Solid solution strengthening in Nb-Ta and Nb-Mo alloy single crystals
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AND A. A. H E N D R I C K S O N
The s t r e n g t h of Nb, N b - T a , and Nb-Mo a l l o y s w as i n v e s t i g a t e d in t e n s i o n and c o m p r e s s i o n as a function of c o m p o s i t i o n , t e m p e r a t u r e , and s t r a i n r a t e . The r o o m - t e m p e r a t u r e p r o p e r t i e s show m o l y b d en u m to have a s i g n i f i c a n t e f f e c t on the s t r e n g t h of niobium w h i l e the e f f e c t s of t a n t a l u m a r e n e a r l y z e r o . The r o o m - t e m p e r a t u r e s t r e n g t h i s shown to be well c o r r e l a t e d with the a t o m i c m i s f i t p a r a m e t e r . A n a l y s i s of the l o w - t e m p e r a t u r e b e h a v i o r of N b - T a a l l o y s i n d i c a t e s that they b e h a v e s i m i l a r l y .to the p u r e m e t a l w h i l e Nb-Mo a l l o y s show a m a r k e d d e v i a t i o n f r o m t h i s b e h a v i o r . The s t r e n g t h e n i n g p h e n o m e n a o c c u r r i n g in the Nb-Mo a l l o y s at low t e m p e r a t u r e i s i n t e r p r e t e d a s being an i m p u r i t y i n t e r a c t i o n . A t r a n s i t i o n b e t w e e n the m e c h a n i s m o p e r a t i n g in the p u r e m e t a l and the i m p u r i t y i n t e r a c t i o n o c c u r s at low s o l u t e contents.
THEo b j e c t i v e of this
r e s e a r c h w a s to d e t e r m i n e the c a u s e ( s ) of solid s o l u t i o n s t r e n g t h e n i n g e f f e c t s that r e sult f r o m the addition of s u b s t i t u t i o n a l solute a t o m s to bcc m e t a l s . The i d e n t i f i c a t i o n and d e t a i l e d d e s c r i p t i o n of solid solution s t r e n g t h e n i n g m e c h a n i s m s would undoubtedly r e s u l t in e n g i n e e r i n g a l l o y s with i m p r o v e d mechanical properties. Solid solution s t r e n g t h e n i n g e f f e c t s have b e e n studied m o s t e x t e n s i v e l y in c l o s e - p a c k e d m e t a l s b e c a u s e of the r e l a t i v e e a s e in obtaining high p u r i t y s t a r t i n g m a t e r i a l s and in g r o w i n g s i n g l e c r y s t a l s . A l a r g e p o r t i o n of the t h e o r y has t h e r e f o r e b e e n d e v e l o p e d to explain the exp e r i m e n t a l findings f o r fcc and hcp solid s o l u t i o n s . T h i s body of t h e o r y , w h ic h i n v o l v e s m a i n l y v a r i o u s s o l u t e a t o m - d i s l o c a t i o n i n t e r a c t i o n s and t h e i r c o n s e q u e n c e s , should apply e q u a ll y w e l l to bcc m e t a l solid s o l u t i o n s ; solute a t o m s should have s i z e , m o d u l u s , c h e m i c a l and e l e c t r i c a l i n t e r a c t i o n s with d i s l o c a t i o n s in bcc m e t a l s . E v e n though solid s o l u t i o n s t r e n g t h e n i n g e f f e c t s in bcc m e t a l s have not b e e n i n v e s t i g a t e d e x t e n s i v e l y , it i s a p p a r e n t that t h e r e a r e i m p o r t a n t p h e n o m e n o l o g i c a l d i f f e r e n c e s b et w e e n the bcc and fcc c a s e s . The m o s t s t r
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