Solute segregation and brittle fracture in an alloy steel
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BRITTLE
f r a c t u r e in f e r r i t i c s t e e l s n o r m a l l y o c c u r s by cleavage at low t e m p e r a t u r e s or high s t r a i n - r a t e s . However, s e g r e g a t i o n of c e r t a i n solute e l e m e n t s to g r a i n b o u n d a r i e s can change the b r i t t l e f r a c t u r e mode to i n t e r g r a n u l a r . Since this mode o p e r a t e s at lower s t r e s s e s , b r i t t l e f r a c t u r e can then be o b s e r v e d at higher t e m p e r a t u r e s where f a i l u r e by p l a s t i c r u p t u r e would o t h e r w i s e o c c u r . The c l a s s i c a l e x a m p l e of this i s t e m p e r e m b r i t t l e m e n t of alloy s t e e l s . 1 Such e m b r i t t l e m e n t is e v i d e n t l y c a u s e d by a l o w e r i n g of i n t e r g r a n u l a r cohesion, the magnitude of which d e p e n d s on the q u a n t i t y and type of s e g r e g a t e d s o l u t e . The p h y s i c a l b a s i s of this e m b r i t t l e m e n t effect has not yet been s t u d i e d in a r i g o r o u s way. T h i s will r e q u i r e c a l c u l a t i o n s of the c o h e s i v e e n e r g y y as a f u n c tion of the p l a n a r c o n c e n t r a t i o n of the i m p o r t a n t s o l u t e s (which, i n s t e e l s , a r e the G r o u p s IVB to u elem e n t s ) . Methods for making such c a l c u l a t i o n s in 3-d t r a n s i t i o n m e t a l s a r e s t i l l u n d e r d e v e l o p m e n t . 2 Howe v e r , f r o m the m a n y m e t a l l u r g i c a l s t u d i e s of e m b r i t t l e m e n t we expect that the c a l c u l a t i o n s will r e v e a l v a r i a t i o n s in y with c o n c e n t r a t i o n of the v a r i o u s e m b r i t t l i n g e l e m e n t s as shown s c h e m a t i c a l l y in F i g . 1. Such r e d u c t i o n s a r e a n a l o g o u s , but not e q u i v a l e n t , 3 to the r e d u c t i o n s in s u r f a c e t e n s i o n due to solute s e g r e gation m e a s u r e d , for e x a m p l e , by H o n d r o s 4 and Seah and L e a . 5 F o r a f u n d a m e n t a l u n d e r s t a n d i n g of the e m b r i t t l e ment p h e n o m e n o n one would have to couple the t h e o r e t i c a l c a l c u l a t i o n s with m e a s u r e m e n t s of s o m e p r o p e r t y which v a r i e s d i r e c t l y with y, t h e r e b y to deduce how v a r i e s with the q u a n t i t y and type of s e g r e g a t e d s o l u t e . T h e s e m e a s u r e m e n t s could then be c o m p a r e d with the yet to be developed t h e o r y . If they t u r n out to be m u t u alIy c o n s i s t e n t , the t h e o r y would be s u b s t a n t i a t e d , and the p h y s i c a l b a s i s of the e m b r i t t l e m e n t effect could then be said to be u n d e r s t o o d . U n t i l the p r e s e n t t i m e e x p e r i m e n t a l s t u d i e s have u t i l i z e d m a i n l y the n o t c h e d - b a r t r a n s i t i o n t e m p e r a t u r e as the m e a s u r e of the d e g r e e of e m b r i t t l e m e n t . Howe v e r , t h e r e is no r e a s o n to b e l i e v e
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