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Comments on “About the existence of grain boundary cosegregation in pure FeMI solid solutions”
Scripta
METALLURGICA
Vol. 22, pp. 1169-1172, 1988 Printed in the U.S.A.
Pergamon Press plc All rights r e s e r v e d
COMMENTS ON "ABOUT THE E X I S T E N C E OF GRAIN B O U N D A R Y C O S E G R E G A T I O N IN PURE Fe-M-I SOLID SOLUTIONS"
Max-Planck-Institut General
Electric
H.J. Grabke fur E i s e n f o r s c h u n g
Co.,
Corp.
Research
and C.L. B r i a n t GmbH, D - 4 O O O DUsseldorf, Fed. R e p . G e r m a n y and and D e v e l o p m e n t , S c h e n e c t a d y NY 12301, U S A
(Received February 15, 1988) (Revised March 7, 1988)
Introduction In their p a p e r (I) B e r n a r d i n i and Gas c r i t i c i z e the s t a t e m e n t in an earlier p a p e r of G r a b k e et al. (2): ' c o s e g r e g a t i o n has not b e e n o b s e r v e d in s i m p l e c l e a n ternary systems Fe-M-I'. They try to prove that the systems F e - M n - P and F e - N i - S b are cl e a r e x a m p l e s for G u t t m a n n ' s m o d e l of c o s e g r e g a t i o n and that the w o r k of G r a b k e (2,3) and B r i a n t (4,5,6) on these systems is in error. However, this c r i t i c i s m is b a s e d on very w e a k e v i d e n c e as we w i l l show. M o s t a l l o y i n g effects in grain b o u n d a r y s e g r e g a t i o n in steels can now be c l e a r l y e x p l a i n e d by the site c o m p e t i t i o n of the e m b r i t t l i n g elements such as P, S and Sb and n o n - e m b r i t t l i n g e l e m e n t s such as C, N and B (7) and by the effects of the all o y i n g elements Cr, Mn, Ni etc. on the a c t i v i t i e s and s o l u b i l i t i e s of these elements. The m o s t i m p o r t a n t case of site c o m p e t i t i o n was first d e s c r i b e d by E r h a r t and G r a b k e (8), i.e. the e q u i l i b r i u m of m u t u a l d i s p l a c e m e n t of P and C at grain boundaries: P
(segregated)
+ C
(dissolved)
= P
(dissolved)
+ C
(segregated)
It was also p o i n t e d out that an a d d i t i o n of Cr, Mn and other c a r b i d e forming elements shifts this e q u i l i b r i u m to m o r e P - s e g r e g a t i o n . In the p r e v i o u s l i t e r a t u r e this e f f e c t of Cr was a s c r i b e d to c o s e g r e g a t i o n of Cr and P: however, there is no c o s e g r e g a t i o n , no m u t u a l e n h a n c e m e n t of Cr- and P - s e g r e g a t i o n in Fe-Cr-P alloys, c o m p a r e d to F e - C r and Fe-P (8). These results are a c c e p t e d now, at least by several authors (9,10). B e l o w it is e m p h a s i z e d again that the same c o n s i d e r a t i o n s apply in the s y s t e m Fe-Mn-P. Comments
on the S y s t e m F e - M n - P
Data on the grain b o u n d a r y s e g r e g a t i o n of P in F e - M n - P alloys h a v e r e c e n t l y b e e n p r e s e n t e d b y several authors: W e n g and M c M a h o n (10), Tatsumi, O k u m u r a and Y a m a m o t o (11) and G r a b k e et al. (2). The results are clear and i n d i s p u t a b l e in the case of high c a r b o n contents, a b o ut O.2%C in (11) and a b o u t O . 1 5 % C in (2) o p For such specimens, t e m p e r e d at 600 C, the grain b o u n d a r y c o n c e n t r a t i o n of n creases w i t h i n c r e a s i n g M n - c o n t e n t . This e f f e c t is clearly r e l a t e d to the dec r e a s e in C - a c t i v i t y and - s o l u b i l i t y in such alloys with i n c r e a s i n g M n - c o n t e n t . That c a r b i d e (Fe,Mn) 3C is formed for w h i c h the s o l u b i l i t y p r o d u c t d e c r e a s e s w i t h i n c r e a s i n g M n - c o n t e n t . The c o r r e s p o n d i n g d e c r e a s e in the c o n c e n t r a t i o n of dissolved c a r b o n leads to a d e c r e a s e of c a r b o n s e g r e g a t i o n and an i n c r e a s e of P - s e g regation. In their very careful study T a t s u m i et al. (11) p r o v e d the d e c r e a s e of d i s s o l v e d c a r b o n by i n t e r n a l f r i c t i o n m e a s u r e m e n t s and the d e c r e a s e of C segreg a t i o n by AES g r a i n b o u n d a r y analysis.
