• Their theory states that the y i e l d stress i s proportional to the exponential of the r e c i p r o c a l temperature. They have -assumed that the isothermal s t r e s s - s t r a i n r e l a t i o n i s a f f e c t e d by a rate process which has a heat of a c t i v a t i o n . Data which are i n agreement with t h i s theory must produce a s t r a i g h t l i n e r e l a t i o n s h i p when l o g y i e l d stress i s p l o t t e d versus the r e c i p r o c a l absolute temperature. B. Previous work on Vanadium 1 2 In reviews of previous work on vanadium, Snowball and Fraser have summarized e a r l i e r experimental r e s u l t s i n the following manner: At -196°C, i n v e s t i g a t o r s reported a f i v e - f o l d increase i n y i e l d strength over that at room temperature. In some cases, at temperatures near -196°C, the y i e l d strength trend reversed and began to decrease as the - k -temperature continued to decrease. I t i s possible that t h i s type of anomalous behaviour was due to a form of embrittlement or to t r i a x i a l stresses introduced through non-axial loading. Other experimental evidence ind i c a t e d that hydraulic t e n s i l e machines tend to suppress y i e l d point phenomena, and that the e f f e c t i s best observed on "hard" screw-driven t e n s i l e machines. I t has also been proven that misalignment or non-axial loading of the specimen w i l l also tend to suppress the y i e l d point. A d u c t i l e - t o - b r i t t l e t r a n s i t i o n has been found to occur over a wide range of temperatures. Factors that a f f e c t -the t r a n s i t i o n temperature are s t r a i n rate and composition. In a few cases where the vanadium was doped with hydrogen, a d u c t i l i t y recovery at very low temperatures was detected. D u c t i l i t y was evaluated by measuring reduction i n area as pronounced l o c a l i z e d necking occurred at low temperatures. The e f f e c t of increasing the hydrogen content was to r a i s e the temperature at which the minimum occurred. Cleavage f r a c t u r e was found to occur'below the d u c t i l i t y t r a n s i t i o n temperature on the ^100^ and ^-loj planes. Mechanical twinning was found to occur on |jL12j- planes. 1 In a recent i n v e s t i g a t i o n , Snowball , who worked with zone-s r e f i n e d single c r y a t a l s of vanadium, found that his y i e l d point data d i d not f i t a s t r a i g h t l i n e when the y i e l d stress was p l o t t e d versus the r e c i p r o c a l of the absolute temperature, as suggested by Fisher''. However hi s data d i d f i t two s t r a i g h t l i n e s i f the logarithm of the y i e l d stress was p l o t t e d versus the r e c i p r o c a l of the absolute temperature (Figure 1.), as suggested by Zener and Hollomon . Zener and Hollomon's r e l a t i o n , however, should f i t one s t r a i g h t l i n e over the complete range of temperature. In explanation of t h i s anomalous behaviour, Snowball"^ stated' that the vanadium used i n h i s i n v e s t i g a t i o n was purer by a f a c t o r of ten than any reported by e a r l i e r i n v e s t i g a t o r s and postulated that d i f f e r e n t i n t e r s t i t i a l elements were responsible f o r the y i e l d i n g phenomena over the two d i s t i n c t temperature ranges. These r e s u l t s w i l l be more f u l l y discussed l a t e r i n t h i s t h e s i s . Results of work on p o l y c r y s t a l l i n e vanadium s t r i p reported 2 by Fraser y i e l d a s t r a i g h t l i n e on a p l o t of y i e l d stress versus the r e c i p r o c a l of the absolute temperature. However, i f the above r e s u l t s are p l o t t e d i n comparison with those of Snowball (Figure 1.), there i s an i n d i c a t i o n that the anomolous e f f e c t may e x i s t i n the p o l y c r y s t a l l i n e material. Figure 1. Log P l o t of Y i e l d Stress versus Reciprocal Temperature f o r Single and P o l y c r y s t a l s of Vanadium C. Previous Work on Tantalum g In a .summary of recent l i t e r a t u r e , Lawson reports that tantalum also e x h i b i t s increasing y i e l d stress with decreasing temperature. The d u c t i l i t y , however, does not undergo a sharp t r a n s i t i o n s i m i l a r to vanadium, but decreases gradually, r e s u l t i n g i n a l e s s d u c t i l e f a i l u r e at low temperatures. Anomalouspeaks i n the y i e l d stress-temperature r e l a t i o n s h i p at approximately 100°C. have been ascribed to s t r a i n aging. Cleavage induced by impact was found to take place c h i e f l y on -Q-loJ" planes and secondarily on {lOo} P^ 3 1 1 6 1 3* Twinning took place on ( l - " -^ planes. Lawson found that the add i t i o n of i n t e r s t i t i a l elements to tantalum tended to.1 increase y i e l d strength values, decrease the room temperature d u c t i l i t y , and introduce a s l i g h t l y more pronounced d u c t i l e to l e s s - d u c t i l e t r a n s i t i o n . D. Previous Work on Niobium Several recent i n v e s t i g a t i o n s of the t e n s i l e properties of niobium tested below room temperature have indi c a t e d a strong temperature dependence of y i e l d s t r e s s . A d u c t i l i t y t r a n s i t i o n has also been observed. Mincher and Sheely"^ have investigated the mechanical properties of niobium over the temperature range -196°C. to 1093°C» The r a p i d increase i n y i e l d strength below 0°C has been interpreted i n terms of an increasing importance i n Peirels-Nabarro forces and a decreasing importance i n i n t e r s t i t i a l atoms i n the locking of d i s l o c a t i o n s . Unfortunately t h i s data was not s u f f i c i e n t l y d e t a i l e d to determine whether an abrupt decrease i n d u c t i l i t y occurred. In a more d e t a i l e d i n v e s t i g a t i o n "between -I96 and +2kO°C., 1 1 Dyson, Jones and Tegart concluded that high p u r i t y niobium i s d u c t i l e ,0 down to -I96 C. The y i e l d stress v a r i a t i o n with the decreasing temperature was t y p i c a l of other i n v e s t i g a t i o n s . grain s i z e on the p h y s i c a l properties' of niobium. A d u c t i l i t y t r a n s i t i o n and twinning were detected. Twinning, which occurred r e l u c t a n t l y , d i d not appear to be e s s e n t i a l to cleavage. The cleavage plane was determined as J100J . Wessel and Lawthers l®have investigated the d u c t i l e - t o - b r i t t l e t r a n s i t i o n of niobium containing over 4 0 0 parts per m i l l i o n t o t a l i n t e r -s t i t i a l content. The t r a n s i t i o n temperature was determined to be between o o - 1 0 0 and - 2 0 0 C. The y i e l d strength dependence of temperature was c h a r a c t e r i s t i c of body-centsred^ cubic metals. The cleavage plane was reported as J 1 0 0 ] . 1 2 Adams, Roberts and Smallman have determined the e f f e c t of - 8 -I I I . EXPERIMENTAL A. Materials 1. High P u r i t y Niobium Rod The niobium rod used f o r t h i s i n v e s t i g a t i o n was obtained from the Wah Chang Corporation i n the form of 0.200 inch diameter annealed, centerless-ground rod. The niobium was supplied from two production l o t s , both having the same analysis as shown i n Table I. TABLE I. Analysis of As-Received Niobium Element Parts Per M i l l i o n C 1+0 0 172 N 60 H 3. ,6 T o t a l 275. ,6 Metallographic examination of the as-received metal d i d not reveal the presence of a second phase. The diamond pyramid hardness of the as-received rod was 115. 2. Oxygen, Hydrogen and Nitrogen Gases Oxygen, hydrogen and nitrogen gases were used f o r i n t e r s t i t i a l a d dition i n t h i s work. In each case the gas used was that which was commercially a v a i l a b l e . Suitable p u r i f y i n g treatments, which w i l l be discussed l a t e r , were incorporated i n t o the experimental process. B. P u r i f i c a t i o n P u r i f i c a t i o n of the niobium rod was c a r r i e d out i n the v e r t i c a l floating-zone electron bombardment zone-refiner described by Snowball 1. The electron bombardment furnace showing the c e n t r a l niobium rod, the c i r c u l a r filament and beam forming plates i s i l l u s t r a t e d i n Figure 2. Figure 2. The Electron Bombardment Furnace of the Zone-Refiner 1. Zone-Refining The as-received rod was prepared f o r zone-refining by d r i l l i n g and tapping an l / 8 inch diameter by l/k inch hole i n the center of one end of the rod. A 1.5 inch long niobium support rod was then screwed i n t o the threaded hole. The other end of the rod was faced o f f squarely i n a lathe. - io -I t was found that i f a short length of bar s i m i l a r to the one described above was used as the top part of the rod when the rod was i n place i n the zone-refiner, the j o i n t that Snowball made at the center of two equal length sections could be eliminated without.introducing l a t e r a l s tresses. The j o i n t was simply made between the 9 inch rod and the one inch s t a r t i n g end, and thereby eliminated any p e r s i s t i n g kinks that may have developed when the center-joining method was used. The s t a r t i n g end acted as a single c r y s t a l seed and could be used i n d e f i n i t e l y . A rate of zone t r a v e l of 10 centimeters per hour was used throughout t h i s i n v e s t i g a t i o n . This rate of t r a v e l required a power input of 170 milliamperes and 2000 v o l t s or 3^ -0 watts to maintain an 1/8 inch zone i n the 0.200 inch diameter rod. Power reductions at both ends of the rod were necessary to prevent the zone from growing too large due to reduced heat conduction through the small diameter support rods. -5 An operating vacuum of less than 8 X 10 millimeters of mercury was maintained at a l l times. The vapour pressure of niobium at i t s melting point was s u f f i c i e n t to obscure the view through the glass c y l i n d e r a f t e r a few passes. To eliminate the necessity of opening the system to remove the obscuring m e t a l l i c layer, a mirror periscope was devised so that subsequent m e t a l l i c deposit served to improve the r e f l e c t i n g surfaces rather than to hinder observation of the molten zone. As-received bars 9 inches i n length were given two passes. The result; was a smooth, lustrous bar of r e l a t i v e l y uniform diameter. The bottom 1 l/h inches of the bar was discarded to insure that the specimen cut from the bottom of the bar would not be contaminated. - 11 -2. I n t e r s t i t i a l Content V a r i a t i o n with Zone-Refining The i n t e r s t i t i a l content v a r i a t i o n with the degree of zone-r e f i n i n g was determined by removing samples from a single bar as. i t was given ten successive passes. The r e s u l t s f o r oxygen, hydrogen, nitrogen and carbon are given i n graphical form i n Figure 3° T k e dotted l i n e s represent the l i m i t of the experimental er r o r quoted by the analyst, Ledoux and Company, of ± 15 percent. In view of the expected downward trend of the curves, there i s reason to suspect that the sample taken a f t e r si x passes became contaminated. Oxygen, hydrogen and nitrogen concentrations are e f f e c t i v e l y reduced while carbon remains unaffected. The i n t e r s t i t i a l content of the metal used i n t h i s i n v e s t i g a t i o n a f t e r two passes i s shown i n Table I I . TABLE I I . I n t e r s t i t i a l Content of Zone-Refined Niobium Element Parts per M i l l i o n C kO 0 40 N 42 H 2 T o t a l 12T Hence the t o t a l i n t e r s t i t i a l content of the uncontaminated material used i n t h i s i n v e s t i g a t i o n was 124 parts per m i l l i o n which i s l e s s than one-half the o r i g i n a l value. 