A study of intergranular fracture in iron using auger spectroscopy
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has long b e e n known h2 that i r o n can exhibit i n t e r g r a n u l a r e m b r i t t l e m e n t at low t e m p e r a t u r e s but the detailed m e c h a n i s m for this b e h a v i o r is not understood. The e m b r i t t l e m e n t s e e m s to be dependent on s m a l l v a r iations in c a r b o n content and on heat t r e a t m e n t . 3 Rellick and McMahon 4 have suggested that the e m b r i t t l e m e n t m a y be due to oxygen s e g r e g a t e d to the g r a i n b o u n d a r i e s whilst c a r b o n is d i s p e r s e d within the g r a i n s , this being achieved by quenching f r o m about 970 K. Quenching f r o m s u c c e s s i v e l y lower t e m p e r a t u r e s r e s u l t e d in i n c r e a s i n g d u c t i l i t y which was a t t r i b u t e d to c a r b o n s e g r e g a t i n g to the b o u n d a r i e s and i n t e r a c t i n g with the oxygen. It was also shown that the addition of a l u m i n u m p r e v e n t e d e m b r i t t l e m e n t i r r e s p e c t i v e of quenching t e m p e r a t u r e , p r e s u m a b l y by acting as an oxygen s c a v e n g e r . Jolly and Goux S have o b s e r v e d s i m i l a r effects but show evidence that s u l f u r r a t h e r than oxygen was the e m b r i t tling e l e m e n t . Honda and Taga s i n v e s t i g a t e d the effect of c a r b o n and oxygen on the f r a c t u r e of e l e c t r o l y t i c i r o n , slowly cooled f r o m 973 K. The t e n s i l e p r o p e r t i e s were independent of oxygen content in the r a n g e 0.002 to 0.016 pct but the f r a c t u r e changed f r o m t r a n s g r a n u l a r to i n t e r g r a n u l a r below a c r i t i c a l c a r b o n content of 0.002 pct. E m b r i t t l e m e n t can r e s u l t f r o m the a d s o r p t i o n of m i n o r i m p u r i t i e s at g r a i n b o u n d a r i e s , with c o n s e q u e n t r e d u c t i o n of the i n t e r f a c e e n e r g y . The s u b j e c t has r e cently b e e n r e v i e w e d by Hondros. 7 Changes in the g r a i n b o u n d a r y and s u r f a c e e n e r g y can also lead to r e d u c e d c r e e p f r a c t u r e r e s i s t a n c e since the n u c l e a t i o n and growth of c a v i t i e s depends on the e n e r g y r e q u i r e d to c r e a t e new s u r f a c e s . For i n s t a n c e T i p l e r and McLean s have shown that the addition of 0.3 at. pct a n t i m o n y to copper halved the c r e e p f r a c t u r e elongation of that m a terial. Until r e c e n t l y it was difficult or i m p o s s i b l e to obtain d i r e c t evidence of g r a i n b o u n d a r y s e g r e g a t i o n , but the technique of Auger e l e c t r o n s p e c t r o s c o p y can now be used for this p u r p o s e . This technique, for example, B. D POWELLand H. MYKURAare Senior Research Fellow, Department of Physics, and Chairman of tile School of Materials Science, respectively, Umverslty of Warwick, Coventry, England. H J WESTWOOD, formerly at the Umvers~tyof Waterloo, is now at the Dobson Research Labs, Ontario Hydro, Toronto, Canada D M. R. TAPLIN
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