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Less clear are the results for very low c a r b o n contents. Tatsumi et al. (11) re p o r t e d no e f f e c t of Mn on the grain b o u n d a r y s e g r e g a t i o n of P at 7OOOC in the range 0.5 to 1.5%Mn, w i t h o u t g i v i n g further details. A l s o in the w o r k of Grabke et al. (2) no e f f e c t of 0.5 to 9 . 4 % M n could be seen at 5OO°C. Only in the third paper (10) has an increase of P grain b o u n d a r y c o n c e n t r a t i o n b e e n o b s e r v e d at 48OOC. In the c r i t i c i s m of B e r n a r d i n i and Gas (I), they s u s p e c t that the p r e s e n c e of about 30 ppm C in the specimens of the study (2) d e c r e a s e the P s e g r e g a t i o n and somehow conceal the s u p p o s e d d e p e n d e n c e on the m a n g a n e s e c o n c e n t r a t i o n . However, the c o n d i t i o n s in that study are very w e l l defined: at 500°C the m a x i m u m solub i l i t y of C in e q u i l i b r i u m w i t h Fe3C is b e l o w 20 p p m (12). So, for all the samples i n v e s t i g a t e d (see table 1 of the paper) the m a x i m u m s o l u t i o n c o n c e n t r a t i o n of C was e s t a b l i s h e d d u r i n g a n n e a l i n g so that the c o r r e s p o n d i n g g r a i n b o u n d a r y concentrations of C and P w e r e constant, i n d e p e n d e n t of c a r b o n and m a n g a n e s e c o n t e n t (13). (At the low carbon c o n c e n t r a t i o n s Mn is not i n v o l v e d in carbide p r e c i p i t a tion). B e r n a r d i n i and Gas are r i g h t that the P - s e g r e g a t i o n is d e c r e a s e d in comp a r i s o n to the study on the pure s y s t e m Fe-P (8) : however, if there was a s t r o n g c o s e g r e g a t i o n of Mn and P it should have b e e n o b s e r v e d also in the p r e s e n c e of some carbon. In contrast, the conditions in the study of W e n g and M c M a h o n (10) seem to be less well defined. The results w e r e o b t a i n e d with d e c a r b u r i z e d samples b u t it is not clear to w h a t e x t e n t the d e c a r b u r i z a t i o n was s u c c e s s f u l since the c h e m i c a l analysis after d e c a r b u r i z a t i o n gave results "which s c a t t e r e d i r r a t i o n a l l y b e t w e e n O and 28 ppm C and t h e r e f o r e w e r e judged to be u n r e l i a b l e " (10). The d e c a r b u r i z a tion w i t h w e t h y d r o g e n (saturated w i t h H20 at r o o m temperature) at 65OOC, may r a t h e r be u n r e l i a b l e since u n d e r these c o n d i t i o n s s e l e c t i v e e x t e r n a l or i n t e r n a l o x i d a t i o n of the m a n g a n e s e is p o s s i b l e in the F e - M n alloys. The o x i d a t i o n may p r e v e n t d e c a r b u r i z a t i o n and/or a f f e c t the m a t e r i a l s properties. H e n c e the d a t a in (10) are d o u b t f u l b e c a u s e p o s s i b l e d i f f e r e n c e s in the C - c o n t e n t b e t w e e n O to 28 p p m could cause g r e a t d i f f e r e n c e s in P - s e g r e g a t i o n . The further c r i t i c a l comments of B e r n a r d i n i and Gas (I) do not apply - the grain b o u n d a r i e s in the study in fact are not s a t u r a t e d w i t h P. There is no sense in d i s c u s s i n g the p o s s i b l e role of sulfur: it is s c a v e n g e d in the b u l k and h a d not been d e t e c t e d at the grain b o u n d a r i e s in the study d i s c u s s e d (2). W h i l e there is a g r e e m e n t that M n - s e g r e g a t i o n tO the grain b o u n d a r i e s is o b s e r v e d (2,10,11), there is no clear e v i d e n c e for an e n h a n c e m e n t of P - s e g r e g a t i o n by the s e g r e g a t e d Mn. It was not the aim of the p a p e r (2) to d i s p r o v e c o s e g r e g a t i o n of Mn and P b u t to find the r e a s o n for the a g g r a v a t i n g role of Mn in the e m b r i t t l e m e n t of steels. For that e f f e c t three reasons are given: the i n f l u e n c e of Mn on the c a r b o n - a c t i v ity and -solubility, the g r a i n b o u n d a r y s e g r e g a t i o n of Mn itself, and the accele r a t i n g e f f e c t of M n on the bulk and grain b o u n d a r y d i f f u s i o n of P in ferrite, the perhaps p o s s i b l e c o s e g r e g a t i o n c e r t a i n l y plays no i m p o r t a n t role. Comments