200 Number of Zone Refining Passe Figure 3- - . I n t e r s t i t i a l Removal as a Function of Number of Zone-Refining Passes - 13 -C. Single C r y s t a l Growth and Orientation Single c r y s t a l s were grown i n the manner described by Snowball, The s t a r t i n g end mentioned previously was used repeatedly as a seed to i n i t i a t e p r e f e r e n t i a l growth during zone-refining. W CU si a c •H I 2 3 4 5 7 8 inches Figure k. T y p i c a l Single Crystals Produced During Zone-Refining The o r i e n t a t i o n of the seed and .direction of p r e f e r e n t i a l growth resu l t e d i n single c r y s t a l s with an a x i a l d i r e c t i o n of L^lOjj " f i g u r e k. shows t y p i c a l single c r y s t a l s produced by t h i s technique. Conventional Laue b a c k - r e f l e c t i o n x-ray exposures were used to determine whether the e n t i r e zone-refined bar was a single c r y s t a l . Exposures of the top, middle and bottom of the bar were compared i n the determination. The a x i a l o r i e n t a t i o n of the bar was also checked i n each case. Figure 5* i l l u s t r a t e s the type of p i c t u r e obtained at any p o s i t i o n along the bar. The a x i a l d i r e c t i o n of the bar i s p a r a l l e l to the h o r i z o n t a l axis of the photograph. * Figure 5. A T y p i c a l X-Ray Photograph of a Single C r y s t a l D. I n t e r s t i t i a l Additions to Single Crystals The i n t e r s t i t i a l s oxygen, hydrogen and nitrogen were introduced i n t o as-zone-refined single c r y s t a l s of t e n s i l e specimen length by means of gas absorption and d i f f u s i o n at elevated temperatures. I n t e r s t i t i a l addition w i l l hereinafter be r e f e r r e d to as "Doping". 1. Doping Furnace Doping of the zone-refined s i n g l e c r y s t a l s was c a r r i e d out i n a h o r i z o n t a l tube furnace that could a l t e r n a t i v e l y be connected to a 2 vacuum pump, as described by Fraser , or to a gas r e s e r v o i r as described by Lawson^. The vacuum system i s capable of reducing the pressure to les s -6 than 10" millimeters of mercury and was used at t h i s capacity only when the gas content of the r e s e r v o i r was being changed. The mechanical fore pump was used to f l u s h the furnace tube subsequent to the charging of specimens. - 15 The ^ - l i t e r gas r e s e r v o i r was f i l l e d with the desired gas to a pressure s l i g h t l y greater than atmospheric so that the doping could be c a r r i e d out under s t a t i c conditions, with the assurance that inward leaking of a i r would not occur. 2. Gas P u r i f i c a t i o n The nitrogen used i n t h i s i n v e s t i g a t i o n was reported by the manufacturer to be 99°5 percent pure, with the balance predominantly oxygen. The r e s i d u a l oxygen was removed by passing the nitrogen over hot^pre-reduced copper chips and through s i l i c a g e l and sulphuric a c i d drying agents. Hydrogen was deoxidized by using a standard "Deoxo " c a t a l y t i c u n i t and was d r i e d with s i l i c a g e l and sulphuric a c i d . Oxygen, reported by the manufacturer to be 99°99 percent pure, was simply d r i e d p r i o r .to use with s i l i c a g e l and sulphuric a c i d . In each case the p u r i f i c a t i o n was c a r r i e d out as the r e s e r v o i r was f i l l e d . A mercury manometer was incorporated i n t o the gas flow to in d i c a t e that the p u r i f y i n g chain was always at a s l i g h t p o s i t i v e pressure^, thereby eliminating any inward leaking of a i r that might occur while f i l l i n g the evacuated r e s e r v o i r . Each time the doping gas was changed;, the r e s e r v o i r was evacuated to le s s than 10"5 millimeters of mercury and then flushed once with p u r i f i e d gas. 3• Doping Treatments Single c r y s t a l s which had been produced as previously described were cut to t e n s i l e specimen length and wrapped with molybdenum wire i n t o bundles of three i n such a manner that the specimens d i d not touch eaeli other or the w a l l of the furnace tube. With the specimens i n place the tube was flushed with p u r i f i e d gas three times using the fore pump only. The tube was then f i l l e d to s l i g h t l y greater than atmospheric pressure - 1 6 -and brought to equilibrium with the gas r e s e r v o i r . Due to the nature of the re a c t i o n of oxygen with niobium, i t was necessary to reduce the oxygen pressure to 2 5 microns f o r the doping period. To achieve t h i s condition the r e s e r v o i r valve was adjusted to leak at a rate that would maintain a pressure of 2 5 microns with the fore pump connected to the c i r c u i t . Hydrogen and nitrogen doping was c a r r i e d out at 7 6 5 millimeters of mercury. Long homogenization periods were required f o r oxygen and nitrogen treatments. These treatments were c a r r i e d out at 2 5 microns i n the same manner as described above f o r oxygen doping. Hydrogenated specimens were not homogenized at a reduced pressure due to tjie r a p i d outgassing of hydrogen from niobium. k. Hardness Measurements Cor r e l a t i o n of hardness values of as-zone-refined and doped specimens with i n t e r s t i t i a l content was not succes s f u l . Experimentation l e d to the conclusion that a 3 or k degree misalignment with the a x i a l d i r e c t i o n of the single c r y s t a l when performingtransverse hardness measure-ments l e d to wide v a r i a t i o n s i n hardness values. As well, the hardness values varied as the specimen was rotated about i t s a x i a l direction^with v a r i a t i o n s i n hardness values as large as 8 percent. However, at a f i x e d orientation,' i t was evident that the hardness values were r e l a t i v e , and t h i s f a c t was u t i l i z e d i n determining the e x i s t -ence of concentration gradients subsequent to doping. In t h i s way i t was possible to insure that concentration gradients were removed during the machining of the t e n s i l e shape from the single c r y s t a l . - 17 -E. Tens i l e Specimen Preparation 1. Machining Six t e n s i l e specimen, blanks 1»30 inches i n length were cut from each single c r y s t a l using a jeweler's saw. The blanks were then mounted i n a lathe. The surface speed and t o o l design were very c r i t i c a l f a c t o rs i n the successfulmachining.ofniobium. G a l l i n g was avoided by r o t a t i n g the specimen at approximately kOO rpm and using a .080 inch diameter t o o l . The t o o l was held i n the v e r t i c a l p o s i t i o n and was hollow-ground at a r e l i e f angle of 50 degrees perpendicular to the axis of the specimen. T r i c h l o r o -ethylene was used as a l u b r i c a n t . The gauge section of the f i n i s h e d specimen was 0.125 inches i n diameter and va r i e d from 0.8 to 0.9 inches i n length. Due to the low t e n s i l e strength of high p u r i t y niobium at room temperature the maximum depth of cut per pass was approximately 0.003 inches. The doped t e n s i l e specimen blanks exhibited superior machining c h a r a c t e r i s t i c s compared to the pure sing l e c r y s t a l s ; however, i t was necessary to sharpen the t o o l more frequently. The machined specimens were polished with 0 through 0000 emery paper p r i o r to removal from the lat h e . 2. Surface Preparation Machined and mechanically polished t e n s i l e specimens were el e c t r o p o l i s h e d i n a s o l u t i o n of 85' percent concentrated sulphuriq ,acid and 15 percent h y d r o f l u o r i c a c i d (35 percent). The specimen was a x i a l l y rotated i n the s o l u t i o n which was contained i n a 2.5 inch diameter s t a i n l e s s s t e e l beaker. A current density of approximately 1.15 amperes per square inch provided an acceptable surface. This treatment removed 0.003 inches from the diameter of each specimen. - 18 -3- X-Ray Laue X-ray exposures indi c a t e d that the mechanically deformed la y e r that developed during machining was completely removed by e l e c t r o -p o l i s h i n g . F. Tens i l e Testing 1. T e n s i l e Apparatus Single c r y s t a l specimens were tested on the modified Instron t e n s i l e t e s t i n g machine described by Snowball"'". Figure 6. i l l u s t r a t e s the s p l i t - j a w gripping of the specimen. The jaws containing the specimen were threaded i n t o the u n i v e r s a l heads as shown i n Figure 7» A s t r a i n rate of 0.057 per minute was used throughout t h i s i n v e s t i g a t i o n . This s t r a i n rate was the r e s u l t of a crosshead speed of 0.05 in/min. and an average specimen gauge length of 0.875 i n . Load elongation curves were recorded auto-graphically. Examples of the curves are shown i n Figures 12, 15, 23 and 28. 2. Temperature Measurement and Control Temperature measurements were made with a copper-constantan thermocouple wrapped securely around the gauge length of the t e n s i l e specimen as shown i n Figure 7* The thermocouple emf was measured on a Pye potentiometer, r e s u l t i n g i n an accuracy of temperature measurement within ± 1°C. Temperatures between + 25° and -150°C. were obtained by immersing the t e n s i l e apparatus i n a dewar f l a s k containing petroleum ether (30° -60°C. f r a c t i o n ) . The petroleum ether was cooled by passing l i q u i d nitrogen through the bath u n t i l the desired temperature was reached. An e l e c t r i c a l l y driven s t i r r e r maintained a homogeneous temperature throughout the bath. - 19 -Figure 7. Specimen and Universal Heads i n P o s i t i o n f o r Tensile Testing - 20 -Temperature v a r i a t i o n during a t e n s i l e t e s t was found to be l e s s than one degree centigrade, however the temperature recorded f o r the t e s t was measured at the y i e l d point. Temperatures of - I 8 3 0 and -197°C were attained through the use of l i q u i d oxygen and nitrogen r e s p e c t i v e l y . Temperatures between - I 8 3 0 and -150°C. were attained by f i r s t immersing the t e n s i l e apparatus i n l i q u i d nitrogen, allowing equilibrium to be established and then removing the l i q u i d nitrogen and allowing the specimen to warm slowly i n the empty dewar f l a s k . The temperature v a r i a t i o n during the t e s t was approximately 3°C» This l e d to a s l i g h t l y higher experimental e r r o r than i f the specimen were i n a f l u i d bath. Consequently t h i s temperature range was not investigated extensively. In order to determine the r e l a t i o n between the temperatures a,t the surface and the center of the specimen, a .030 inch diameter hole was d r i l l e d along the specimen axis to the center of the gauge length. The hole was f i l l e d with mercury and an insulated thermocouple was placed i n the mercury. Inside and outside temperature v a r i a t i o n s were simultaneously measured as the specimen was immersed i n or removed from a low temperature bath. The r e s u l t s , presented g r a p h i c a l l y i n Figure 8. i n d i c a t e that the v a r i a t i o n between i n t e r n a l and external temperatures was l e s s than one degree centigrade a f t e r a period of two