Effect of normal stress on yield asymmetry in high purity tantalum crystals
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T H E f a c t that the low t e m p e r a t u r e y i e l d s t r e s s in b c c m e t a l s does not o b e y the law of a c o n s t a n t c r i t i c a l r e s o l v e d s h e a r s t r e s s is now well d o c u m e n t e d . The a s y m m e t r y of the low t e m p e r a t u r e y i e l d s t r e s s with c r y s t a l o r i e n t a t i o n h a s b e e n r e p o r t e d in i r o n , m o l y b d e n u m , t u n g s t e n , niobium, and t a n t a l u m ( s e e f o r e x a m p l e , ChristianS). Although s e v e r a l m o d e l s have b e e n p r o p o s e d to e x p l a i n t h i s a s y m m e t r y , the m o s t p r o m i s i n g m o d e l s a r e b a s e d on the b e h a v i o r of e x t e n d e d s c r e w d i s l o c a t i o n s . 4 This e x t e n d e d d i s l o c a t i o n is n o n - p l a n a r and s e s s i l e in its low e n e r g y f o r m , and m u s t be t r a n s f o r m e d into a h i g h e r e n e r g y g l i s s i l e f o r m b e f o r e m o tion can o c c u r . The m o d e l can q u a l i t a t i v e l y account f o r the low m o b i l i t y of s c r e w d i s l o c a t i o n s , the t e m p e r a t u r e d e p e n d e n c e of the y i e l d s t r e s s , and the a s y m m e t r y of both the y i e l d s t r e s s and s l i p p l a n e s . The b e h a v i o r of the e x t e n d e d s c r e w d i s l o c a t i o n c o r e u n d e r the influence of a s h e a r s t r e s s has b e e n r e p o r t e d f r o m c o m p u t e r s t u d i e s f o r a v a r i e t y of i n t e r i o n i c p o t e n t i a l s . 2 A p p l i c a t i o n of such s t u d i e s to t r a n s i t i o n m e t a l s is u n c e r t a i n . In addition, t e n s i o n o r c o m p r e s s i o n t e s t s involve not only the p r e s e n c e of a s h e a r s t r e s s , but a l s o the p r e s e n c e of a n o r m a l s t r e s s . S e v e r a l i n v e s t i g a t o r s have shown that the d i r e c t i o n and m a g n i t u d e of the n o r m a l s t r e s s m a y a f f e c t the o r i e n t a t i o n d e p e n dence of the y i e l d " s t r e s s . Ravi and G i b a l a 5 found a s t r e n g t h d i f f e r e n t i a l (between s p e c i m e n s of the s a m e o r i e n t a t i o n t e s t e d in t e n s i o n and c o m p r e s s i o n ) that r e v e r s e d in sign a s the t e m p e r a t u r e was l o w e r e d f r o m 200 K to 77 K f o r n i o b i u m - o x y g e n a l l o y c r y s t a l s . S i m i l a r r e s u l t s w e r e r e p o r t e d b y Sherwood e t a l . 6 f o r v a r i ous z o n e - m e l t e d r e f r a c t o r y m e t a l c r y s t a l s . T a k e u c h i e t a l . 7 o b s e r v e d a s t r e n g t h d i f f e r e n t i a l in t a n t a l u m c r y s t a l s at 77 K. T h e s e d i f f e r e n c e s b e t w e e n the y i e l d s t r e s s m e a s u r e d in t e n s i o n and c o m p r e s s i o n have u s u a l l y b e e n a t t r i b u t e d to a n o r m a l s t r e s s effect, but G. L. WEBB, formerly Graduate Student in Metallurgy and Materials Science at Case Western Reserve University, is now Senior Engineer, Westin
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