on the S y s t e m F e - N i - S b
B e r n a r d i n i and Gas (1) c l a i m that c o s e g r e g a t i o n of Sb and Ni has b e e n o b s e r v e d in steels. However, the data in the papers (14,15,16) w h i c h p u r p o r t to show cosegregation are a c t u a l l y taken from steels of d i f f e r e n t C-contents. B r i a n t (6) has sh o w n that both, Sb and Ni, c o m p e t e with c a r b o n for sites at the grain boundaries. In a f o r t h c o m i n g paper (17) it w i l l be shown that all of the changes in Ni and Sb s e g r e g a t i o n can be e x p l a i n e d through their i n t e r a c t i o n w i t h c a r b o n and the effects of Ni and Sb on each others s o l u b i l i t i e s . B e r n a r d i n i and Gas are c o n c e r n e d a b o u t the lower grain b o u n d a r y c o n c e n t r a t i o n s in the p a p e r of B r i a n t and R i t t e r (4) c o m p a r e d w i t h their p r e v i o u s d a t a (18). However, the alloys w i t h 0.07 Sb show i d e n t i c a l values of s e g r e g a t i o n and the data for the O.1 Sb alloys are r e a s o n a b l e s i m i l a r and p r o b a b l y w i t h i n the s c a t t e r of such data. This p o i n t of d i s c u s s i o n could be c o n s i d e r e d in another w a y as well. The i n d i v i d u a l data points in figure I in the p a p e r (18) of Gas, G u t t m a n n and B e r n a r d i n i r e p r e s e n t values o b t a i n e d from i n d i v i d u a l grain b o u n d a r y facets -
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o b v i o u s l y these are all the d a t a they obtained. In this case the authors are basing their c o n c l u s i o n s on very limited data, and this fact could also e x p l a i n the small d i f f e r e n c e in Sb s e g r e g a t i o n b e t w e e n r e f e r e n c e s (4) and (18). There is strong e v i d e n c e that Ni additions to an alloy increase Sb segregation, but the q u e s t i o n is w h e t h e r or not Sb s e g r e g a t i o n enhances Ni s e g r e g a t i o n . If it does not, the theory of c o s e g r e g a t i o n does not apply. R e l e v a n t to this point, B e r n a r d i n i and Gas (1) fail to discuss two studies (4,6) in w h i c h equal Ni segreg a t i o n was o b s e r v e d in c o r r e s p o n d i n g steels at d i f f e r e n t Sb segregation. Both studies show that Sb s e g r e g a t i o n has no e f f e c t on Ni s e g r e g a t i o n . Thus,
the s y s t e m F e - N i - S b
also is not at all a clear e x a m p l e
of c o s e g r e g a t i o n .
Conclusions One m u s t not think that we g e n e r a l l y do not b e l i e v e in the theory of s y n e r g i s t i c c o s e g r e g a t i o n (19,20); such a p h e n o m e n o n may w e l l be possible, b u t o b v i o u s l y it is not easy to find it and prove it. In contrast, in the 1970's, in the s t a r t of A E S - s t u d i e s on grain b o u n d a r y s e g r e g a t i o n and e m b r i t t l e m e n t of steels, 'cosegregation' was used as an e x p l a n a t i o n t h r o u g h o u t the l i t e r a t u r e for all effects of al l o y i n g elements, and its i m p o r t a n c e was g r o s s l y o v e r e s t i m a t e d . On the other hand, it is not easy to imagine the process of c o s e g r e g a t i o n . C e r t a i n l y , t h e r e may be an a t t r a c t i v e c h e m i c a l i n t e r a c t i o n of a l l o y i n g elements M and n o n - m e t a l elements I in iron and o t h e r m e t a l s o l i d solutions. However, w h y s h o u l d M and I diffuse to the grain b o u n d a r y , b e f o r e i n t e r a c t i n g and forming, e.g., a cluster or G.P. zone in the m a t r i x ? Only if one of the p a r t n e r s h a d 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 before, u n d e r d i f f e r e n t conditions, by n o n - e q u i l i b r i u m s e g r e g a t i o n or in a range of h i g h e r temperatures, can it be p e r c e i v e d that the grain b o u n d a r y s e g r e g a t i o n of the o t h e r p a r t n e r w i l l be e n h a n c e d u n d e r conditions, for example, of lower temperatures. But again that w o u l d not be s i m u l t a n e o u s s e g r e g a t i o n - no c o s e g r e g a t i o n in the sense of G u t t m a n n s ' s theory (19,20). A l s o s e g r e g a t i o n leading to p r e c i p i t a t i o n at grain b o u n d a r i