Creep deformation of TD-nickel chromium
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AND
L. d. EBERT
The c r e e p b e h a v i o r of t h o r i a d i s p e r s e d n i c k e l - c h r o m i u m ( T D - N i C r ) was e x a m i n e d at 1093~ (2000~ M a j o r e m p h a s i s was p l a c e d on 1) the e f f e c t s of the m a t e r i a l and the t e s t r e l a t e d v a r i a b l e s ( g r a i n s i z e , t e m p e r a t u r e , s t r e s s , s t r a i n and s t r a i n r a t e ) on the d e f o r m a t i o n c h a r a c t e r i s t i c s , and 2) the e v a l u a t i o n of single c r y s t a l T D - N i C r m a t e r i a l p r o d u c e d by a d i r e c tional r e c r y s t a l l i z a t i o n technique. C r e e p a c t i v a t i o n e n t h a l p i e s w e r e found to i n c r e a s e with i n c r e a s i n g g r a i n s i z e r e a c h i n g m a x i m u m v a l u e s for the s i n g l e c r y s t a l T D - N i C r . S t r e s s exponent of the s t e a d y s t a t e c r e e p r a t e was a l s o s i g n i f i c a n t l y h i g h e r for the s i n g l e c r y s t a l m a t e r i a l a s c o m p a r e d to that d e t e r m i n e d for the p o l y c r y s t a l l i n e T D - N i C r . 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 T D - N i C r was a n a l y z e d in t e r m s of two p a r a l l e l - c o n c u r r e n t p r o c e s s e s : 1) 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 and 2) d i s l o c a t i o n motion. The c h a r a c t e r i s t i c s of the d i s l o c a t i o n m o t i o n d e f o r m a t i o n mode (as o b s e r v e d in the s i n g l e c r y s t a l T D - N i C r ) s u g g e s t that s t r o n g p a r t i c l e - d i s l o c a t i o n i n t e r a c t i o n s a r e p r e s e n t . The r e l a tive c o n t r i b u t i o n s of d i s l o c a t i o n m o t i o n and g r a i n b o u n d a r y s l i d i n g in T D - N i C r w e r e e s t i m a t e d . In c r e e p , g r a i n b o u n d a r y s l i d i n g was found to p r e d o m i n a t e for the s m a l l , e q u i a x e d g r a i n s t r u c t u r e s , w h e r e a s the d i s l o c a t i o n d e f o r m a t i o n m o d e b e c a m e s i g n i f i c a n t for only the l a r g e g r a i n T D - N i C r and the s i n g l e c r y s t a l m a t e r i a l . T H E f i r s t g e n e r a t i o n of m o d e r n e n g i n e e r i n g l y s i g n i f icant dispersion strengthened materials were those which contained a u n i f o r m d i s p e r s i o n of t h o r i a (ThO2) p a r t i c l e s in a c o m m e r c i a l l y p u r e n i c k e l m a t r i x , i.e. T D - N i c k e l . While d i s p e r s i o n s t r e n g t h e n i n g s u c c e s s fully i n c r e a s e d the t e m p e r a t u r e r a n g e of u s a b l e s t r e n g t h s of n i c k e l up to 1316~ (2400~ the p o o r oxidation r e s i s t a n c e of nickel: at t h e s e t e m p e r a t u r e s s e r i o u s l y l i m i t e d the e n g i n e e r i n g a p p l i c a t i o n of TDNickel. Consequently, r e s e a r c h e f f o r t s w e r e d i r e c t e d to the d e v e l o p m e n t of a " s e c o n d g e n e r a t i o n " of t h o r i
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