Effects of stress coarsening on coherent particle strengthening
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AND
R. P. GAMBLE
We have d e t e r m i n e d the effects of s t r e s s c o a r s e n i n g of c o h e r e n t , 7 ' [Ni3 (A1, X)] p r e c i p i t a t e s on the yield b e h a v i o r of n i c k e l - b a s e s u p e r a l l o y single c r y s t a l s . In so doing, we have also d e t e r m i n e d the i n f l u e n c e of c o h e r e n t p r e c i p i t a t e shape on yield s t r e n g t h , and the s t r e n g t h e n i n g r o l e of d i s l o c a t i o n s u b s t r u c t u r e s i n n i c k e l - b a s e s u p e r a l l o y s . The 7 ' m o r p h o l o g i e s c o n s i d e r e d a r e l a r g e p l a t e s , long r o d s , and the f a m i l i a r cuboids. T h e s e m o r p h o l o g i e s w e r e obtained by a n n e a l i n g the c r y s t a l s u n d e r applied t e n s i l e s t r e s s , applied c o m p r e s s i v e s t r e s s , and z e r o s t r e s s c o n d i t i o n s , r e s p e c t i v e l y . The (100} t e n s i l e t e s t axes w e r e p e r p e n d i c u l a r to the 7 ' p l a t e s and p a r a l l e l to the 7' r o d s . Between r o o m t e m p e r a t u r e and 1400~ the 7 ' rod and 7 ' plate m o r phologies a r e found to i n c r e a s e the yield s t r e n g t h of the c r y s t a l s over that due to the 7 ' cuboids by about 10 k s i (10 pct) and 30 k s i (35 pct), r e s p e c t i v e l y . Above 1400~ the yield s t r e n g t h s of the c r y s t a l s a r e found to be e s s e n t i a l l y i n d e p e n d e n t of p r e c i p i t a t e m o r p h o l o g y . The i n c r e a s e in yield s t r e n g t h in going f r o m c r y s t a l s with 7 ' cuboids to those with 7 ' r o d s is shown to be due to d i s l o c a t i o n n e t w o r k s , which a r e p r o d u c e d as a n a t u r a l c o n s e q u e n c e of a n n e a l i n g u n d e r s t r e s s . A s i m p l e a n a l y s i s of the i n t e r a c t i o n b e t w e e n d i s l o c a t i o n s and the p r e c i p i t a t e s show that the i n c r e a s e in yield s t r e n g t h in going f r o m 7 ' r o d s to 7' p l a t e s is due to d i s l o c a t i o n line t e n s i o n and c o n s i s t e n t with the d i f f e r e n c e in the shape of these c o h e r e n t and o r d e r e d p r e c i p i t a t e s . The r e s u i t s of this i n v e s t i g a t i o n a r e c o m f o r t i n g in that they show c r e e p a p p l i c a t i o n s should not d e grade the s t r e n g t h of n i c k e l - b a s e s u p e r a l l o y single c r y s t a l s . The r e s u l t s a r e also d i s c u s s e d with r e s p e c t to p o l y c r y s t a l l i n e n i c k e l - b a s e s u p e r a l l o y s and c o m p o s i t e s t r u c t u r e s of 7 and 7 ' phases.
STRESS-induced
c h e m i c a l effects a r e of i n t e r e s t in s t r u c t u r a l m a t e r i a l s s u b j e c t e d to load at high t e m p e r a t u r e s for long t i m e s . Of s p e c i a l i m p o r t a n c e is the i n f l u e n c e of applied s t r e s s on the c o a r s e n i n g (Ostwaldr i p e n i n g ) b e h a v i o r of s t r e n g t h e n i n g p r e c i p i t a t e s . A c
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