e s m u s t be c l e a r l y d i s t i n g u i s h e d from cosegregation. Even the p o s s i b l e f o r m a t i o n of a b i d i m e n s i o n a l i n t e r f a c i a l c o m p o u n d MI x is to be d i s t i n g u i s h e d from s y n e r g i s t i c c o s e g r e g a t i o n . The theory of syne r g i s t i c c o s e g r e g a t i o n is b a s e d on the regular s o l u t i o n model, w i t h free e n e r g i e s of s e g r e g a t i o n as the d r i v i n g forces, in c o n t r a s t to the f o r m a t i o n of a two- or three d i m e n s i o n a l c o m p o u n d w i t h an o r d e r e d s t r u c t u r e and a s p e c i f i c v a l u e of the Gibbs' free energy of formation. T h e s e points, on the p a r t i a l e n h a n c e m e n t of segregation, the f o r m a t i o n of cluster and G.P. zones in the b u l k and two- or three d i m e n s i o n a l p r e c i p i t a t e s at interfaces will be d i s c u s s e d in m o r e d e t a i l in a f o r t h c o m i n g article. We e m p h a s i z e that m o s t a l l o y i n g effects in grain b o u n d a r y s e g r e g a t i o n can be exp l a i n e d now by e q u i l i b r i a of m u t u a l d i s p l a c e m e n t (site competition) of e m b r i t t ling and n o n - e m b r i t t l i n g elements I, and the i n f l u e n c e of the a l l o y i n g e l e m e n t s M on the a o t i v i t i e s and s o l u b i l i t i e s of these elements. Additionally, the segreg a t i o n to d i s l o c a t i o n s w h i c h has b e e n p r o v e d for P r e c e n t l y (21) - and w h i c h is in c o m p e t i t i o n w i t h the s e g r e g a t i o n to grain b o u n d a r i e s m u s t be considered. The d i s l o c a t i o n s can be a f f e c t e d by the p r e s e n c e of a l l o y i n g e l e m e n t s and of c e r t a i n carbides and nitrides. For e x a m p l e it has b e e n s h o w n r e c e n t l y that at r e l a t i v e l y high t e m p e r a t u r e s Mo2C carbides s t a b i l i z e d i s l o c a t i o n s (22,23) w h i c h can absorb a lot of P: this is the m e c h a n i s m of the w e l l - k n o w n m o l y b d e n u m effect, l e a d i n g to a d e c r e a s e of P s e g r e g a t i o n to grain b o u n d a r i e s . So there are s e v e r a l o t h e r mechanisms i m p o r t a n t in the grain b o u n d a r y s e g r e g a t i o n in alloys, w h e r e a s c o s e g r e g a t i o n has not even b e e n p r o v e d c o n v i n c i n g l y as yet. References I. 2. 3. 4. 5. 6.
J. B e r n a r d i n i H.J. Grabke, (1987) 1329 W. Wei, H.J. C.L. Briant, C.L. Briant, C.L. Briant,
and P. Gas, K. Hennesen,
S c r i p t a Metall., this issue R. M~ller, W. Wei, S c r i p t a Metall.
Grabke, C o r r o s i o n Sci. 26 (1986) 223 A.M. Ritter, A c t a Metall. 32 (1984) 2031 R.P. Messmer, A c t a Metall. 32 (1984) 2043 A c t a Metall. 35 (1987) 149
21
1172
7.
8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.
COMMENTS ON COSEGREGATION
Vol.
22, No.
H.J. Grabke 'Segregation at interfaces' in Eds. R.M. Latanision and R.H. Jones 'Chemistry and Physics of Fracture' Proceed. NA~O Adv.Res. Workshop 1986, Martinus Nijhoff Publ. Dordrecht 1987, p. 388-415 H. Erhart, H.J. Grabke, Metals Sci. 15 (1981) 401 S. Suzuki, M. Obata, K. Abiko, H. Kimura, Scripta Metall. 17 (1983) 1325 Weng Yu-Quing, C.J. McMahon Jr., Mat. Sci. Techn. 3 (1987) 207 K. Tatsumi, N. Okumura, M. Yamamoto, presented at Interface Sci. and Engg. '87 Lake Placid N.Y. J.C. Swartz, Trans. Metall. Soc. AIME 245 (1969) 1083 H. H~nsel, Dissertation, Univ. Dortmund, 1986, H. H~nsel, H.J. Grabke, Scripta Metall..20 (1986) 1641 R.A.-Mulford, C.J. McMahon Jr., D.P. Pope, H.C. Feng, Metall. Trans. A 7 (1976) 1269 C.J. McMahon Jr., Mater. Sci. Engg. 25 (1976) 233 J.Q. Clayton, J.F. Knott, Proceed. 4th Int. Conf. on Fracture, Univ. of Waterloo Press 1977, Vol.2, 287 C. Bri&nt, Mat. Sci. Techn. to be published P. Gas, M. Guttmann, J. Bernardini, Acta Metall. 30 (1982) 1309 M..Guttmann, Surface Sci. 53 (1975) 213 M. Guttmann, Metal Sci. 10 (1976) 337 H. Viefhaus, J. Wittig, K. Hennesen, H.J. Grabke, to be published R. M~ller, S.S. Brenner, H.J. Grabke, Scripta Metall. 20 (1986) 587 H.J. Grabke, R. M611er, H. Erhart, H. Viefhaus, Surface and Interface Analysis 10 (1987) 202
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Vol. 22, pp. 1169-1172, 1988 Printed in the U.S.A.