minutes had elapsed. Experimentally, i n t e r n a l s t r u c t u r a l equilibrium conditions were found to e x i s t within 13 to 15 minutes a f t e r immersion i n the low temp-erature bath. Testing p r i o r to t h i s period r e s u l t e d i n .abnormally high t e n s i l e strengths. This e f f e c t w i l l be more f u l l y discussed l a t e r i n the t h e s i s . - 21 -Time (seconds) Figure 8. Relationship Between Internal and External Specimen Temperature as a Function of Time. Bath Temperature Change, +38°C. - 22 -3. Y i e l d Point Measurements The upper y i e l d stress, hereinafter r e f e r r e d to as y i e l d stress, was c a l c u l a t e d from the highest value reached p r i o r to the i n i t i a l load drop. Y i e l d points were not observed f o r high p u r i t y single c r y s t a l s above 0°C. In t h i s instance, the y i e l d stress values reported were cal c u l a t e d from a 0.2 percent o f f s e t . A l l doped specimens exhibited y i e l d points above 0°C. IV. EXPERIMENTAL OBSERVATIONS AND RESULTS - 23 -The e f f e c t of temperature and i n t e r s t i t i a l content on the mechanical properties of high p u r i t y niobium single c r y s t a l s has been o o determined between +25 and -197 C. A. High P u r i t y Single Crystals High purity single c r y s t a l s containing 2 ppm H, kO ppm 0, k2 ppm N, and kO ppm C were tested i n tension between +25° and -197°C 1. Y i e l d Stress versus Temperature Y i e l d stress as a function of temperature i s shown gr a p h i c a l l y 5 i n Figure 9» T n e continuous r e l a t i o n s h i p that both Fis h e r and Zener g and Holloman suggest does not appear on a p l o t of t h i s type. Replot-t i n g the y i e l d stress versus the r e c i p r o c a l of the absolute temperature r e s u l t s i n two i n t e r s e c t i n g s t r a i g h t l i n e s (Figure 10). A logarithmic p l o t of y i e l d stress versus the r e c i p r o c a l of the absolute temperature (Figure l l ) , also y i e l d s two i n t e r s e c t i n g s t r a i g h t l i n e s with some deviation i n the region of the i n t e r s e c t i o n . These r e s u l t s i n d i c a t e that the temperature dependence of y i e l d stress varies anomalously at approximately -100°C. Explanations of t h i s anomalous behaviour w i l l be discussed l a t e r i n the text. The anomalous r e s u l t i s s i m i l a r to that found by Snowball"'" f o r single c r y s t a l s of vanadium. Snowball, however, d i d not f i n d that h i s data f i t t e d two st r a i g h t l i n e s on a.plot of y i e l d stress versus r e c i p r o c a l absolute temperature. His data y i e l d e d two i n t e r s e c t i n g curves on a plo t of y i e l d stress versus r e c i p r o c a l temperature, and two i n t e r s e c t i n g -straight l i n e s on a p l o t of Mog y i e l d stress versus r e c i p r o c a l temperature (analogous to the curves of t h i s i n v e s t i g a t i o n ) . -. 2k -1301 [ 1 1 1 r - r 120 _ 110 k-I 1 J 1 1 1 -J I I I 1 l I -200 -180 -l60 -IkO -120 -100 -80 - 6 0 - k O - 2 0 0 + 2 0 + k 0 Temperature (°c). Figure 9. Y i e l d Stress versus Temperature f o r High Pu r i t y Single Crystals Figure 10. Y i e l d Stress versus the Reciprocal Temperature f o r High P u r i t y Single C r y s t a l s Temperature (°C.) Figure 11. Log Y i e l d Stress versus Reciprocal Temperature f o r High Pu r i t y Single Crystals-- 27 -T y p i c a l autograph!cally recorded load-elongation curves are shown i n Figure 12. Only the curve at +22°C. shows a p o s i t i v e work hardening exponent, and d i s t i n c t y i e l d points were not found at temperatures above 0°C. The general increase i n y i e l d drop that occurs as the temperature i s decreased 6 agrees with r e s u l t s reported by Wessel . 2. Elongation versus Temperature Elongation as a function of temperature i s shown i n Figure 13• The d u c t i l i t y t r a n s i t i o n i s of the d u c t i l e - t o - l e s s - d u c t i l e type and occurs at approximately -60°C. Below -l80°C. a further d u c t i l i t y decrease i s evident. The d i s t i n c t d u c t i l i t y t r a n s i t i o n i s r e f l e c t e d by the load elongation curves of Figure 12.. In two cases, necking occurred i n two separate regions of the specimen gauge length; an example of double necking i s shown i n Figure lk a). Values of elongation f o r these specimens were approximately 65 percent and are not shown i n Figure 12-3. Reduction i n Area Reduction i n area was estimated at 100 percent f o r a l l specimens except those tested at -186° and -197°C where the reduction i n area was estimated to be between 80 and 90 percent. The marked differ e n c e between elongation and reduction i n area below the t r a n s i t i o n temperature i s due to l o c a l i z e d necking, which i s i l l u s t r a t e d i n Figures lk b) and lk c ) . In a l l cases the o r i g i n a l c y l i n d r i c a l area reduced to a sharp knife edge. k. Mechanism of Deformation The a c t i v e s l i p system was found by Laue b a c k - r e f l e c t i o n X-ray i o n photographs to be ^ - 1 ^ 0"""^ "^  ' 1do^ a ^ o v e an& ^ e l ° w i " n e t r a n s i t temperature. This i s i n agreement with Snowball's"'* r e s u l t s f o r vanadium single c r y s t a l s . In both cases the a x i a l d i r e c t i o n was Q-lcQ ' 13 Elongation (percent) Figure 12. Load-Elongation Curves f o r High P u r i t y Single Crystals Figure 13» Percent Elongation versus Temperature f o r High Purity Single Crystals - 30 Figure Ik. a) Double Necking i n Pure Niobium Single Crystals b) Uniform: Elongation at 25 C. c) Localized Necking at -197°C. (x 3 ) - 31 -The jog that occured j u s t p r i o r to the y i e l d .point on the load-elongation curve at -197°C», shown i n Figure 12. was accompanied by a loud c l i c k . These i n d i c a t i o n s are t y p i c a l of twinning, however evidence 12 of a twin band could not be found on the specimen surface. Adams et a l reported that niobium appeals to be reluctant to twin, but that increased s t r a i n rates tend to i n i t i a t e twinning at low temperatures. B. Single C r y s t a l s Doped with Hydrogen Single c r y s t a l s with hydrogen contents of 100 ppm and 300 ppm o ° were tested i n tension between +25 and -197 0. Example load-elongation curves f o r these specimens are shown i n Figure 15. 1. E f f e c t of Hydrogen on Y i e l d Stress Curves showing the e f f e c t of hydrogen content on y i e l d stress are compared with the curve f o r high p u r i t y single c r y s t a l s i n Figure l6. The general e f f e c t was a higher y i e l d stress at a l l temperatures. At temperatures below the anomaly, the temperature dependence of y i e l d stress was constant f o r both l e v e l s of hydrogen content, r e s u l t i n g i n p a r a l l e l curves at higher l e v e l s of s t r e s s . At temperatures above the anomaly, the temperature dependence of y i e l d stress decreased with increasing hydrogen content, r e s u l t i n g i n n o n - p a r a l l e l curves at higher l e v e l s of s t r e s s . o Below -175 0. the e f f e c t of hydrogen was detrimental, and the y i e l d stress values dropped below those f o r the high p u r i t y specimens. This decrease i n y i e l d stress at low temperatures has been reported by 6 16 Wessel f o r niobium and molybdenum,and by Loomis and Carlson f o r vanadium. Wessel explained t h i s phenomenon as premature abrupt y i e l d i n g i n i t i a t e d by dislocation-induced microcracking, and an associated - 32 -- 33 --200 -180 -l60 -140 -120 -100 ^80 ^60 -40 ^20 0 +20 +40 Temperature (°C). Figure l6. Y i e l d Stress versus Temperature f o r Single Crystals Doped with Hydrogen d u c t i l i t y decrease. Loomis and Carlson, on the other hand, reported a d u c t i l i t y recovery associated with decreasing y i e l d s t r e s s . The e f f e c t of hydrogen content on the y i e l d stress i s also i l l u s t r a t e d i n Figure YJ9 which i s a p l o t of log y i e l d stress Versus r e c i p r o c a l absolute temperature. Above the t r a n s i t i o n temperature the e f f e c t of increasing hydrogen content i s to decrease the slope of the curve. Below the t r a n s i t i o n , the o r i g i n a l slope, i s retained at a higher l e v e l of s t r e s s . Assuming that the t r a n s i t i o n temperature can be determined by extending the two s t r a i g h t portions of the curve u n t i l they i n t e r s e c t , increased hydrogen content lowers the t r a n s i t i o n temperature. However, t h i s trend i s not w e l l established since the differences i n temperature on a r e c i p r o c a l p l o t are very small. The anomalous t r a n s i t i o n temperature f o r the pure material was -100°C. and decreased to -108°C. and to -117°C. as the hydrogen content was increased. 2. E f f e c t of Hydrogen on D u c t i l i t y Elongation as a function of temperature f o r the two l e v e l s of hydrogen content i s compared with that f o r high p u r i t y single c r y s t a l s i n Figure 18. The hydrogen content of 100 ppm appears to have lowered the t r a n s i t i o n temperature while the hydrogen content of 300 ppm has r a i s e d the t r a n s i t i o n temperature. Increased hydrogen content reduced the l e v e l of d u c t i l i t y both above and below the t r a n s i t i o n temperature. A second d u c t i l i t y decrease i s evident below -170°C. corresponding to the curve f o r high purity niobium. The point at +2U°C. and 56 $ elongation exhibited" double necking, which resulted i n excessive elongation.' It. i s expected, that" the ac t u a l . '- 35 -Temperature (°C.) +227 +60 -23 -73 -107 -130 -148 -162 -173 -182 -190 -196 ra ft 0 0 o w w CD SH + > C O 100 90 80 70 60 50 ko 30 20 10 9 8 7 6 l T T T T T o -O 300 ppm Hydrogen • 100 ppm Hydrogen — High Purity Single Crystals 10 11 12 l / T (l/°K X 1 0 J ) 13 14 Figure 17 •• Log Y i e l d Stress versus Reciprocal Temperature f o r Single Crystals Doped with Hydrogen compared with High Pu r i t y Single C r y s t a l s . Figure 18. Percent Elongation versus Temperature f o r Single Crystals Doped with Hydrogen - 37 -d u c t i l i t y of the specimen may be misrepresented by l o c a l i z e d necking. As a consequence, reduction i n area has been determined as a function of temperature and i s i l l u s t r a t e d i n Figure 19. Both curves show minima and maxima at low temperatures. Specimens with a hydrogen content of 100 ppm have a minimum at approximately -130°C. and a maximum at approximately o -170 C. Specimens with a hydrogen .content of 300 ppm show a minimum at o 0 approximately -115 0 , and a maximum at -170 C. I t i s noted that the d u c t i l i t y recovery maxima occur at the same temperature and that the second d u c t i l i t y decrease below -170°C. i s coincident with the estimated d u c t i l i t y decrease of the pure material. D u c t i l i t y r e s u l t s f o r niobium reported i n the l i t e r a t u r e " ^ ' """""" are not d e t a i l e d enough to detect a lk recovery i n reduction i n area as shown i n Figure 19. Eustice and Carlson ? however, have c a r r i e d out a d e t a i l e d low temperature study of the e f f e c t of hydrogen on the t e n s i l e properties of p o l y c r y s t a l l i n e vanadium and have detected a d u c t i l i t y recovery (Figure 20) between -120° and - 1 9 7°C Uniform elongation was used as the c r i t e r i o n f o r d u c t i l i t y i n the paper by Eustice and Carlson, but, as well, data regarding reduction i n area was presented g r a p h i c a l l y f o r one l e v e l of hydrogen content. The d u c t i l i t y minimum l e v e l s were the same i n both cases. 