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Grain boundary plane and intergranular segregation in nickel-sulfur system
Scripta METALLURGICA
Vol. 21, pp. 475-478, 1987 Printed in the U.S.A.
Pergamon Journals, Ltd. All rights reserved
GRAIN BOUNDARY PLANE AND INTERGRANULAR SEGREGATION IN NICKEL-SULFUR SYSTEM. D. BOUCHET, L. PRIESTER Laboratolre de M@tallurgle S t r u c t u r a l e - U . A . CNRS 1 1 0 7 Unlverslt@ P a r l s - S u d , Bat 413, 9 ] 4 0 5 ORSAY CEDEX
(Received November 14, 1986) (Revised January 23, 1987) INTRODUCTION The study by TEM of the c o r r e l a t i o n between the s e g r e g a t i o n and the grain b o u n d a r y structure in nickel has been made possible by m e a n s of the Intergranular m i c r o e t c h l n g t e c h n i q u e which allows a vlsualisation of the grain b o u n d a r i e s c o n t a i n i n g the highest quantity of s e g r e g a t e d sulfur for a given specimen (I). The first experimental results d i s p l a y e d the p r e p o n d e r a n t influence of grain b o u n d a r y plane on s e g r e g a t i o n , some facets b e i n g m i c r o e t c h e d , s o m e others not, for the same £ = 3 case. The grain b o u n d a r y plane Is characterized by Its I n t e r p l a n a r s p a c i n g d ( h k l ) / a which Is p r o p o r t i o n a l to the p l a n a r atom density. For an asymmetrical b o u n d a r y , we c o n s i d e r e d , after WOLF ( 2 ) , the mean value ( d / a ) m ( 1 ) . The p r o p o s e d interpretation took Into a c c o u n t the excess v o l u m e values c a l c u l a t e d by the hard s p h e r e model for a FCC system ( 3 ) . The present p a p e r Is an e x t e n s i o n , of t h e s e e x p e r i m e n t s and c o n s i d e r a t i o n s , to other grain b o u n d a r y types. For details on t e c h n i q u e s and m a t e r i a l s , see the previous p a p e r ( 1 ) . EXPERIMENTAL RESULTS The Investigated material was an ultra high purity polycrystalllne nickel (FCC) c o n t a i n i n g a b o u t 10ppm of sulfur. The s p e c i m e n s were t h e r m a l - t r e a t e d 7 days at 625°C, after h o m o g e n e l z a t l o n and cold rolling, in o r d e r to Induce maximal I n t e r g r a n u l a r sulfur s e g r e g a t i o n ( 4 ) . About 30 grain b o u n d a r i e s (different from i: = 3) were investigated. Two results were significant : ] - M i c r o e t c h e d b o u n d a r i e s a r e most often of a g e n e r a l type but can also be special b o u n d a r i e s (3 • £ ~; 19 a c c o r d i n g to the BRANDON c r i t e r i o n ( 5 ) . An e x a m p l e of a m i c r o e t c h e d s p e c i a l b o u n d a r y (E = 11) Is shown on figure 1. The relation between g e o m e t r i c a l speciality and o b s e r v e d p r o p e r t y turns out to be, once again, questionable. 2 - The ( d / a ) m values a r e much lower for m l c r o e t c h e d b o u n d a r i e s than for non m i c r o e t c h e d ones. We never observed any p r e f e r e n t i a l thinning of a grain b o u n d a r y the plane of which has a ( d / a ) m value higher than O. 150. The analysis of a curved I: = 3 b o u n d a r y tends to support these two previous assertions ( d / a ) m i n c r e a s e s when moving from the m l c r o e t c h e d part to the non m i o r o e t c h e d one (figure 2 ) . DISCUSSION The value of i n t e r p l a n a r s p a c i n g of a g e n e r a l grain b o u n d a r y plane seems to be an interesting c r i t e r i o n for its p r o p e n s i t y to s e g r e g a t i o n a l t h o u g h the physical m e a n i n g of ( d / a ) m for an a s y m m e t r i c a l b o u n d a r y ts not very explicit ( 2 ) . As for special b o u n d a r i e s , the c o i n c i d e n c e indices I: and = must also be taken Into a c c o u n t as shown in the r_ = 3 c a s e ( 1 ) . T h e s e two p a r a m e t e r s d e t e r m i n e , for each c o i n c i d e n c e d e s c r i p t i o n , the excess v o l u m e that a c c o u n t s for the size effect of grain b o u n d a r y structure on s e g r e g a t i o n . Excess v o l u m e values c o m i n g from the hard s p h e r e s model were c a l c u l a t e d for symmetrical tilt ( a r o u n d <110>, <100>, and <111> rotation axis) b o u n d a r i e s with the highest planar c o i n c i d e n c e (= = 1) ( 3 ) . On figure 3, we have plotted : the d e c r e a s i n g d / a values of the lower planar indices in the FCC system ; the E Indices which are a s s o c i a t e d with such grain b o u n d a r y planes ; the excess v o l u m e values Vex, for e a c h of these £. Open c i r c l e s show the r. which have no possibilities of substitutional sites ( 3 ) .