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COMMENTS ON "ABOUT THE E X I S T E N C E OF GRAIN B O U N D A R Y C O S E G R E G A T I O N IN PURE Fe-M-I SOLID SOLUTIONS"
Max-Planck-Institut General
Electric
H.J. Grabke fur E i s e n f o r s c h u n g
Co.,
Corp.
Research
and C.L. B r i a n t GmbH, D - 4 O O O DUsseldorf, Fed. R e p . G e r m a n y and and D e v e l o p m e n t , S c h e n e c t a d y NY 12301, U S A
(Received February 15, 1988) (Revised March 7, 1988)
Introduction In their p a p e r (I) B e r n a r d i n i and Gas c r i t i c i z e the s t a t e m e n t in an earlier p a p e r of G r a b k e et al. (2): ' c o s e g r e g a t i o n has not b e e n o b s e r v e d in s i m p l e c l e a n ternary systems Fe-M-I'. They try to prove that the systems F e - M n - P and F e - N i - S b are cl e a r e x a m p l e s for G u t t m a n n ' s m o d e l of c o s e g r e g a t i o n and that the w o r k of G r a b k e (2,3) and B r i a n t (4,5,6) on these systems is in error. However, this c r i t i c i s m is b a s e d on very w e a k e v i d e n c e as we w i l l show. M o s t a l l o y i n g effects in grain b o u n d a r y s e g r e g a t i o n in steels can now be c l e a r l y e x p l a i n e d by the site c o m p e t i t i o n of the e m b r i t t l i n g elements such as P, S and Sb and n o n - e m b r i t t l i n g e l e m e n t s such as C, N and B (7) and by the effects of the all o y i n g elements Cr, Mn, Ni etc. on the a c t i v i t i e s and s o l u b i l i t i e s of these elements. The m o s t i m p o r t a n t case of site c o m p e t i t i o n was first d e s c r i b e d by E r h a r t and G r a b k e (8), i.e. the e q u i l i b r i u m of m u t u a l d i s p l a c e m e n t of P and C at grain boundaries: P
(segregated)
+ C
(dissolved)
= P
(dissolved)
+ C
(segregated)
It was also p o i n t e d out that an a d d i t i o n of Cr, Mn and other c a r b i d e forming elements shifts this e q u i l i b r i u m to m o r e P - s e g r e g a t i o n . In the p r e v i o u s l i t e r a t u r e this e f f e c t of Cr was a s c r i b e d to c o s e g r e g a t i o n of Cr and P: however, there is no c o s e g r e g a t i o n , no m u t u a l e n h a n c e m e n t of Cr- and P - s e g r e g a t i o n in Fe-Cr-P alloys, c o m p a r e d to F e - C r and Fe-P (8). These results are a c c e p t e d now, at least by several authors (9,10). B e l o w it is e m p h a s i z e d again that the same c o n s i d e r a t i o n s apply in the s y s t e m Fe-Mn-P. Comments
on the S y s t e m F e - M n - P
Data on the grain b o u n d a r y s e g r e g a t i o n of P in F e - M n - P alloys h a v e r e c e n t l y b e e n p r e s e n t e d b y several authors: W e n g and M c M a h o n (10), Tatsumi, O k u m u r a and Y a m a m o t o (11) and G r a b k e et al. (2). The results are clear and i n d i s p u t a b l e in the case of high c a r b o n contents, a b o ut O.2%C in (11) and a b o u t O . 1 5 % C in (2) o p For such specimens, t e m p e r e d at 600 C, the grain b o u n d a r y c o n c e n t r a t i o n of n creases w i t h i n c r e a s i n g M n - c o n t e n t . This e f f e c t is clearly r e l a t e d to the dec r e a s e in C - a c t i v i t y and - s o l u b i l i t y in such alloys with i n c r e a s i n g M n - c o n t e n t . That c a r b i d e (Fe,Mn) 3C is formed for w h i c h the s o l u b i l i t y p r o d u c t d e c r e a s e s w i t h i n c r e a s i n g M n - c o n t e n t . The c o r r e s p o n d i n g d e c r e a s e in the c o n c e n t r a t i o n of dissolved c a r b o n leads to a d e c r e a s e of c a r b o n s e g r e g a t i o n and an i n c r e a s e of P - s e g regation. In their very careful study T a t s u m i et al. (11) p r o v e d the d e c r e a s e of d i s s o l v e d c a r b o n by i n t e r n a l f r i c t i o n m e a s u r e m e n t s and the d e c r e a s e of C segreg a t i o n by AES g r a i n b o u n d a r y analysis.
