The temperature dependence of stacking fault energy in Fe-Cr-Ni alloys
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T H E s t a c k i n g fault e n e r g y is an i m p o r t a n t m a t e r i a l s p a r a m e t e r , d e t e r m i n i n g to v a r i o u s d e g r e e s such p r o p e r t i e s as s t r e s s c o r r o s i o n c r a c k i n g s u s c e p t i b i l i t y , ~ c r e e p r e s i s t a n c e , 2 and v a r i o u s m e c h a n i c a l p r o p e r t i e s . H o w e v e r , j u d g m e n t s of the m e r i t s of a p a r t i c u l a r m a t e r i a l in an e l e v a t e d t e m p e r a t u r e a p p l i c a t i o n can often be made only on the b a s i s of r o o m t e m p e r a t u r e m e a s u r e m e n t s of the s t a c k i n g fault e n e r g y . It h a s been shown, in f act , that the i n t r i n s i c s t a c k i n g fault e n e r g y (~i) of s e v e r a l a l l o y s a-s v a r i e s with t h e r m a l h i s t o r y : A r e v e r s i b l e i n c r e a s e in 7i with i n c r e a s e d t e m p e r a t u r e has been o b s e r v e d in f c c N i - C o a l l o y s , a I r r e v e r s i b l e c h a n g e s on a n n e a l i n g have been r e p o r t e d in f c c A g - Sn 4 and c o p p e r - b a s e d a l l o y s , s In the c a s e of the c o m m e r cially i m p o r t a n t F e - C r - N i s y s t e m ( s t a i n l e s s s t e e l s ) the t w i n - d e n s i t y m e a s u r e m e n t s of Cowan and Staehle ~ indicate the i m p o r t a n c e of c o n s i d e r i n g the t e m p e r a t u r e d e p e n d e n c e of s t a c k i n g f a u lt e n e r g y . T h e y found that the f r e q u e n c y of twinning i n c r e a s e d with i n c r e a s i n g n i c k e l c o n c e n t r a t i o n in a s e r i e s of t h e s e a l l o y s , s u g g e s t i n g that the twin boundary e n e r g y ?,t d e c r e a s e d with i n c r e a s i n g n i c k e l c o n c e n t r a t i o n . D i s l o c a t i o n node m e a s u r e m e nts,7 h o w e v e r , had p r e v i o u s l y i n d ic a t ed an i n c r e a s e in ~i with n i c k e l content. Since ~i oc )'t in many m a t e r i a l s , 8 t h e s e o b s e r v a t i o n s a r e in c o n f l i c t . Cowan and Staehle e m p h a s i z e d , h o w e v e r , that Vi had been d e t e r m i n e d at r o o m t e m p e r a t u r e w h e r e a s twin f r e q u e n c y d e t e r m i n a t i o n s a r e m a d e upon m i c r o s t r u e t u r e s d e ve ! o p ed at h i g h e r (1090~ a n n e a l i n g t e m p e r a t u r e s . T h e y s u g g e s t e d that the dependence of 7~ on T may have d i f f e r e d f r o m one a l l o y to the next, a c c o u n t i n g f o r t h e i r ob se r v a t i o n s . In the p r e s e n t w o r k , the v a r i a t i o n of Vi with t e m p e r a ture f o r two F e - C r - N i a l l o y s h a s b e e n d e t e r m i n e d f r o m d i s l o c a t i o n node m e a s u r e m e n t s o v e r the r a n g e 25 ~ to 325~ T h e s e t e m p e r a t u r e s a r e f a r b e lo w the usu al ann e a l i n g point f o r s t a i n l e s s s t e e l s , y e t a s i g n i f i c a n t r e v e r s i b l e t e m p e r a t u r e dependence is o b s e r v e d . R. M. LATANISION, formerly NAS-NAE
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