475 0036-9748/87 $3.00 + .00 Copyright (c) 1987 Pergamon Journals Ltd.
476
INTERGRANULAR
SEGREGATION
Vol.
21, No. 4
FIGURE l Observation by TEM of t h e microetchings of a special grain boundary (E = l l ) and of a general one (G). (d/a)m values are writen with i t a l i c characters.
a
b
FIGURE 2 TEM micrographies with d/a values ( i t a l i c characters) of a curved E = 3 boundary : a microetched part b - non microetched part (prolongation of the region a). Note the two general boundaries are microetched.
FIGURE 3 (next page) Expression of excess volumes and total numbers of sites ( i n t e r s t i t i a l plus substitutional) as a function of interplanar spacings and t h e i r corresponding ~ f o r ~ " = I . A transition zone is experimentally located around 0.150 : for d/a values which are above the dashed l i n e , the segregation becomes very low.
Vol.
21, No.
4
INTERGRANULAR SEGREGATION
477
o
i ........ I~ .........
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478
INTERGRANULAR SEGREGATION
Vol.
21, No. 4
the total number of possible sites (N T) (Interstitial plus substitutional) for each £ ( 3 ) . Firstly, it may be seen that excess volume is globally an inverse function of d/a, as we may expect : the lower the planar density, the higher the excess volume is. Secondly, according to our experimental data, it is possible to define a transition zone, located around d / a = 0. 150, (see horizontal dashed line on figure 3) above which the segregation becomes very unlikely, i . e . above which mlcroetchlng does not occur. Three convergent remarks can be made concerning the coincidence boundaries the d / a of which is greater than 0. 150 : their excess volumes are among the lowest ones , their structures admit no substitutional sites (except £ = 27a) their total numbers of possible sites NT are the lowest ; -
-
-
-
Thus a few grain boundaries appear to be "special" insofar as their propensity to segregation is the weakest ; - f o r r.3 (111), r . l l ( 3 1 1 ) , r.19a ( 3 3 1 ) , E3 ( 2 1 1 ) , and r_9 (221) boundaries, segregation must be very unlikely : - f o r r.5 (210), ~27a ( 5 1 1 } , and r.5 (310) boundaries, a strong segregation is not expected. (~ONCLUSIQNS This study tends to support the postulate that the grain boundary plane characterized by the interplanar spacing rather than the r. criterion is the fundamental parameter describing the propensity for segregation of a grain boundary. The boundaries containing the highest level of segregation are those with the lowest d / a values. This conclusion, which shows the importance of the size effect on intergranular segregation, raises some questions. First of all, the present approach does not take Into account the relaxed grain boundary structure : to what extent is the size effect correctly described by the excess volume deduced from the hard spheres model ? Furthermore, what is the role of the solute-solvent interaction on segregation ? Although the hard sphere model does not allow substitutional sites for segregation in a r_ = 5 (310) tilt boundary, the simulation of the sulfur segregation in this boundary in nickel, by the tight binding model, leads to an attractive energy ( 6 ) . Also, the respective influences of the sterlc and electronic factors of segregation are still Controversial. In some cases, the energy associated with each site of the structural unit must be considered. In other cases, the propensity for segregation is governed by the size effects ( 7 ) . According to our experimental data, segregation depends essentially on the Interplanar spacings of the atomic planes parallel to the grain boundary plane ; this is in good agreement with a recent theoretical classification of grain boundaries which is proposed as a guide-line to investigate Intargranular properties ( 8 ) .
(1) (2) (3)
D. BOUCHET, L. PRIESTER Scrlpta Met. vol 20, 961, (1986) D. WOLF Journal de Physique n" 4, Colloque C4, 197, ('1985) H.J. FROST, M.F. ASHBY, F. SPAEPEN Report of Division of Applied Sciences Harward University, June 1982 (4) L. BEAUNIER, D. BOUCHET, C. COLLIEX, P. TREBBIA, C. VIGNAUD Journal de Physique n*4, Colloque C4, 505, (1985) (5) D.G. BRANDON Acta Met, Vol 14, 1479, (1966) (6) A. LARERE, K.I. MASUDA-JINDO, R. YAMAMOTO, M. DOYAMA "Grain boundary related phenomena* Proceedings of JIMIS 4, 229, (1986) (7) V. VITEK, G.J. WANG, Journal de Physique C6, 43, 147 (1982) (8) V. PAIDAR Phys. Stat. S o l . ( a ) 92, 115 (1985).
structure and
Vol. 21, pp. 475-478, 1987 Printed in the U.S.A.