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Less clear are the results for very low c a r b o n contents. Tatsumi et al. (11) re p o r t e d no e f f e c t of Mn on the grain b o u n d a r y s e g r e g a t i o n of P at 7OOOC in the range 0.5 to 1.5%Mn, w i t h o u t g i v i n g further details. A l s o in the w o r k of Grabke et al. (2) no e f f e c t of 0.5 to 9 . 4 % M n could be seen at 5OO°C. Only in the third paper (10) has an increase of P grain b o u n d a r y c o n c e n t r a t i o n b e e n o b s e r v e d at 48OOC. In the c r i t i c i s m of B e r n a r d i n i and Gas (I), they s u s p e c t that the p r e s e n c e of about 30 ppm C in the specimens of the study (2) d e c r e a s e the P s e g r e g a t i o n and somehow conceal the s u p p o s e d d e p e n d e n c e on the m a n g a n e s e c o n c e n t r a t i o n . However, the c o n d i t i o n s in that study are very w e l l defined: at 500°C the m a x i m u m solub i l i t y of C in e q u i l i b r i u m w i t h Fe3C is b e l o w 20 p p m (12). So, for all the samples i n v e s t i g a t e d (see table 1 of the paper) the m a x i m u m s o l u t i o n c o n c e n t r a t i o n of C was e s t a b l i s h e d d u r i n g a n n e a l i n g so that the c o r r e s p o n d i n g g r a i n b o u n d a r y concentrations of C and P w e r e constant, i n d e p e n d e n t of c a r b o n and m a n g a n e s e c o n t e n t (13). (At the low carbon c o n c e n t r a t i o n s Mn is not i n v o l v e d in carbide p r e c i p i t a tion). B e r n a r d i n i and Gas are r i g h t that the P - s e g r e g a t i o n is d e c r e a s e d in comp a r i s o n to the study on the pure s y s t e m Fe-P (8) : however, if there was a s t r o n g c o s e g r e g a t i o n of Mn and P it should have b e e n o b s e r v e d also in the p r e s e n c e of some carbon. In contrast, the conditions in the study of W e n g and M c M a h o n (10) seem to be less well defined. The results w e r e o b t a i n e d with d e c a r b u r i z e d samples b u t it is not clear to w h a t e x t e n t the d e c a r b u r i z a t i o n was s u c c e s s f u l since the c h e m i c a l analysis after d e c a r b u r i z a t i o n gave results "which s c a t t e r e d i r r a t i o n a l l y b e t w e e n O and 28 ppm C and t h e r e f o r e w e r e judged to be u n r e l i a b l e " (10). The d e c a r b u r i z a tion w i t h w e t h y d r o g e n (saturated w i t h H20 at r o o m temperature) at 65OOC, may r a t h e r be u n r e l i a b l e since u n d e r these c o n d i t i o n s s e l e c t i v e e x t e r n a l or i n t e r n a l o x i d a t i o n of the m a n g a n e s e is p o s s i b l e in the F e - M n alloys. The o x i d a t i o n may p r e v e n t d e c a r b u r i z a t i o n and/or a f f e c t the m a t e r i a l s properties. H e n c e the d a t a in (10) are d o u b t f u l b e c a u s e p o s s i b l e d i f f e r e n c e s in the C - c o n t e n t b e t w e e n O to 28 p p m could cause g r e a t d i f f e r e n c e s in P - s e g r e g a t i o n . The further c r i t i c a l comments of B e r n a r d i n i and Gas (I) do not apply - the grain b o u n d a r i e s in the study in fact are not s a t u r a t e d w i t h P. There is no sense in d i s c u s s i n g the p o s s i b l e role of sulfur: it is s c a v e n g e d in the b u l k and h a d not been d e t e c t e d at the grain b o u n d a r i e s in the study d i s c u s s e d (2). W h i l e there is a g r e e m e n t that M n - s e g r e g a t i o n tO the grain b o u n d a r i e s is o b s e r v e d (2,10,11), there is no clear e v i d e n c e for an e n h a n c e m e n t of P - s e g r e g a t i o n by the s e g r e g a t e d Mn. It was not the aim of the p a p e r (2) to d i s p r o v e c o s e g r e g a t i o n of Mn and P b u t to find the r e a s o n for the a g g r a v a t i n g role of Mn in the e m b r i t t l e m e n t of steels. For that e f f e c t three reasons are given: the i n f l u e n c e of Mn on the c a r b o n - a c t i v ity and -solubility, the g r a i n b o u n d a r y s e g r e g a t i o n of Mn itself, and the accele r a t i n g e f f e c t of M n on the bulk and grain b o u n d a r y d i f f u s i o n of P in ferrite, the perhaps p o s s i b l e c o s e g r e g a t i o n c e r t a i n l y plays no i m p o r t a n t role. Comments