3- E f f e c t of Hydrogen on the Deformation Mechanism The s l i p system at room temperature was found to be ^111^ [ l l ^ J the same as that f o r high p u r i t y single c r y s t a l s . At -25°C. the mechanism of deformation was both s l i p and cleavage. S l i p occurred on the ^111^ J112J system p r i o r to cleavage which occurred on the jiooj and j ^ i o j planes, with no apparent preference f o r e i t h e r family of planes. Examples of reduction i n area and fr a c t u r e at +25° and -25°C. are i l l u s t r a t e d i n Figure 2 1 a ) andb) r e s p e c t i v e l y . - 38 -Figure 19« Percent Reduction i n Area versus Temperature f o r Single Crystals Doped with Hydrogen Figure 20. Temperature Versus D u c t i l i t y of Vanadium at D i f f e r e n t Hydrogen Concentrations. A f t e r Eustice and Carlson^- . - ko -Figure 22. Twin Lines Observed i n Specimen Doped with 300 ppm Hydrogen (-197°C.) (X 10) - 41 -At -197 and -170 C. twinning took place on the e l a s t i c portion of the load-elongation curve (Figure 16, page 33)• Twinning was extensive p r i o r to cleavage f a i l u r e f o r "both l e v e l s of hydrogen. Cleavage f a i l u r e (Figure 21'c)) occurred on a single [112} plane. The twin plane was also determined as ( l l 2 ^ which conforms with r e s u l t s reported i n the l i t e r a t u r e "^ > Twin l i n e s which were observed on a specimen surface i n the region adjacent to the f r a c t u r e are i l l u s t r a t e d i n Figure 22. Schadler detected s i m i l a r l i n e s on tungsten single c r y s t a l s . C. Single Crystals Doped with Oxygen Single c r y s t a l s containing 800 and 2000 ppm oxygen were tested o o i n tension between +25 and -197 C. Example load-elongation curves are shown i n Figure 23-1. E f f e c t of Oxygen on Y i e l d Stress Curves showing the e f f e c t of oxygen content on y i e l d stress are compared with the curve f o r high p u r i t y single c r y s t a l s SB i n Figure 24. Here a marked increase i n y i e l d stress i s evident. At temperatures below the anomaly the temperature dependence has been decreased s l i g h t l y by increasing oxygen contents, r e s u l t i n g i n no n - p a r a l l e l curves at higher l e v e l s of s t r e s s . At temperatures above the anomaly, the temperature dependence of y i e l d stress decreased i n i t i a l l y . An increase i n oxygen content from 800 ppm to - 2000 ppm d i d not produce a f u r t h e r decrease i n the temperature dependence of y i e l d stress, which r e s u l t e d i n two p a r a l l e l curves. In contrast to the s i m i l a r curve f o r hydrogen discussed previously, there i s not an apparent drop i n y i e l d stress at the lowest temperature. - 42 Figure 23. Load-Elongation Curves f o r Single Crystals Doped with Oxygen - 4 3 -Figure 2k. Y i e l d Stress versus Temperature f o r Single Crystals Doped with Oxygen - kk -The e f f e c t of oxygen content on the y i e l d stress i s also i l l u s t r a t e d i n Figure 25, which i s a log p l o t of y i e l d stress versus temperature. At temperatures higher than the t r a n s i t i o n temperature, the e f f e c t of oxygen has caused a decrease i n slope, the slope remaining constant over the range of oxygen content investigated. Below the t r a n s i t i o n temperature a small change i n slope has occurred. Unfortunately the data i s not d e t a i l e d enough to e s t a b l i s h whether or not the change i n slope i s r e a l . The point of i n t e r s e c t i o n of the curves i n i t i a l l y decreases from -100° to -113°C. hut a further increase i n oxygen content d i d not produce a corresponding reduction i n temperature. 2. E f f e c t of Oxygen on D u c t i l i t y M u l t i p l e cleavage r e s u l t e d i n the los s of several pieces of the specimen and prohibited the measurement of elongation at t e s t temperature below -100°C. As a consequence, the d u c t i l i t y v a r i a t i o n with temperature i s not presented. Measurements of reduction i n area adjacent to the fracture were recorded and are shown i n Figure 26. I t i s apparent that increased oxygen content increases the d u c t i l i t y t r a n s i t i o n temperature and that the trans-i t i o n i s of the d u c t i l e - t o - b r i t t l e type. The curve f o r specimens containing 2000 ppm oxygen exhibited an 8 percent recovery i n reduction i n area at -197°C. which was not exhibited by specimens containing 800 ppm; however, only one specimen containing 2000 ppm was tested at -197°C. and the r e s u l t cannot be considered conclusive. -• 45 -Temperature (°C.) +227 +50 -23 -73 -107 -130 -148 -l62 -173 T T T -182 -190 -I96 -202 T 200 150 L 100 L. 80 h 60 50 -40 -30 H 20 10 T T / / / O 2000 ppm Oxygen Q 800 ppm Oxygen Pure Single Crystals / / / / J _ J L ± 3 ' 4 5 6 7 8 9 10 I/T (I/°K x 103) 11 12 13 14 Figure 25. Log Y i e l d Stress versus Reciprocal Temperature f o r Single Crystals Doped with Oxygen - ke -Figure 26. Percent Reduction i n Area versus Temperature f o r Single C r y s t a l s Doped with Oxygen - 47 -3- E f f e c t of Oxygen on the Deformation Mechanism The s l i p system at room temperature was ^.1.1^ '(^-•'-^} > a s w a s ^ n e case f o r high p u r i t y and hydrogen-doped single c r y s t a l s . For c r y s t a l s containing 800 ppm oxygen, cleavage occurred "below -115°C. For c r y s t a l s containing 2000 ppm oxygen, cleavage occurred below -70°C. Some degree of necking occurred i n both cases, as indicated by the measurable reductions i n area. The cleavage plane was found to be 1^00 at a l l temperatures below those mentioned above f o r both oxygen contents. The degree of p l a s t i c deformation and cleavage at +25° and -138°C. i s i l l u s t r a t e d i n Figures 27 a) and b) r e s p e c t i v e l y . Figure 27. a) P l a s t i c Deformation and, b) Cleavage of Single Crystals Doped with 2000 ppm Oxygen (X 3) Twin markings were not found on any of the specimens, regardless of t e s t temperature. This i s i n agreement with the f a c t that £lOoJ? the cleavage plane, i s not a recognized twin plane i n body centered cubic l a t t i c e s , - 48 -D. Single Crystals Doped with Nitrogen Single c r y s t a l s containing 70 and 85 ppm nitrogen were tested i n tension between +25° and -197°C. Example load-elongation curves f o r these specimens are shown i n Figure 28. 1. E f f e c t of Nitrogen on Y i e l d Stress Curves showing the e f f e c t of. nitrogen content on y i e l d stress are compared with the curve f o r high p u r i t y single c r y s t a l s i n Figure 29. The i general e f f e c t was a higher y i e l d stress at a l l temperatures. At temperatures below the anomaly, the temperature dependence of y i e l d stress was constant f o r both l e v e l s of nitrogen content, r e s u l t i n g i n p a r a l l e l curves at higher l e v e l s of s t r e s s . At temperatures above the anomaly the temperature dependence of y i e l d stress decreased i n i t i a l l y . An increase i n nitrogen content from 70 to 85 ppm d i d not produce a fur t h e r decrease i n temperature dependence of y i e l d . s t r e s s , This r e s u l t e d i n two p a r a l l e l curves. The e f f e c t of nitrogen content on the y i e l d stress i s also i l l u s t r a t e d i n Figure 30, which i s a l o g p l o t of y i e l d stress versus temp-erature. At temperatures higher than the t r a n s i t i o n temperature, the e f f e c t of nitrogen has caused a decrease i n slope, the slope remaining constant over the range of nitrogen content investigated. The point of i n t e r s e c t i o n of the curves i n i t i a l l y decreased from -100° to 110°C, but a further increase i n nitrogen content d i d not produce a corresponding reduction i n temperature. - k9 -Figure 28. Load-Elongation Curves f o r Single Crystals Doped with Nitrogen - 50 -Figure 29. Y i e l d Stress versus Temperature f o r Single Crystals Doped with Nitrogen - 51 -+227 +60 -23 i 5 0 | — j — Temperature (°C. ) -73 -107 -130 -148 -162 T T T •173 -182 -190 -196 w ft 00 o W u +> CQ 100 90 80 70 60 50 40 30 20 10U T O 85 PP m Nitrogen 70 ppm Nitrogen __. High P u r i t y Single Crystals 4 7 8 9 l/T (l/°K x 10 3 ; 10 11 12 13 Figure 30. Log Y i e l d Stress versus Reciprocal Temperature f o r Single Crystals Doped with Nitrogen - 52. -2. E f f e c t of Nitrogen on D u c t i l i t y Figure 31 i s a comparison of the p l o t of elongation versus temp-erature f o r high p u r i t y single c r y s t a l s and specimens containing two l e v e l s of nitrogen. Both nitrogen contents have lowered the t r a n s i t i o n temperature by the same amount. This r e s u l t i s contrary to normal behaviour where increased i n t e r s t i t i a l content r a i s e s the t r a n s i t i o n temperature. The elongation has been decreased both at high and low temperatures to approximately the same degree, regardless of the nitrogen content. The t r a n s i t i o n i s of the d u c t i l e - t o - b r i t t l e type. The reduction i n area as a function of temperature has been o estimated at 100 percent f o r a l l specimens, except those at -170 and -197°Q where the reduction i n area has been estimated between 80 and 90 percent. Deformed specimens are s i m i l a r i n appearance to those i l l u s t r a t e d i n Figure 14 b) and c ) , page 30. Double necking d i d not occur i n single c r y s t a l s doped with nitrogen. 3. E f f e c t of Nitrogen on the Deformation Mechanism Nitrogen apparently does not a f f e c t the deformation mechanism. The s l i p system over the range of temperature investigated was ^111^ (^ --^ -j the same as that f o r high p u r i t y s i n g l e c r y s t a l s . Twinning occurred i n a spepimen containing 85 ppm which was tested at -197°C A specimen containing 7'0 PPm.» also tested at -197°C> di d not show evidence of twinning. Excessive twinning at the beginning of p l a s t i c flow obscured a true y i e l d point on the -197°C load-elongation curve shown i n Figure 20, page kty. The twins observed on" the specimen surface were s i m i l a r to those shown i n Figure 22, page 40. Cleavage was not detected as a deformation mechanism f o r specimens containing nitrogen over the ranges of concentration and temperature investigated. - 53 -O 85 ppm Nitrogen • 70 ppm Nitrogen High P u r i t y Single Crystals - 2 0 0 ""i5o ""io"o ^ik~o =020 - 1 0 0 - 0 6 rg75 ^40 ^20 0 +20 Temperature (°C) Figure 31- Percent Elongation versus Temperature f o r Single Crystals Doped with Nitrogen - 54 -E. Dependence of Y i e l d Stress on the Eate of Change of Testing Temperature The dependence of y i e l d stress on the time that the specimen was allowed to remain i n the low temperature bath was determined. The minimum length of time required to obtain the equilibrium y i e l d stress was found to be approximately 13 minutes. In order to determine the length of time required f o r thermal equilibrium to be established i n t e n s i l e specimens, i n t e r n a l and external temperature measurements were recorded as a function of time. Measurements were begun when the specimen was immersed i n l i q u i d nitrogen and were made according to the method out l i n e d i n Section I I I of the t h e s i s . Experiments were conducted i n which the rate of warming over a f i x e d temperature range was varied. The te s t s were conducted between the o ^o l i q u i d baths at -197 and -146 C^ only because i t was more convenient to vary the rate of change of temperature by increasing the temperature. I t was found that i f a specimen was allowed to come to i n t e r n a l . o \ s t r u c t u r a l equilibrium i n l i q u i d nitrogen (.