The effect of impurity interstitials on the lattice resistance to dislocation motion
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AND
T. W. CADMAN
A c o m p u t e r s i m u l a t i o n m o d e l was d e v e l o p e d to d e t e r m i n e the n a t u r e of the s y n e r g i s t i c e f f e c t b e t w e e n the l a t t i c e r e s i s t a n c e and i m p u r i t y i n t e r s t i t i a l a t o m s to d i s l o c a t i o n m o t i o n . If the i n t e r a c t i o n b e t w e e n an i m p u r i t y i n t e r s t i t i a l and a s c r e w d i s l o c a t i o n in a b c c l a t t i c e with a l a r g e P e i e r l s b a r r i e r is t r e a t e d in an e l a s t i c c o n t i n u u m m a n n e r , s t r e n g t h e n i n g w i l l o c c u r . T h i s s t r e n g t h e n i n g o c c u r s at a l l i n t e r s t i t i a l c o n c e n t r a t i o n s and at a l l t e m p e r a t u r e s in the l o w - t e m p e r a t u r e r e g i o n .
THE e l a s t i c
i n t e r a c t i o n of an i m p u r i t y i n t e r s t i t i a l a t o m (a t e t r a g o n a l d i s t o r t i o n ) with a d i s l o c a t i o n in a bc c l a t t i c e has been the s u b j e c t of n u m e r o u s i n v e s t i g a t i o n s i-4 T h e r e a s o n f o r this i n t e r e s t i s the e f f e c t i m p u r i t y i n t e r s t i t i a l s have on the y i e l d s t r e s s of bcc m e t a l s . H o w e v e r , the d e t a i l e d m e c h a n i s m o r m e c h a n i s m s by which the i m p u r i t y i n t e r s t i t i a l s m a y a f f e c t the y i e l d s t r e s s is s t i l l the s u b j e c t of s o m e c o n t r o v e r s y . 5 One m e c h a n i s m i s b a s e d on the a s s u m p t i o n that t h e r e is i n t r i n s i c s t r e n g t h e n i n g due to the l a t t i c e r e s i s t a n c e and that the e f f e c t due to i n t e r s t i t i a l s can be additive; 6-9 a n o t h e r m e c h a n i s m a s s u m e s that the l a t t i c e r e s i s t a n c e is z e r o . I~ O t h e r i n v e s t i g a t o r s have contended that i m p u r i t y i n t e r s t i t i a l s cannot have a d i r e c t e f f e c t on the m a c r o s c o p i c yieId s t r e s s of b c c m e t a l s . 5,11 None of t h e s e s t u d i e s has b e e n b a s e d on a c o n s i d e r a t i o n of the r a n d o m a r r a y of i n t e r s t i t i a l s with a p o s i t i v e and n e g a t i v e i n t e r a c t i o n b e t w e e n the i n t e r s t i t i a l and the d i s l o c a t i on . U r a k a m i and F i n e 12 d e v e l o p e d a t h e o r y of s o l i d s o l u tion weakening b a s e d on the i n t e r a c t i o n of a s p h e r i c a l d e f e c t , which i n d u c e s a t o r q u e on a s c r e w d i s l o c a t i o n . A s a r e s u l t of the t o r q u e the d e f e c t can aid in double kink n u c l e a t i o n , which can lead to s o l i d s o l u t i o n w e a k e n i n g . Sato and M e s h i i 13 a s s u m e d that an i n t e r s t i t i a l a t o m could be t r e a t e d as a s p h e r i c a l d e f e c t , i . e . the i n t e r s t i t i a l p r o d u c e s a t o r q u e on the s c r e w d i s l o c a t i o n . T h e y showed that f o r s p e c i f i c i n t e r s t i t i a l c o n c e
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