on the S y s t e m F e - N i - S b
B e r n a r d i n i and Gas (1) c l a i m that c o s e g r e g a t i o n of Sb and Ni has b e e n o b s e r v e d in steels. However, the data in the papers (14,15,16) w h i c h p u r p o r t to show cosegregation are a c t u a l l y taken from steels of d i f f e r e n t C-contents. B r i a n t (6) has sh o w n that both, Sb and Ni, c o m p e t e with c a r b o n for sites at the grain boundaries. In a f o r t h c o m i n g paper (17) it w i l l be shown that all of the changes in Ni and Sb s e g r e g a t i o n can be e x p l a i n e d through their i n t e r a c t i o n w i t h c a r b o n and the effects of Ni and Sb on each others s o l u b i l i t i e s . B e r n a r d i n i and Gas are c o n c e r n e d a b o u t the lower grain b o u n d a r y c o n c e n t r a t i o n s in the p a p e r of B r i a n t and R i t t e r (4) c o m p a r e d w i t h their p r e v i o u s d a t a (18). However, the alloys w i t h 0.07 Sb show i d e n t i c a l values of s e g r e g a t i o n and the data for the O.1 Sb alloys are r e a s o n a b l e s i m i l a r and p r o b a b l y w i t h i n the s c a t t e r of such data. This p o i n t of d i s c u s s i o n could be c o n s i d e r e d in another w a y as well. The i n d i v i d u a l data points in figure I in the p a p e r (18) of Gas, G u t t m a n n and B e r n a r d i n i r e p r e s e n t values o b t a i n e d from i n d i v i d u a l grain b o u n d a r y facets -
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o b v i o u s l y these are all the d a t a they obtained. In this case the authors are basing their c o n c l u s i o n s on very limited data, and this fact could also e x p l a i n the small d i f f e r e n c e in Sb s e g r e g a t i o n b e t w e e n r e f e r e n c e s (4) and (18). There is strong e v i d e n c e that Ni additions to an alloy increase Sb segregation, but the q u e s t i o n is w h e t h e r or not Sb s e g r e g a t i o n enhances Ni s e g r e g a t i o n . If it does not, the theory of c o s e g r e g a t i o n does not apply. R e l e v a n t to this point, B e r n a r d i n i and Gas (1) fail to discuss two studies (4,6) in w h i c h equal Ni segreg a t i o n was o b s e r v e d in c o r r e s p o n d i n g steels at d i f f e r e n t Sb segregation. Both studies show that Sb s e g r e g a t i o n has no e f f e c t on Ni s e g r e g a t i o n . Thus,
the s y s t e m F e - N i - S b
also is not at all a clear e x a m p l e
of c o s e g r e g a t i o n .
Conclusions One m u s t not think that we g e n e r a l l y do not b e l i e v e in the theory of s y n e r g i s t i c c o s e g r e g a t i o n (19,20); such a p h e n o m e n o n may w e l l be possible, b u t o b v i o u s l y it is not easy to find it and prove it. In contrast, in the 1970's, in the s t a r t of A E S - s t u d i e s on grain b o u n d a r y s e g r e g a t i o n and e m b r i t t l e m e n t of steels, 'cosegregation' was used as an e x p l a n a t i o n t h r o u g h o u t the l i t e r a t u r e for all effects of al l o y i n g elements, and its i m p o r t a n c e was g r o s s l y o v e r e s t i m a t e d . On the other hand, it is not easy to imagine the process of c o s e g r e g a t i o n . C e r t a i n l y , t h e r e may be an a t t r a c t i v e c h e m i c a l i n t e r a c t i o n of a l l o y i n g elements M and n o n - m e t a l elements I in iron and o t h e r m e t a l s o l i d solutions. However, w h y s h o u l d M and I diffuse to the grain b o u n d a r y , b e f o r e i n t e r a c t i n g and forming, e.g., a cluster or G.P. zone in the m a t r i x ? Only if one of the p a r t n e r s h a d 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 before, u n d e r d i f f e r e n t conditions, by n o n - e q u i l i b r i u m s e g r e g a t i o n or in a range of h i g h e r temperatures, can it be p e r c e i v e d that the grain b o u n d a r y s e g r e g a t i o n of the o t h e r p a r t n e r w i l l be e n h a n c e d u n d e r conditions, for example, of lower temperatures. But again that w o u l d not be s i m u l t a n e o u s s e g r e g a t i o n - no c o s e g r e g a t i o n in the sense of G u t t m a n n s ' s theory (19,20). A l s o s e g r e g a t i o n leading to p r e c i p i t a t i o n at grain b o u n d a r i e s m