-197 C) f o r 15 minutes and then immersed i n a bath of petroleum ether at -146°C^ allowed to e s t a b l i s h thermal equilibrium f o r two minutes and then tested, a y i e l d strength 17-7 percent higher than that usually recorded at -146 C. was observed. Y i e l d strength as a function of time from removal from the l i q u i d nitrogen bath u n t i l observance of the y i e l d point at -146°C• i s shown i n Figure 32. The mathematical form of the curve may be approximated by a hyperbola. In each case the rate of warming was c o n t r o l l e d so that i t was constant. The specimen was allowed to e s t a b l i s h thermal equilibrium during every t e s t . The data i s r e p l o t t e d i n Figure 33 where the y i e l d strength appears as a function of the rate of warming.• - 55 -Elapsed Time to Y i e l d (seconds) Figure 32. Y i e l d Strength as a Function of Elapsed Time to Observance of the Y i e l d Point During a Rapid Change i n Testing Temp-erature f o r High P u r i t y Single C r y s t a l s . (1+6°C, Constant Interval) - 56 -Figure 33* Y i e l d Strength as a Function of Warming Rate f o r High P u r i t y Single C r y s t a l s . (46°C. Constant Interval) - 57-V. DISCUSSION The r e s u l t s of t h i s i n v e s t i g a t i o n i n d i c a t e that high p u r i t y niobium single c r y s t a l s e x h i b i t both a marked temperature dependence of upper y i e l d stress and a d u c t i l e - t o - b r i t t l e t r a n s i t i o n . The v a r i a t i o n of y i e l d stress with temperature was found to be anomalous at approximately -100°C. Increased i n t e r s t i t i a l content within the range investigated d i d not eliminate the anomaly or appreciably a f f e c t the temperature at which i t occurred. A. Temperature Dependence of Y i e l d Stress Two current theories of the temperature dependence of the y i e l d stress were b r i e f l y mentioned i n the review of previous work and w i l l be presented i n more d e t a i l here. Fis h e r has studied the mechanism of d i s l o c a t i o n break-away by an examination of delay time. He considered a C o t t r e l l model of an anchored d i s l o c a t i o n , and estimated the energy required to extend a portion of the d i s l o c a t i o n beyond the l i n e of solute atoms so that the loop could grow with decreased energy. He then postulated that the minimum energy required must be supplied by thermal f l u c t u a t i o n s . In a c a l c u l a t i o n of delay time f o r p o l y c r y s t a l l i n e i r o n , Fisher a r r i v e d at an expression which r e l a t e d stress and temperature i n the following way: ( T T / G = constant where CT = y i e l d stress T = absolute temperature G = r i g i d i t y modulus. In other words, a p l o t of <J~ versus l / T should y i e l d a s t r a i g h t l i n e . Experimental evidence reported i n the l i t e r a t u r e f i t s the Fisher expression very well above approximately - 125°C. However, at lower temperatures there are marked deviations from the t h e o r e t i c a l curve of y i e l d stress versus r e c i p r o c a l temperature. 58 A p l o t of y i e l d stress versus r e c i p r o c a l temperature f o r high pu r i t y niobium single c r y s t a l s (Figure 10, page 25•) also shows a marked deviation below about -100°C. However the r e s u l t s also f i t a s t r a i g h t l i n e at temperatures below -100°C. This would imply that the value of the constant i n Fisher's expression has changed at -100°C. 5 Fisher has indicated i n h i s o r i g i n a l paper that i t i s expected that the above equation w i l l f a i l at low temperatures due to the approximate method of i n t e r p r e t i n g d i s l o c a t i o n break-away... The r e s u l t s of t h i s invest-i g a t i o n suggest that the r e l a t i o n i s v a l i d at low temperatures i f the value of the constant i s changed at approximately -100°C. A possible explanation i s that the mechanism of s o l u t e - d i s l o c a t i o n i n t e r a c t i o n has changed at the break i n the curve of Figure 10. 8 Zener and Hollomon predicted that the i n i t i a t i o n of p l a s t i c flow i s thermally act i v a t e d . They r e l a t e d the s t r e s s - s t r a i n r e l a t i o n and the s t r a i n rate through a parameter P: CP = CP (P, € ) and, P = A «£ exp(Q/RT) • where; £ = s t r a i n rate Q = a c t i v a t i o n energy T = absolute temperature A = frequency f a c t o r The implication i s that the y i e l d stress depends upon a dimensionless parameter which i s the r a t i o of the s t r a i n rate to the rate of an unknown rate process which has an a c t i v a t i o n energy Q; assumed to be i n s e n s i t i v e to temperature. • I f the s t r a i n (£ ) and s t r a i n rate ( £ ) are held constant then: CT cx exp(l/T) and a p l o t of log (T* versus l/T would y i e l d a s t r a i g h t l i n e . Zener and Hollomon have determined a c t i v a t i o n energies between 90 and 120 c a l o r i e s per gram mole f o r s t e e l . A l o g p l o t of y i e l d stress versus r e c i p r o c a l temperature f o r high purity niobium s i n g l e c r y s t a l s (Figure 11, page 26.) r e s u l t s i n two s t r a i g h t l i n e s of d i f f e r e n t slope. Therefore i t i s apparent that there i s a change i n the nature of the rate process involved, r e s u l t i n g i n a change i n act-i v a t i o n energy ..at a c r i t i c a l temperature. . Hence, i f the slopes of the curves of Figure 11. are a c t i v a t i o n energies, the change i n a c t i v a t i o n energy from the high temperature range to the low temperature range i s 1360 to 230 " c a l o r i e s per mole". However, these values are not true act-i v a t i o n energies, since stress i s not interpreted i n the present work as a rate or f l u x f u n c t i o n . In a recent i n v e s t i g a t i o n of the p h y s i c a l properties of high p u r i t y vanadium single c r y s t a l s , Snowball 1 p l o t t e d both y i e l d stress and log y i e l d stress versus r e c i p r o c a l temperature i n an e f f o r t to c o r r e l a t e 5 8 h i s data with the theories of Fisher , and Zener and Hollomon . The data does not f i t Fisher's theory at a l l . However i t does f i t the Zener-Hollomon theory to the extent that two i n t e r s e c t i n g s t r a i g h t l i n e s are obtained. Snowball's r e s u l t s are then analogous to the r e s u l t s of t h i s i n v e s t i g a t i o n . A comparison of the two curves with approximate r e p l o t s of data of other inv e s t i g a t o r s , shown i n Figure 3^ > indicates that the anomalous trend of y i e l d strength with decreasing temperature i s not l i m i t e d to single c r y s t a l s nor to a p a r t i c u l a r metal. I f the curves of Wessel, Bechtold, and Clough and Pavlovic (Figure 3+.) are r e p l o t t e d as y i e l d strength versus r e c i p r o c a l temperature, the break i n the curve p e r s i s t s . 11 17 Other recent in v e s t i g a t i o n s ' also show a s i m i l a r behaviour. - 6o -Temperature (°C.) +227 +60 -23 -73 -107 -130 -148 -I63 -173 -182 -190 — I 1 1 1 1 1 1 1 1 i r -2 3 h 5 6 7 8 9 10 I T 1 2 l/T (l/°K X 10 3) Figure 3^ * Log Y i e l d 'Stress versus Reciprocal Temperature f o r Niobium, Vanadium, Tantalum. - 61 -I t i s proposed that the other inv e s t i g a t o r s d i d not detect the anomaly on a y i e l d stress versus temperature p l o t due to the wide i n t e r v a l s between t e s t temperatures. As well, the data was apparently not correlated to the theories of Fisher or Zener and Hollomon, where l i n e a r deviations become more obvious. Snowball^ has offered an explanation of the discontinuous r e l a t i o n -ship between y i e l d stress and temperature i n terms of d i f f e r e n t d i s l o c a t i o n locking e f f e c t s of each species of i n t e r s t i t i a l atom. He suggested that the low temperature slope may be governed by carbon and that the high temp-erature slope may be governed by nitrogen. Carbon was selected as most l i k e l y because the carbon content of the vanadium used by Snowball was very much lower than that used by any other i n v e s t i g a t o r . The assumption made was that an increase i n carbon content would r a i s e the low temperature curve, thereby lowering the temperature of the ,o anomaly u n t i l i t was below -I96 C. and hence went undetected. However, Figure 3^. i l l u s t r a t e s that the anomaly does e x i s t i n the work of other-inve s t i g a t o r s where the carbon content was s u b s t a n t i a l l y higher (Table III) than i n the vanadium used by Snowball. TABLE I I I . Analysis of Metal Used by Investigators shown i n Figure 34. Investigator Metal and State Carbon, ppm Nitrogen, ppm Bechtold Ta, p o l y c r y s t a l l i n e 100 100 Wessel Nb, p o l y c r y s t a l l i n e 120 120 Clough and Pavlovic V, p o l y c r y s t a l l i n e 460 500 Snowball V, single c r y s t a l s 70 90 This i n v e s t i g a t i o n Nb, sin g l e c r y s t a l s 40 40 - 62 -This apparent inconsistency may be explained by the presence of a second species of atom, say nitrogen, i n an amount s u f f i c i e n t to r a i s e the high temperature curve to a p o s i t i o n where the temperature of the i n t e r s e c t i o n of the curves e i t h e r remained f i x e d or increased. I t should be remembered that Snowball d i d not f i r m l y e s t a b l i s h the species of the atoms thought to be responsible f o r the e f f e c t . B. E f f e c t of I n t e r s t i t i a l Content on the Temperature Dependence of Y i e l d Stress Graphs of log y i e l d stress versus r e c i p r o c a l temperature are employed to i l l u s t r a t e the e f f e c t s of varying i n t e r s t i t i a l contents, since st r a i g h t l i n e p l o t s are b e t t e r suited f o r v i s u a l examination i n t h i s instance. Comparisons w i l l be made by r e f e r r i n g to Figures 1$, page 35> 25, page 45; and 30> page 51; f o r hydrogen, oxygen and nitrogen r e s p e c t i v e l y . Hydrogen, oxygen and nitrogen a l l produced an increase i n y i e l d strength over the temperature range investigated i n t h i s work. Over the range of concentration of the three i n t e r s t i t i a l s investigated, the anomaly was not eliminated, nor was the temperature at which i t occurred a l t e r e d more than 15°C. In each case the change i n the point of i n t e r s e c t i o n was to a lower temperature. The movement of the i n t e r s e c t i o n with respect to temperature was governed by the r e l a t i v e y i e l d strength increases above and below the anomaly. The r e l a t i v e slopes of the high and low temperature portions of the curve also governed the movement of the point of i n t e r -section when the i n t e r s t i t i a l content was increased. At temperatures below the anomaly the slope of the o r i g i n a l curve was not a l t e r e d by hydrogen or nitrogen but was s l i g h t l y reduced by oxygen, although the data f o r oxygen i s i n s u f f i c i e n t to make a f i r m statement. However, i t may be concluded that the temperature dependence of y i e l d stress below the anomaly i s not a f f e c t e d by hydrogen or nitrogen and possibly oxygen. The graphical comparisons of Figure 3 -^j and the r e s u l t s of t h i s i n v e s t i g a t i o n - 63 -suggest that carbon would not a f f e c t the temperature dependence of y i e l d strength at temperatures below the anomaly. At temperatures above the anomaly the slope was a l t e r e d i n each case, depending on the element and l e v e l of i n t e r s t i t i a l content. The curves f o r oxygen and nitrogen i n d i c a t e that there i s a l i m i t i n g value of i n t e r s t i t i a l content and that once t h i s value i s exceeded, the slope i s not a f f e c t e d f u r t h e r . In t h i s case, i t i s apparent that the l i m i t i n g l e v e l f o r hydrogen was not reached at a l e v e l of 300 ppm. The change of slope may be i n t e r p r e t e d as a decrease i n the temperature dependence-of y i e l d strength. Above the l i m i t i n g value, the temperature dependence became constant with increasing i n t e r s t i t i a l content. Interpretation of the apparent l i m i t i n g a f f e c t above the anomaly involves three assumptions: 1. The f i r s t requires a proportional f i l l i n g of the two types of i n t e r s t i t i a l s i t e s , (-g- 0) and \ 0), which are 23 described by Barrett . The r a t i o of occupancy of (-g- jj- 0) to (•§ 0) must be close to 1 f o r the proposed mechanism to be v a l i d . 