Sulfur segregation to grain boundaries in Ni 3 Al and Ni 3 (AI,Ti) alloys
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S O L U T E and i m p u r i t y s e g r e g a t i o n to g r a i n b o u n d a r i e s in p o l y c r y s t a l l i n e solids is known to be an i m p o r t a n t factor in d e t e r m i n i n g the e n g i n e e r i n g p e r f o r m a n c e of these m a t e r i a l s . I n t e r g r a n u l a r f r a c t u r e r e s u l t i n g f r o m i m p u r i t y s e g r e g a t i o n to g r a i n b o u n d a r i e s is one of the most s t r i k i n g e x a m p l e s of such p h e n o m e n a . I~ I n t e r granular corrosion, 4 stress rupture, ~ recrystallization, and g r a i n growth 6 a r e a m o n g other p h e n o m e n a likely to be affected by i m p u r i t y s e g r e g a t i o n to g r a i n boundaries, B e c a u s e of t h e i r technological i m p o r t a n c e for hight e m p e r a t u r e a p p l i c a t i o n s , n i c k e l - b a s e alloys have been studied e x t e n s i v e l y with r e s p e c t to the effect of t r a c e i m p u r i t i e s on h i g h - t e m p e r a t u r e ductility. S e v e r a l s t u d i e s 7-9 have shown that e l e m e n t s such as S, As, Se, Sn, Sb, Te, T1, Pb, and Bi a r e h a r m f u l to the ductility of n i c k e l - b a s e alloys, even when p r e s e n t at levels of a few ppm. F i g u r e 1 shows the r e s u l t s of L o z i n s k i y e t al 9 and i l l u s t r a t e s the d r a s t i c effect of a few ppm S on the h i g h - t e m p e r a t u r e d u c t i l i t y of Ni-S a l l o y s . E x a m i n ing these r e s u l t s along with the phase e q u i l i b r i a between Ni and S~~ (Fig. 2), one concludes that the effects of up to 70 at. ppm S in the t e m p e r a t u r e r a n g e between 800 and 1000~ cannot be a t t r i b u t e d to p r e c i p i tation of Ni3S2; hence, s e g r e g a t i o n without p r e c i p i t a tion of the S a p p e a r s to be an explanation for the low d u c t i I i t i e s in Fig. 1. In the p r e s e n t study, s u l f u r s e g r e g a t i o n to the g r a i n b o u n d a r i e s in Ni3A1 and Ni3(A1, Ti) alloys was o b s e r v e d u s i n g A u g e r E l e c t r o n Spectroscopy (AES). The exp e r i m e n t a l l y o b s e r v e d s e g r e g a t i o n has b e e n c h a r a c t e r i z e d in t e r m s of effective v a l u e s of solute b i n d i n g e n e r g y for g r a i n b o u n d a r i e s , As as defined by a t r e a t m e n t due to M c L e a n , ~2 The effective b i n d i n g e n e r g y of s u l f u r to the g r a i n b o u n d a r i e s , however, is t e m p e r a t u r e dependent, in c o n t r a s t to the a s s u m p t i o n made by M c L e a n . The sign and magnitude of the t e m p e r a t u r e dependence of A~eff can be explained by a s t r a i g h t f o r w a r d e x t e n s i o n of M c L e a n ' s a p p r o a c h that p a r t i a l l y a c c o u n t s for the d i s o r d e r e d n a t u r e of high angle g r a i n b o u n d a r i e s .
SOLUTE SEGREGATION TO GRAIN BOUNDARIES The s e g r e g a t i o n of solute atoms to g r a i n b o u n d a r y r e g i o n s in p o l y c r y s t a l l i n e m e t a l s has b e e n t r e a t
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