A theoretical model for the elevated temperature deformation of dispersion hardened metals
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L. J. E B E R T
A t h e o r e t i c a l m o d e l is p r e s e n t e d to d e s c r i b e the e l e v a t e d t e m p e r a t u r e (above ~Tm)' d e f o r m a tion of d i s p e r s i o n h a r d e n e d m e t a l s such a s T D - n i c k e l and SAP a l l o y s . The m o d e l is b a s e d on two p r o p o s a l s : 1) Both d i s l o c a t i o n glide and c l i m b a r e influenced b y m a t r i x s t r e s s e s at s m a l l , i n c o h e r e n t , s e c o n d p h a s e d i s p e r s e d p a r t i c l e s p r o d u c e d b y s u r f a c e t e n s i o n e f f e c t s at the p a r t i c l e - m a t r i x i n t e r f a c e . 2) Two c o n c u r r e n t p r o c e s s e s m a y c o n t r i b u t e to the e l e v a t e d t e m p e r a t u r e d e f o r m a t i o n of p o l y c r y s t a l l i n e d i s p e r s i o n h a r d e n e d m e t a l s , d i s l o c a t i o n m o t i o n and diffusion c o n t r o l l e d g r a i n b o u n d a r y s l i d i n g . The m o d e l m a y e x p l a i n the o r i g i n s of high a p p a r e n t a c t i v a t i o n e n t h a l p i e s and l a r g e s t r e s s s e n s i t i v i t i e s which have b e e n o b s e r v e d in d i s p e r s i o n h a r d e n e d m e t a l s . It m a y a l s o p r o v i d e g u i d e l i n e s for o p t i m i z a t i o n of e l e v a t e d temperature strength.
D I S P E R S I O N h a r d e n e d m e t a l s such a s T D - n i c k e l and SAP a l l o y s a r e known for t h e i r i m p r o v e d e l e v a t e d t e m p e r a t u r e (above 89 properties, as comp a r e d to t h o s e of the b a s e m e t a l . H o w e v e r , no t h e o r e t i c a l d e s c r i p t i o n for the e l e v a t e d t e m p e r a t u r e d e f o r m a tion of t h e s e m a t e r i a l s is c u r r e n t l y a v a i l a b l e of s u i t a b l e g e n e r a l i t y to e x p l a i n a l l b e h a v i o r a l a s p e c t s . D i s l o c a tion m o d e l s 1-6 cannot a c c o u n t for g r a i n s i z e and s h a p e s e n s i t i v i t i e s which t h e s e m a t e r i a l s p o s s e s s . 1,7-14 G r a i n b o u n d a r y s l i d i n g m o d e l s %1s do not p r e d i c t the r o l e of the d i s p e r s e d p h a s e in the i m p r o v e m e n t of e l e v a t e d t e m p e r a t u r e p r o p e r t i e s . 9 " t ' 1 % 17 N e i t h e r c l a s s of m o d e l s can s a t i s f a c t o r i l y cope with the high a c t i v a t i o n e n t h a l p i e s t,7, lo, 16, is and l a r g e s t r e s s s e n s i t i v i t i e s 8, lo which have b e e n m e a s u r e d in d i s p e r s i o n h a r d e n e d a l l o y s . The p r e s e n t t h e o r e t i c a l m o d e l e v o l v e d f r o m an e x a m i n a t i o n of g r a i n s i z e and s h a p e e f f e c t s on the e l e v a t e d t e m p e r a t u r e t e n s i l e and c r e e p d e f o r m a t i o n of T D - n i c k e l . 19 The m o d e l r e s u l t s f r o m two p r o p o s a l s with r e g a r d to the e l e v a t e d t e m p e r a t u r e d e f o r m a t i o n of d i s p e r s i o n h a r d e n e d m e t a l s . PROPOSAL
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