2. The second requires that there i s a d i f f e r e n c e i n i n t e r -a ction between d i s l o c a t i o n s and i n t e r s t i t i a l s i n the two d i f f e r e n t l a t t i c p o s i t i o n s . This i s not unreasonable as the (-|- \ 0) p o s i t i o n s are the smaller of the two and hence w i l l cause more d i s t o r t i o n . 3» In order that the concentration l e v e l s required to produce a l a t t i c e d i s t o r t i o n can be attained within the range of concentration investigated i n t h i s work, i t i s necessary f o r i n t e r s t i t i a l segregation to take place. I t i s expected that the required segregation w i l l take place around d i s l o c a t i o n p . - 6k -Then, at a c e r t a i n concentration of solute i n the smaller (•§-§• 0) holes, the stress l e v e l due to l a t t i c e d i s t o r t i o n would p r o h i b i t f u r t h e r f i l l i n g of the (•§• \ 0) holes, and only (•§• \ 0) holes would continue to be f i l l e d . Thereafter an increase i n concentration would produce a pro-p o r t i o n a l l y constant change i n y i e l d s t r e s s . When lower t e s t temperatures were employed, the 0) equilibrium concentration l e v e l would be reduced, producing a correspondingly smaller increase i n y i e l d stress before the saturation l e v e l was reached. At temperatures below the break i n the curve, the saturation concentrations would have to be lower than those of the high p u r i t y single c r y s t a l s i n order that the proposed mechanism be v a l i d . The f a c t that both slopes are a f f e c t e d by each i n t e r s t i t i a l employed finds agreement with Snowball's"^ proposal i n that the curves are displaced v e r t i c a l l y , but h i s proposal that the two slopes are displaced by d i f f e r e n t impurities does not appear to be c o r r e c t . C. E f f e c t of I n t e r s t i t i a l Content on D u c t i l i t y Figures 18, page 36; and 31* page 53j i l l u s t r a t e ; , the e f f e c t s of hydrogen and nitrogen, r e s p e c t i v e l y , on elongation. Figures 19; page 38,' and 26, page k6'} i l l u s t r a t e the e f f e c t of hydrogen and oxygen on the reduction i n area. The e f f e c t of hydrogen on the d u c t i l i t y t r a n s i t i o n temperature i s not c l e a r from Figure 18, but a general decrease i n the values of elongation both above and below the t r a n s i t i o n temperature i s apparent. Nitrogen e x h i b i t s abnormal behaviour i n that the t r a n s i t i o n temperature i s lowered by increased nitrogen content whereas normally, increased i n t e r s t i t i a l contents r a i s e the t r a n s i t i o n temperature. This e f f e c t cannot be s a t i s f a c t o r i l y explained at the present time. -65 Both hydrogen and oxygen induce a recovery i n reduction i n area, hydrogen producing the most pronounced e f f e c t . This recovery due to ' lk hydrogen has been reported i n vanadium by Eustice and Carlson ; and 19 Roberts and Rogers and i s a t t r i b u t e d to slow s t r a i n aging embrittlement. Both hydrogen and oxygen r a i s e the reduction i n area t r a n s i t i o n temp-erature which has been reported to occur i n other body-centred cubic metals. I t should be noted that there i s not an apparent r e l a t i o n s h i p between the d u c t i l i t y t r a n s i t i o n temperature and the temperature of the anomalous break i n the curve. D. Deformation Mechanisms Cleavage of the high p u r i t y niobium si n g l e c r y s t a l s was not detected at any temperature; however, both hydrogen and oxygen i n i t i a t e d cleavage at approximately -25°;'-:to -115°C. Nitrogen d i d not i n i t i a t e cleavage i n the concentration range investigated. The observance of ^.1^ ( l 1 2 } slip^and both [lOo] and [jLloJ cleavage i h doped niobium single c r y s t a l s agrees with the r e s u l t s of several i n v e s t i g a t i o n s 1 2 ' ^ ' 2 0 . Cleavage of hydrogenated specimens at -197°C on a [112] plane i s be l i e v e d to have been i n i t i a t e d by twinning. This explains why the y i e l d stress values f o r these specimens are much lower than expected. Additions of hydrogen-and nitrogen-induced twinning on {112] planes at -186° and -197°C. Oxygen altogether i n h i b i t e d the formation of twins at low temperatures, which i s i n agreement with the fin d i n g s of 21 Leadbetter and Argent ". Twinning and the e f f e c t of i n t e r s t i t i a l impurities on twinning i s not f u l l y understood i n the body-centred cubic metals, but the r e s u l t s of the present work i n d i c a t e that not a l l i n t e r s t i t i a l s encourage twinning. - 66 -E. Load-Elongation Curves The observed i r r e g u l a r nature of the higher temperature curves 26 has also been reported by Schoeck and Seeger . They have interpreted t h i s type of occurrence as the Snoek-effect, implying that the i n t e r -s t i t i a l atoms responsible are r e d i s t r i b u t i n g at a rate that would permit the re-anchoring of d i s l o c a t i o n s . I f t h e i r i n t e r p r e t a t i o n i s correct, i t i s an i n d i c a t i o n that hydrogen and nitrogen are mobile down to approx-imately -75°C> which i s unusual. F. Explanation of Anomalous Behaviour of the Temperature Dependence of Y i e l d Stress Two possible mechanisms w i l l be considered i n an e f f o r t to explain q u a l i t a t i v e l y the anomalous dependence of y i e l d stress on temperature. The f i r s t , based on i n t e r s t i t i a l orderingjWOuld involve a more d e t a i l e d i n v e s t i g a t i o n before i t could be regarded with any degree of c e r t a i n t y . The second i s based on d i s l o c a t i o n - s o l u t e i n t e r a c t i o n and i s to be regarded as the most s a t i s f a c t o r y of the two explanations. 1. I n t e r s t i t i a l Ordering I t has been proposed 1 that the anomalous behaviour of the r e -l a t i o n s h i p between y i e l d stress and temperature may be q u a l i t a t i v e l y explained by assuming that ordering or a movement of i n t e r s t i t i a l atoms away from d i s l o c a t i o n s takes place i n the region of the break i n the y i e l d s t r e s s -temperature curve. This depletion of the atmosphere surrounding a d i s -l o c a t i o n would then r e s u l t i n a lower stress necessary to free the d i s -l o c a t i o n , thereby decreasing the temperature dependence. The; y i e l d stress-temperature curve would then become " f l a t t e r " at the temperature at which the ordering takes place. Further reductions i n temperature would produce a curve analogous to the f i r s t . The fore-going d e s c r i p t i o n i s i l l u s t r a t e d i n Figure 35• where region "A" i s the temperature i n t e r v a l over which the ordering takes place. - 67 -Temperature (°C.) Figure 35. Hypothetical Curve of Stress versus RuuijuU. u i j i l Abeolutc Temperature •H o 50 IOO 150 200 250 300 350 Temperature (°K.) Figure 36. Temperature V a r i a t i o n of the Temperature-Dependent Part of the L a t t i c e F r i c t i o n Stress. A f t e r Heslop and Petch . - 68 -The magnitude of the thermal f l u c t u a t i o n s which contribute to the t o t a l energy required f o r d i s l o c a t i o n release .is. assumed to decrease continuously between +25° and. -197°C i n the c l a s s i c a l manner and there-fore are not connected with the anomaly. The primary mechanism which impedes the movement of d i s l o c a t i o n s i n the temperature region below i n t e r v a l "A" (Figure 35 •) i s thought to 2k be the Peierls-Nabarro s t r e s s . Heslop and Petch have suggested that with decreasing temperatures, d i s l o c a t i o n s i n body-centred l a t t i c e s become progressively narrower^, and as a r e s u l t , i n c r e a s i n g l y d i f f i c u l t to move. As a manifestation of t h i s they showed^in t h e i r analysis of the y i e l d point of i r o n , a highly temperature dependent term (Figure 36.) which they r e l a t e d to forces opposing d i s l o c a t i o n motions, the P e i e r l s -7 10 Nabarro fo r c e s . C o t t r e l l , and Mincher and Sheely have u t i l i z e d the r e s u l t s of Heslop and Petch i n explanation of the extreme temperature dependence of the y i e l d stress at low temperatures i n body-centred cubic metals. Although the emphasis has been placed upon a highly temperature dependent f r i c t i o n a l s tress, i t i s assumed that segregated solute atoms w i l l also tend to i n t e r f e r e with the movement of d i s l o c a t i o n s . The extent of interference i s expected to be reduced as the i n t e r s t i t i a l atoms are f a r t h e r from the d i s l o c a t i o n s . 28 Recent experimental r e s u l t s of Seraphim indi c a t e that ordering occurs i n tantalum containing i n t e r s t i t i a l nitrogen. Conc-entration l e v e l s as low as 65 ppm nitrogen have given i n d i c a t i o n s that a super l a t t i c e e x i s t s i n tantalum. However, the data presented by qm Seraphim ss not complete, e s p e c i a l l y data regarding the temperature dependence of the existence of the super l a t t i c e . - 69 -There are several inconsistencies with the above theory which indicate that without more experimental evidence the theory can not be accepted as v a l i d . The following experimental and t h e o r e t i c a l f a c t s are the most l i k e l y to v i t i a t e the theory. i ) Most ordering processes r e s u l t i n increases rather than i n decreases i n y i e l d strength. i i ) E n e r g e t i c a l l y i t i s u n l i k e l y at low temperatures that i n t e r s t i t i a l atoms would move away from d i s l o c a t i o n s , unless high values of compressional stresses e x i s t due to i n t e r s t i t i a l m i s f i t . i i i ) At the temperatures involved, .diffusion of atoms f o r more than a distance equal to a few l a t t i c e parameters i s not very probable. i v ) Preliminary e l e c t r i c a l resistance measurements of niobium single c r y s t a l s by C l a y t o n 2 ^ and those f o r vanadium by Snowball 1 e x h i b i t a change i n slope at the temperature at which the y i e l d stress-temperature curves i n t e r s e c t . However, the change i s i n a d i r e c t i o n that would i n d i c a t e that disordering, rather than ordering is.'taking, .place. v) Another inconsistency with the above theory i s that, upon a rapid change of bath temperature from -197° "to -l46°C y an- .unusually high y i e l d strength i s observed. As the rate of change of temperature between the two baths decreases, the y i e l d strength observed also decreases, approaching the equilibrium value. This result; i s i n d i c a t i v e of ordering but the degree of ordering must increase l i n e a r l y with decreasing temperature. However, the theory requires that ordering take place within a small temperature range. 