Pergamon Journals, Ltd. All rights reserved
GRAIN BOUNDARY PLANE AND INTERGRANULAR SEGREGATION IN NICKEL-SULFUR SYSTEM. D. BOUCHET, L. PRIESTER Laboratolre de M@tallurgle S t r u c t u r a l e - U . A . CNRS 1 1 0 7 Unlverslt@ P a r l s - S u d , Bat 413, 9 ] 4 0 5 ORSAY CEDEX
(Received November 14, 1986) (Revised January 23, 1987) INTRODUCTION The study by TEM of the c o r r e l a t i o n between the s e g r e g a t i o n and the grain b o u n d a r y structure in nickel has been made possible by m e a n s of the Intergranular m i c r o e t c h l n g t e c h n i q u e which allows a vlsualisation of the grain b o u n d a r i e s c o n t a i n i n g the highest quantity of s e g r e g a t e d sulfur for a given specimen (I). The first experimental results d i s p l a y e d the p r e p o n d e r a n t influence of grain b o u n d a r y plane on s e g r e g a t i o n , some facets b e i n g m i c r o e t c h e d , s o m e others not, for the same £ = 3 case. The grain b o u n d a r y plane Is characterized by Its I n t e r p l a n a r s p a c i n g d ( h k l ) / a which Is p r o p o r t i o n a l to the p l a n a r atom density. For an asymmetrical b o u n d a r y , we c o n s i d e r e d , after WOLF ( 2 ) , the mean value ( d / a ) m ( 1 ) . The p r o p o s e d interpretation took Into a c c o u n t the excess v o l u m e values c a l c u l a t e d by the hard s p h e r e model for a FCC system ( 3 ) . The present p a p e r Is an e x t e n s i o n , of t h e s e e x p e r i m e n t s and c o n s i d e r a t i o n s , to other grain b o u n d a r y types. For details on t e c h n i q u e s and m a t e r i a l s , see the previous p a p e r ( 1 ) . EXPERIMENTAL RESULTS The Investigated material was an ultra high purity polycrystalllne nickel (FCC) c o n t a i n i n g a b o u t 10ppm of sulfur. The s p e c i m e n s were t h e r m a l - t r e a t e d 7 days at 625°C, after h o m o g e n e l z a t l o n and cold rolling, in o r d e r to Induce maximal I n t e r g r a n u l a r sulfur s e g r e g a t i o n ( 4 ) . About 30 grain b o u n d a r i e s (different from i: = 3) were investigated. Two results were significant : ] - M i c r o e t c h e d b o u n d a r i e s a r e most often of a g e n e r a l type but can also be special b o u n d a r i e s (3 • £ ~; 19 a c c o r d i n g to the BRANDON c r i t e r i o n ( 5 ) . An e x a m p l e of a m i c r o e t c h e d s p e c i a l b o u n d a r y (E = 11) Is shown on figure 1. The relation between g e o m e t r i c a l speciality and o b s e r v e d p r o p e r t y turns out to be, once again, questionable. 2 - The ( d / a ) m values a r e much lower for m l c r o e t c h e d b o u n d a r i e s than for non m i c r o e t c h e d ones. We never observed any p r e f e r e n t i a l thinning of a grain b o u n d a r y the plane of which has a ( d / a ) m value higher than O. 150. The analysis of a curved I: = 3 b o u n d a r y tends to support these two previous assertions ( d / a ) m i n c r e a s e s when moving from the m l c r o e t c h e d part to the non m i o r o e t c h e d one (figure 2 ) . DISCUSSION The value of i n t e r p l a n a r s p a c i n g of a g e n e r a l grain b o u n d a r y plane seems to be an interesting c r i t e r i o n for its p r o p e n s i t y to s e g r e g a t i o n a l t h o u g h the physical m e a n i n g of ( d / a ) m for an a s y m m e t r i c a l b o u n d a r y ts not very explicit ( 2 ) . As for special b o u n d a r i e s , the c o i n c i d e n c e indices I: and = must also be taken Into a c c o u n t as shown in the r_ = 3 c a s e ( 1 ) . T h e s e two p a r a m e t e r s d e t e r m i n e , for each c o i n c i d e n c e d e s c r i p t i o n , the excess v o l u m e that a c c o u n t s for the size effect of grain b o u n d a r y structure on s e g r e g a t i o n . Excess v o l u m e values c o m i n g from the hard s p h e r e s model were c a l c u l a t e d for symmetrical tilt ( a r o u n d <110>, <100>, and <111> rotation axis) b o u n d a r i e s with the highest planar c o i n c i d e n c e (= = 1) ( 3 ) . On figure 3, we have plotted : the d e c r e a s i n g d / a values of the lower planar indices in the FCC system ; the E Indices which are a s s o c i a t e d with such grain b o u n d a r y planes ; the excess v o l u m e values Vex, for e a c h of these £. Open c i r c l e s show the r. which have no possibilities of substitutional sites ( 3 ) .