u s t be c l e a r l y d i s t i n g u i s h e d from cosegregation. Even the p o s s i b l e f o r m a t i o n of a b i d i m e n s i o n a l i n t e r f a c i a l c o m p o u n d MI x is to be d i s t i n g u i s h e d from s y n e r g i s t i c c o s e g r e g a t i o n . The theory of syne r g i s t i c c o s e g r e g a t i o n is b a s e d on the regular s o l u t i o n model, w i t h free e n e r g i e s of s e g r e g a t i o n as the d r i v i n g forces, in c o n t r a s t to the f o r m a t i o n of a two- or three d i m e n s i o n a l c o m p o u n d w i t h an o r d e r e d s t r u c t u r e and a s p e c i f i c v a l u e of the Gibbs' free energy of formation. T h e s e points, on the p a r t i a l e n h a n c e m e n t of segregation, the f o r m a t i o n of cluster and G.P. zones in the b u l k and two- or three d i m e n s i o n a l p r e c i p i t a t e s at interfaces will be d i s c u s s e d in m o r e d e t a i l in a f o r t h c o m i n g article. We e m p h a s i z e that m o s t a l l o y i n g effects in grain b o u n d a r y s e g r e g a t i o n can be exp l a i n e d now by e q u i l i b r i a of m u t u a l d i s p l a c e m e n t (site competition) of e m b r i t t ling and n o n - e m b r i t t l i n g elements I, and the i n f l u e n c e of the a l l o y i n g e l e m e n t s M on the a o t i v i t i e s and s o l u b i l i t i e s of these elements. Additionally, the segreg a t i o n to d i s l o c a t i o n s w h i c h has b e e n p r o v e d for P r e c e n t l y (21) - and w h i c h is in c o m p e t i t i o n w i t h the s e g r e g a t i o n to grain b o u n d a r i e s m u s t be considered. The d i s l o c a t i o n s can be a f f e c t e d by the p r e s e n c e of a l l o y i n g e l e m e n t s and of c e r t a i n carbides and nitrides. For e x a m p l e it has b e e n s h o w n r e c e n t l y that at r e l a t i v e l y high t e m p e r a t u r e s Mo2C carbides s t a b i l i z e d i s l o c a t i o n s (22,23) w h i c h can absorb a lot of P: this is the m e c h a n i s m of the w e l l - k n o w n m o l y b d e n u m effect, l e a d i n g to a d e c r e a s e of P s e g r e g a t i o n to grain b o u n d a r i e s . So there are s e v e r a l o t h e r mechanisms i m p o r t a n t in the grain b o u n d a r y s e g r e g a t i o n in alloys, w h e r e a s c o s e g r e g a t i o n has not even b e e n p r o v e d c o n v i n c i n g l y as yet. References I. 2. 3. 4. 5. 6.
J. B e r n a r d i n i H.J. Grabke, (1987) 1329 W. Wei, H.J. C.L. Briant, C.L. Briant, C.L. Briant,
and P. Gas, K. Hennesen,
S c r i p t a Metall., this issue R. M~ller, W. Wei, S c r i p t a Metall.
Grabke, C o r r o s i o n Sci. 26 (1986) 223 A.M. Ritter, A c t a Metall. 32 (1984) 2031 R.P. Messmer, A c t a Metall. 32 (1984) 2043 A c t a Metall. 35 (1987) 149
21
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8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.
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H.J. Grabke 'Segregation at interfaces' in Eds. R.M. Latanision and R.H. Jones 'Chemistry and Physics of Fracture' Proceed. NA~O Adv.Res. Workshop 1986, Martinus Nijhoff Publ. Dordrecht 1987, p. 388-415 H. Erhart, H.J. Grabke, Metals Sci. 15 (1981) 401 S. Suzuki, M. Obata, K. Abiko, H. Kimura, Scripta Metall. 17 (1983) 1325 Weng Yu-Quing, C.J. McMahon Jr., Mat. Sci. Techn. 3 (1987) 207 K. Tatsumi, N. Okumura, M. Yamamoto, presented at Interface Sci. and Engg. '87 Lake Placid N.Y. J.C. Swartz, Trans. Metall. Soc. AIME 245 (1969) 1083 H. H~nsel, Dissertation, Univ. Dortmund, 1986, H. H~nsel, H.J. Grabke, Scripta Metall..20 (1986) 1641 R.A.-Mulford, C.J. McMahon Jr., D.P. Pope, H.C. Feng, Metall. Trans. A 7 (1976) 1269 C.J. McMahon Jr., Mater. Sci. Engg. 25 (1976) 233 J.Q. Clayton, J.F. Knott, Proceed. 4th Int. Conf. on Fracture, Univ. of Waterloo Press 1977, Vol.2, 287 C. Bri&nt, Mat. Sci. Techn. to be published P. Gas, M. Guttmann, J. Bernardini, Acta Metall. 30 (1982) 1309 M..Guttmann, Surface Sci. 53 (1975) 213 M. Guttmann, Metal Sci. 10 (1976) 337 H. Viefhaus, J. Wittig, K. Hennesen, H.J. Grabke, to be published R. M~ller, S.S. Brenner, H.J. Grabke, Scripta Metall. 20 (1986) 587 H.J. Grabke, R. M611er, H. Erhart, H. Viefhaus, Surface and Interface Analysis 10 (1987) 202
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