2. Dislocation-Solute Interaction I t i s proposed that screw and edge d i s l o c a t i o n s i n t e r a c t with solute atoms over d i f f e r e n t temperature ranges. This proposal does not require d i f f e r e n t i a t i o n between species of atoms i n t e r a c t i n g with - 70 -d i s l o c a t i o n s as suggested by Snowball"'" and therefore conforms to the r e s u l t s of the present i n v e s t i g a t i o n . In h i s o r i g i n a l treatment of the formation of an atmosphere around d i s l o c a t i o n s , C o t t r e l l ^ took the l a t t i c e d i s t o r t i o n around a solute atom to be that due to a center of d i l a t a t i o n . This gave r i s e to an i n t e r a c t i o n with edge d i s l o c a t i o n s only, since the l a t t i c e d i s t o r t i o n around screw d i s l o c a t i o n s i s free of d i l a t a t i o n i f non-linear e l a s t i c e f f e c t s are neglected. In f a c t , due to the tetragonal d i s t o r t i o n around an i n t e r s t i t i a l atom i n body-centred cubic metals, there w i l l also be an i n t e r a c t i o n with the shear stresses around a screw d i s l o c a t i o n . This w i l l lead to the formation of an atmosphere around screw d i s l o c a t i o n s also. The p o s s i b i l i t y of screw d i s l o c a t i o n s taking part i n the y i e l d i n g phenomenon was neglected by F i s h e r ^ . The r e s u l t s of t h i s i n v e s t i g a t i o n i n d i c a t e that the Fisher i n t e r p r e t a t i o n of y i e l d i n g should be modified i f the mobility and locking c h a r a c t e r i s t i c s d i f f e r f o r screw and edge d i s l o c a t i o n s . I t now becomes necessary to e s t a b l i s h the r e l a t i v e m o b i l i t i e s of screw and edge d i s l o c a t i o n s at temperatures above and below the anomaly. The f o l l o w i n g reasons are presented as i n d i c a t i o n s that screw d i s l o c a t i o n s are most l i k e l y to be operative at temperatures above the anomaly. i ) T h e o r e t i c a l calculations of Cochardt 2^ et a l . and Schoeck and Seeger i n d i c a t e that the binding energy of i n t e r s t i t i a l l y saturated screw d i s l o c a t i o n s i s about 20 percent of the l a t t i c e s t r a i n energy while that f o r edge d i s l o c a t i o n s i s about 10 percent of the l a t t i c e s t r a i n energy. The i n t e r a c t i o n energy between both edge and screw d i s l o c a t i o n s and t h e i r associated atmospheres was calculated to be 0.75 eV. However the s p a t i e a l arrangement of the two types of atmospheres are quite d i f f e r e n t and i t i s t h i s f a c t that i s most l i k e l y to support a theory of t h i s type. - 71 -C o t t r e l l ' s o r i g i n a l concept of the atmosphere associated with 2 "5 26 edge d i s l o c a t i o n s has been a l t e r e d ' i n that a high density of i n t e r -s t i t i a l atoms e x i s t s on the s l i p plane on ei t h e r side of the d i s l o c a t i o n as opposed to a sing l e atom l y i n g d i r e c t l y below the d i s l o c a t i o n . A s i m i l a r mathematical an a l y s i s of the atmosphere associated with a screw d i s l o p a t i o n i n d i c a t e s that a c i r c u l a r d i s t r i b u t i o n of i n t e r s t i t i a l atoms e x i s t but that there are three positions of lowest energy r a d i a l l y d i s -o t r i b u t e d at 120 i n t e r v a l s . Therefore i t may reasonably be assumed that screw d i s l o c a t i o n s , due to t h e i r a b i l i t y to move on any plane, are able to become free of t h e i r atmospheres more e a s i l y than edge d i s l o c a t i o n s . An edge d i s -l o c a t i o n can move only on the plane defined by the l i n e of the d i s l o c a t i o n and i t s burgers vector, which implies that the d i s l o c a t i o n must move through the region containing a high density of "locking" atoms. At lower temperatures i t i s l i k e l y that^due to thermal con-t r a c t i o n , the atmosphere of a screw d i s l o c a t i o n w i l l become more evenly d i s t r i b u t e d , r e s u l t i n g i n a decrease i n the ease with which the d i s -l o c a t i o n i s able to escape from i t s atmosphere. Concurrently, a reduction i n temperature i s not l i k e l y to reduce the a b i l i t y of an edge d i s l o c a t i o n to free i t s e l f from i t s atmosphere. The above assumptions imply that screw d i s l o c a t i o n s are more e a s i l y released at high temperatures but the stress required f o r release would increase r a p i d l y with decreasing temperatures. At a c e r t a i n stress l e v e l edge d i s l o c a t i o n s would become f r e e and the required increase i n stress f o r a given decrease i n temperature would become l e s s , decreasing the temperature dependence of y i e l d s t r e s s . Further decreases i n temp-erature would require i n c r e a s i n g l y higher stress l e v e l s to cause y i e l d i n g , - 72 -r e s u l t i n g i n an increased temperature dependence as described by C o t t r e l l . The v a r i a t i o n of y i e l d stress with temperature would then be analagous to that determined i n t h i s i n v e s t i g a t i o n . The highly temperature dependent , Peirels-Nabarro stress i s also expected to cause an increase i n the stress necessary to move a d i s l o c a t i o n as the temperature i s decreased. i i ) I t may be expected that both screw and edge d i s l o c a t i o n s could acquire jogs during the p r e y i e l d p l a s t i c s t r a i n that occurs p r i o r to abrupt y i e l d i n g . The presence of a jog i n a screw d i s l o c a t i o n , as 27 described by Read , would oppose the motion of the d i s l o c a t i o n . The drag created by the jog i s due to the nature of the climbing motion of the jog. Climb then requires the formation of vacancies which involves d i f f u s i o n . As t e n s i l e t e s t i n g temperatures are decreased, movement of jogged screw, d i s l o c a t i o n s become in c r e a s i n g l y d i f f i c u l t . At the same time the movement of edge d i s l o c a t i o n s containing jogs does not require a dif f u s i o n , process. Hence decreasing temperatures w i l l not s i m i l a r l y impede the movement of edge d i s l o c a t i o n s to the same extent. Both types of d i s l o c a t i o n s are subject to the decrease i n thermal f l u c t u a t i o n s as the temperature i s lowered, and would therefore require s i m i l a r increases i n stress to cause y i e l d i n g . In the general case^the movement of d i s l o c a t i o n loops must be considered. Due to the mixed nature of the d i s l o c a t i o n loop (having both screw and edge components) i t i s necessary that the loop take the form of an e l l i p s e as s l i p progresses. This requirement i s not unreasonable and must be assoicated with a theory of t h i s type. - 73 -VI. CONCLUSIONS 1. High p u r i t y niobium single c r y s t a l s of [lio] a x i a l d i r e c t i o n were grown by the floating-zone m e l t - s o l i d i f i c a t i o n .technique. Hydrogen, oxygen and nitrogen were su c c e s s f u l l y removed during zone-refining; carbon.however, was not removed. 2. Tne.hardness of niobium sin g l e c r y s t a l s has been found to vary both with r o t a t i o n about the a x i a l d i r e c t i o n and with r o t a t i o n about an axis perpendicular to the a x i a l d i r e c t i o n of the c r y s t a l . 3« High p u r i t y niobium single crystals', exhibited a pronounced temperature dependence of y i e l d stress below +25°C. The r e l a t i o n s h i p , however, was anomalous at approximately -100°C. where two smooth curves i n t e r s e c t e d . k. The s l i p system f o r high p u r i t y niobium single c r y s t a l s was found to be (^ 111^  {^l 2} over the en t i r e range of temperature. Therefore the abnormal r e l a t i o n between y i e l d stress and temperature could not have been caused by a change i n s l i p system. 5. The y i e l d stress was increased by1 the i n t e r s t i t i a l s hydrogen, oxygen and nitrogen. At temperaturesabove the anomaly the i n i t i a l e f f e c t of the i n t e r s t i t i a l s was to decrease the temperature dependence of y i e l d stress'. Above an undetermined concentration the temperature dependence was not a l t e r e d f u r t h e r . At temperatures below the anomaly the temperature dependence of y i e l d stress was not a l t e r e d within the range of concentration investigated. 6. The i n i t i a l decrease of the temperature dependence of y i e l d stress at temperatures above the anomaly was proposed to be due to a change i n the proportional occupancy of (-§• \ 0) and (•§• \ 0) i n t e r s t i t i a l holes. - 7k -7. Twins were not found in.the high p u r i t y niobium si n g l e c r y s t a l s at any temperature; however increased hydrogen and nitrogen contents promoted twinning at -197°C« Twinning was b e l i e v e d to have encouraged cleavage at -197°0. i n a sample containing 300 ppm. hydrogen. 8. Cleavage was found to be i n i t i a t e d by i n c r e a s i n g i n t e r s t i t i a l content and occurred on {lOo} and [ l i o ] planes without detectable preference f o r e i t h e r set of planes. 9- A p a r t i a l recovery i n reduction i n area was exhibited by specimens containing 100 and 300 ppm hydrogen. This e f f e c t has been reported to occur i n vanadium by another i n v e s t i g a t o r ^ . 10. A d i f f e r e n c e i n the stress required f o r edge and screw d i s l o c a t i o n s to be released from t h e i r atmosphere was proposed as an explanation of the observed y i e l d stress-temperature r e l a t i o n s h i p . The theory requires that only screw d i s l o c a t i o n s are released at temperatures above the anomaly and edge d i s l o c a t i o n s are released at temperatures below the anomaly. - 75 -VII. RECOMMENDATIONS FOR FUTURE WORK I t i s f e l t that the existence of and the e f f e c t of i n t e r s t i t i a l s on the anomaly i n other body-centred cubic metals should be f i r m l y established by d e t a i l e d experiments. The e f f e c t of carbon i n niobium should be determined. Other techniques should be developed f o r studying the-observed e f f e c t . Internal f r i c t i o n and e l e c t r i c a l resistance measurements might prove enlightening and lead to a more d e t a i l e d knowledge of d i s l o c a t i o n - s o l u t e i n t e r a c t i o n . As w e l l as determining the existence of the anomaly i n other metals, the experimentation should be extended to include p o l y c r y s t a l l i n e m a t erial i n order to determine grain boundary e f f e c t s . The e f f e c t observed as a higher than normal y i e l d stress a f t e r a ra p i d increase i n temperature should be thoroughly in v e s t i g a t e d . The mechanism responsible f o r t h i s e f f e c t may be r e l a t e d to the anomalous temp-erature dependence of y i e l d strength. 76 VIII. BIBLIOGRAPHY 1. Snowball, R. F., M.A.Sc. Thesis submitted i n the Department of Mining and Metallurgy, U n i v e r s i t y of B r i t i s h Columbia, October, i960. 2. Fraser, R. M.A.Sc. Thesis submitted i n the Department of Mining and Metallurgy, U n i v e r s i t y of B r i t i s h Columbia, November, i960. 3. C o t t r e l l , A. H., and B i l b y , B. A., " D i s l o c a t i o n Theory of Y i e l d i n g and S t r a i n Aging i n Iron", Proceedings Royal Scoiety, London, A62 (1949) 49. 4. C o t t r e l l , A. H., "Dislocations and P l a s t i c Flow i n C r y s t a l s " , Oxford U n i v e r s i t y Press, London, F i r s t E d i t i o n , (1953) 139• 5. Fisher, J . C , "Application of C o t t r e l l ' s Theory of Y i e l d i n g to Delayed Y i e l d i n S t e e l " , Transactions A.S.M., kj_ (1955) 451-462. 6. Wessel, E. T., "Abrupt Y i e l d i n g and the D u c t i l e - t o - B r i t t l e T r a n s i t i o n i n Body Centered Cubic Metals", J . of Metals, 9_, no. 7, Sec. 2 (1957) 930-935. 7. C o t t r e l l , A. H., "Theory of B r i t t l e Fracture i n Steel and Similar Metals", Transactions A.I.M.E., 212 (1958) 192. 8. Zener, C , Holloman, J . H., " E f f e c t of S t r a i n Rate upon P l a s t i c Flow of S t e e l " , J . of Applied Physics 15_ (1954) 22. 9. Lawson, V. B., M.A.Sc. Thesis submitted i n the Department of Mining and Metallurgy, U n i v e r s i t y of B r i t i s h Columbia, A p r i l , 1961. 10. Mincher, A. L., Sheely, W. F., " E f f e c t of Structure on P u r i t y of the Mechanical Properties of Columbium (Niobium)", Transactions A.I.M.E. 221 (1958) 19. 11. Dyson, B. F., Jones, R. B., Tegart, W.J. McG., "The T e n s i l e Properties of High P u r i t y Niobium at Low Temperatures", J . I n s t i t u t e of Metals 87 (1958) 3^0. 12. Adams, M. A., Roberts, A. C , Smallman, R. E., " Y i e l d and Fracture i n P o l y c r y s t a l l i n e Niobium", Acta. Met. 8 (i960) 328. 13. Loomis, B. A., Carlson, 0. N., "Investigation of the D u c t i l e - t o - B r i t t l e T r a n s i t i o n i n Vanadium", Transactions A.S.M., 52 Preprint 125 (1958). 14. Eustice, A. L., Carlson, 0..N., " E f f e c t of Hydrogen on the T e n s i l e Prop-e r t i e s of Iodide Vanadium", Transactions A.I.M.E. 221 (1961) 238. 15. B a r r e t t , C. S., Bakish, R.', "Twinning and Cleavage i n Tantalum", Trans-actions A.I.M.E., (1958) 122. 16. Schadler, H. W., "Deformation Behaviour of Zpne Melted Tungsten Single C r y s t a l s " , Transactions A.I.M.E., 2l8 (i960) 649. 17. Clough, W. R., Pavlovic, A. S., "The FJLow, Fracture, and Twinning of Commercially Pure Vanadium", Transactions A.S.M., 52, Preprint 125• - 77 -18. Wessel, E. T., Lawthers, D. D., "The D u c t i l e - T o - B r i t t l e T r a n s i t i o n i n Niobium", Westinghouse S c i e n t i f i c Paper 6-9V70I-5-PI, A p r i l 1957. 19« Roberts, B. W., Rogers, H. C , "Observations on Mechanical Properties of Hydrogenated Vanadium", Transactions A.I.M.E., 8 (1956) 1213. 20. Churchman, A.T., "Cleavage Fracture of Niobium", J . I n s t i t u t e of Metals, 88, (1959) 221. 21. Leadbetter, M. J . , Argent, B. B., "The E f f e c t of Oxygen on the Mechanical Properties of Zone-Refined Niobium", J . of Less Common Metals, 3. (1961) 19. 22. Bechtold, J . H., "Tensile Properties of Annealed Tantalum at Low Temp-eratures", Acta. Met. 3 (1955) 2^9. 23. B a r r e t t , C. S., "Structure of Metals", McGraw-Hill Publishing Co., (1952) 223« 2k. Heslop, J . , Petch, N. J . , "The Stress to Move a Free D i s l o c a t i o n i n Alpha Iron", P h i l . Mag. 1 (1956) 867. 25• Cochardt, A. W., Schoek, G., Weidersich, H., "Interaction Between Dis-l o c a t i o n s and I n t e r s t i t i a l Atoms i n Body-Centred "Cubic Metals", Acta. Met., 3 (1955) 533-26. Schoeck, G., Seeger, A., "The Flow Stress of Iron and I t s Dependence on Impurities", Acta. Met., 1 (1959) ^69. 27. • Read, W. T., "Dislocations i n C r y s t a l s " , McGraw-Hill Book Company, (1953) 79-90. 28. Seraphim, D. P., Private Communication, I.B.M. Research Center, Ploughkeepsie, New York. 29. Clayton, J . , Private Communication, Department of Mining and Metallurgy, U n i v e r s i t y of B r i t i s h Columbia. IX. APPENDICES APPENDIX I. THE RESULTS OF TENSILE TESTS High Purity Single Crystals Specimen Temperature 5C. l/T (l/°K X10 3) Elongation Reduction Y i e l d Number (percent) (area io) Stress Nb 5-1 + 21.8 3.39 43.1 100 8,030 Nb 8-1 + 22.0 3-39 41.8 100 9,250 Nb 5-3 - 197 13.15 12.6 85 106,920 Nb 6-1 - 0.8 3.64 66.9 100 11,090 Nb 7-1 - 24.3 4.01 40.6 100 15,520 Nb 7-2 - 76.6 5.09 23.2 100 31,000 39,965 Nb 6-2 - 100 5.77 21.3 100 Nb 8-3 - 125 6.75 16.2 100 47,200 Nb 7-4 - 60.0 4.70 29.0 1Q0 24,140 Nb 8-4 - 85.O 5.32 22.5 100 31,359 Nb 6-5 - 113.5 6.27 18.3 100 42,190 Nb 5-5 - 145.8 7.85 18.7 100 58,400 Nb 5-6 - 186.0 11.50 13.8 95 85,510 Nb 7-6 - 186.0 11.50 13.0 95 88,110 105,380 Nb 6^ 6 - 198 13.30 9.2 100 Nb 8-5 - 187 11.60 13.6 95 85,890 Nb 8-6 - 105 5.95 22.9 100 42,620 Nb 9-1 - 138 7.41 15.9 100 55,300 Nb 9-2 - 125.5 6.75 17.2 100 49.680 Nb 9-4 - 109.2 6.09 27.1 100 40,470 Nb 9-5 - 153.5 8.36 16.8 100 59,710 Nb 9-6 - 169.4 9-66 14.8 100 71,130 Nb 14-4 - 197 13.15 8.5 85 105,250 Nb 12-1 - 197 13.15 - 85 95,930 Nb 13-3 - 197 13.15 - 85 98,380 Nb 13-4 - 45 4-38 39.4 100 20,700 Nb 16-1 + 23-5 3-37 42.8 100 9,200 Nb 16-2 - 197 13.15 10.1 85 95,860 - 80 -B. Single Crystals Doped with Hydrogen 1. 300 Parts Per M i l l i o n Specimen Temperature l/T Elongation Reduction Y i e l d Number °C. (l/°K X103) (percent) (area <j>) Stress Nb 15-1 + 24.0 3-37 56.O 87.6 17,070 Nb 15-4 - 25.0 4.03 33-4 34.4 22,588 Nb .15-5 - 69.O 4.90 11-7 10.4 32,088 Nb 15-2 - 12.0 6-53 9-6 11.67 53,905 Nb 15-3 - 197 13.15 - 0.2 60,717 Nb 15-6 - 141.6 7.60 4.9 66,200 83,417 Nb 16-6 - 197 13.15 l . l NM Nb 16-3 -• 197 13.15 9-9 NM 86,677 Nb 16-4 - 142 7.63 10.7 7.3 60,999 Nb 16-5 - 170.5 9-75 9.7 35.2 83,551 Nb 18-1 - 150 8.13 8.4 12.7 63,369 Nb 18-2 - 163 9.85 " 9-1 29.9 78,940 Nb 18-3 - 115 6.33 10.3 12.6 50,924 Nb 18-4 88 5.4 10.5 5.7 37,560 2. 100 Parts Per M i l l i o n Nb 17-6 - 23 4.0 32.2 65.5 19,096 Nb 17-2 - 197 13.15 1.0 21.2 90,096 Nb 17-1 - 169.3 9:64 12.6 39-8 77,^39 Nb 17-5 - 140 7.52 12.9 15.8 58,179 Nb 17-4 - 70 4-93 25.4 34.8 29,103 Nb 17-3 + 26/0 3-35 32.0 84.1 12,707 Nb 19-1 - 126.8 6.83 12.2 16.4 55,000 Nb 19-2 - 9.4.2 5-59 8.8 22.3 36,088 Nb 19-3 - 160 8.84 13 A 31.5 70,500 - 81 -C. Single Crystals Doped .with Oxygen 1. 800 Parts Per M i l l i o n Specimen Temperature i / l Elongation Reduction Y i e l d Number °C. (l/°K XIO ) (percent) (area <f>) Stress 53,217 129,514 108,401 95,000 79,000 174,334 Nb 26-1 + 24 3.36 DNM 100 * Nb 26-2 - 143-5 7-72 . DNM 36.4 Nb 26-4 - 119 6.48 DNM 51.8 Nb 26-3 - 100 5-78 DNM 95 * Nb 26-5 - 70 4.92 DNM 100 * Nb 26-6 - 197 13.15 DNM 0 2. 2000 Parts Per M i l l i o n Nb 23-1 + 25 3.35 DNM 100 * 67,497 Nb 23-2 - 75 5.05 DNM 100 * 101,693 Nb 23-3 - 138.5 7.43 DNM 0 144,267 Nb 23-4 - 105 5-95 DNM NM 124,003 Nb 23-5 + 170 2.26 DNM 100 * 59,700 89,252 Nb 23-6 - 50 4.48 DNM 90 * Nb 24-4 - 121 6.57 DNM 9.9 139,231 Nb 24-6 - 197 13.15 DNM 7.3 195,002 - 8 2 D. Single Crystals Doped with Nitrogen 1 . 7 0 Parts Per M i l l i o n Specimen Temperature l/T Elongation Reduction Y i e l d Number oc_ ( l / K X 1 0 3 ) (percent) (area $ ) * Stress Nb 1 9 - 5 - 1 9 7 1 3 . 1 5 1 0 . 5 8 5 115,548 Nb 1 9 - 6 - 1 7 2 . 5 9 . 9 4 1 1 . 7 9 5 9 0 , 2 0 0 Nb 2 0 - 1 - 1 5 0 8 . 1 2 1 1 . 4 1 0 0 7 1 , 9 6 2 Nb 2 0 - 4 - 1 2 1 . 5 6 . 6 0 1 1 . 8 1 0 0 5 8 , 7 8 6 Nb 2 0 - 2 - 7 9 5 . 1 5 3 3 - 0 1 0 0 3 3 , 5 3 0 2 1 , 2 5 4 Nb 2 0 - 5 - 2 8 4 . 0 8 3 5 . 1 1 0 0 Nb 2 0 - 3 + 2 5 3 - 3 5 3 5 - 0 1 0 0 1 4 , 4 3 4 Nb 2 0 - 6 - 9 7 - 5 5 . 6 9 2 8 . 6 1 0 0 ' 4 3 , 2 0 0 Nb 1 9 - 4 - 1 3 8 . 5 7 . 4 3 6 . 5 1 0 0 6 3 , 2 0 0 2 . 8 5 Parts Per M i l l i o n Nb 2 1 - 6 I + 2 4 3 . 3 6 3 8 . 7 1 0 0 ' 1 6 , 2 5 4 Nb 2 2 - 5 - 1 9 7 1 3 . 1 5 1 2 . 8 1 0 0 1 0 3 , 4 7 8 Nb 2 2 - 2 - 1 4 2 7 . 6 3 1 3 . 2 1 0 0 7 2 , 0 9 5 Nb 2 2 - 3 - 1 2 0 6 . 5 3 1 4 . 4 1 0 0 6 2 , 0 8 7 Nb 2 1 - 5 - 9 7 - 5 5 . 7 0 2 9 - 7 1 0 0 4 7 , 3 6 3 Nb 2 1 - 4 - 7 0 4 - 9 3 3 0 . 5 1 0 0 3 5 , 3 2 9 Nb 2 2 - 4 - 2 5 4 . 0 3 3 2 . 5 1 0 0 2 2 , 9 5 5 Nb 2 2 - 6 - 1 7 1 . 5 9 . 8 7 9 2 , 3 7 0 - 83 -E. Single Crystals Subjected to a Rapid Change of Testing Temperature S p e c i m e n T e m p e r a t u r e l / T E l o n g a t i o n Reduction Y i e l d Elapsed Number °C. (l/°K X 10 3 ) (percent) (area #) Stress Time Nb 24-2 - 145 7 . 8 l 13.3 100 63,121 200 Nb 12-2 - 146 •*. 5 8.06 15.1 n 67,994 115 Nb 12.3 - 1U6 " 7.87 11.5 11 59,352 315 Nb 12-4 - 146 " 7.87 11.4 11 63,831 170 Nb 12-5 - 146 " 7.87 14 . 2 11 65,634 130 Legend * = Estimated NM = Not Measurable DNM = Did Not Measure - 8k -APPENDIX I I . Example Calculations 1. Y i e l d Stress Exapmle Nb- 7-1 P = load = 197 I t s . A = area = I . 2 6 9 0 X i o " i n 2 • C = stress <r = P/A = 197 = 15,520 p s i 1.2690 X 1 0 " 2 2 . Elongation Example Nb... 7-1 1 = i n i t i a l gauge length = 0.862 ±2 = f i n a l gauge length = 1.212 E l = percent elongation E 1 = l g " 1 1 X 100 = 1.212 - 0.862 x 1 0 0 = U o . 6 $ h 0.862 3 . Reduction i n Area Example Nb 15-4 In a l l cases the reduced area formed.an e l l i p s e M-^  = Major axis = O.1256 i n . M 2 = Minor axis = O.O887 i n . A 1 = O r i g i n a l area = 1.2750 I n 2 A2 = F i n a l area = area of e l l i p s e Ra = percent reduction i n area A 2 ~ TT"M1M2 = 3.1415 X 0.1256 X 0.0887 = O.8355 i n 2 4 Ra = ^ ^ ^ 1.2750 - 0.8355 ( X 100) = 3k.hfo A]_ 1.2750 - 86 -B. C a l c u l a t i o n of Probable Err o r 1. Y i e l d Stress P = 197 ± 1 l b . = 197 ± 0.5 i A = ir/4. (0.1271 * 0.0005) = 1.2690 X I O - 2 * .04 i = 197 * 0.5 g 1.2690 (IO**) * -'04 # = 15,520 + .54 fo = 15,520 * .84 p s i 2. Percent Elongation The a c t u a l e r r o r i s d i f f i c u l t to estimate. The erro r in' measurement _is not greater than * 2 /o. The e r r o r introduced due to the d i f f i c u l t y i n matching the broken ends exactly would not be more than + 3 Therefore the t o t a l e r r o r could not be more than + 5 or - 2 3. Reduction i n Area A X = 1.2750 .04 io A 2 = O.8355 ± .04 i Ra = A - L - A = 1.2750 - 0.8355 * .1 j = 34.4 io * .1 /0 (x 100) 1.2750 Al APPENDIX I I I . - 88 -Doping Treatments Absorption Cycle Homo genization Cycle I n t e r s t i t i a l Content ppm Temp. °C. Pressure Time hrs Temp. °C. Pressure Time hrs Hydrogen 100 1000 1 atm 6.5 cooled to room temp. immed. Hydrogen 300 1000 1 atm 9-5 cooled to room temp, immed. Oxygen 230 1000 25 / i Hg 0.5 1000 5 X 10~6mm Hg 10 Oxygen 350 1000 25 p Hg 1.0 1000 5 X 10~6mm Hg 10 Nitrogen 1000 1 atm 12 1000 25 Ji Hg 7 Recycled 70 1000 1 atm 6 1000 25 )i Hg 9 Nitrogen 1000 1 atm lk 1000 25 p- Hg 7 Recycled 1000 1 atm lk 1000 25 fi Hg 7 Recycled 85 1000 1 atm lk 1000 25 / i Hg 7 Note: Recycling was employed to avoid a high surface concentration. 

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