475 0036-9748/87 $3.00 + .00 Copyright (c) 1987 Pergamon Journals Ltd.
476
INTERGRANULAR
SEGREGATION
Vol.
21, No. 4
FIGURE l Observation by TEM of t h e microetchings of a special grain boundary (E = l l ) and of a general one (G). (d/a)m values are writen with i t a l i c characters.
a
b
FIGURE 2 TEM micrographies with d/a values ( i t a l i c characters) of a curved E = 3 boundary : a microetched part b - non microetched part (prolongation of the region a). Note the two general boundaries are microetched.
FIGURE 3 (next page) Expression of excess volumes and total numbers of sites ( i n t e r s t i t i a l plus substitutional) as a function of interplanar spacings and t h e i r corresponding ~ f o r ~ " = I . A transition zone is experimentally located around 0.150 : for d/a values which are above the dashed l i n e , the segregation becomes very low.
Vol.
21, No.
4
INTERGRANULAR SEGREGATION
477
o
i ........ I~ .........
~ ~
........
I~
t~ .......
~
........
~
+
I
+
--~--~
+
t
~
~ .......
o
+
i- --~' •
"1"
•
PO0+
i
.......
f~
r_, . . . . . . . . .
+
~
=,
•
~l
"
•
~-~
+1 ~
....... ........
~---~l~ ~, . . . . . . . .
'~ . . . . . .
I
~ ~
~ ~;~
~3~
t~"
+
•
oo
~
+ + +
i ~,
•.t-
I
+ + +
I
I
+
I
.0-
I
•
~
• =,
•
•
-
• •
478
INTERGRANULAR SEGREGATION
Vol.
21, No. 4
the total number of possible sites (N T) (Interstitial plus substitutional) for each £ ( 3 ) . Firstly, it may be seen that excess volume is globally an inverse function of d/a, as we may expect : the lower the planar density, the higher the excess volume is. Secondly, according to our experimental data, it is possible to define a transition zone, located around d / a = 0. 150, (see horizontal dashed line on figure 3) above which the segregation becomes very unlikely, i . e . above which mlcroetchlng does not occur. Three convergent remarks can be made concerning the coincidence boundaries the d / a of which is greater than 0. 150 : their excess volumes are among the lowest ones , their structures admit no substitutional sites (except £ = 27a) their total numbers of possible sites NT are the lowest ; -
-
-
-
Thus a few grain boundaries appear to be "special" insofar as their propensity to segregation is the weakest ; - f o r r.3 (111), r . l l ( 3 1 1 ) , r.19a ( 3 3 1 ) , E3 ( 2 1 1 ) , and r_9 (221) boundaries, segregation must be very unlikely : - f o r r.5 (210), ~27a ( 5 1 1 } , and r.5 (310) boundaries, a strong segregation is not expected. (~ONCLUSIQNS This study tends to support the postulate that the grain boundary plane characterized by the interplanar spacing rather than the r. criterion is the fundamental parameter describing the propensity for segregation of a grain boundary. The boundaries containing the highest level of segregation are those with the lowest d / a values. This conclusion, which shows the importance of the size effect on intergranular segregation, raises some questions. First of all, the present approach does not take Into account the relaxed grain boundary structure : to what extent is the size effect correctly described by the excess volume deduced from the hard spheres model ? Furthermore, what is the role of the solute-solvent interaction on segregation ? Although the hard sphere model does not allow substitutional sites for segregation in a r_ = 5 (310) tilt boundary, the simulation of the sulfur segregation in this boundary in nickel, by the tight binding model, leads to an attractive energy ( 6 ) . Also, the respective influences of the sterlc and electronic factors of segregation are still Controversial. In some cases, the energy associated with each site of the structural unit must be considered. In other cases, the propensity for segregation is governed by the size effects ( 7 ) . According to our experimental data, segregation depends essentially on the Interplanar spacings of the atomic planes parallel to the grain boundary plane ; this is in good agreement with a recent theoretical classification of grain boundaries which is proposed as a guide-line to investigate Intargranular properties ( 8 ) .
(1) (2) (3)
D. BOUCHET, L. PRIESTER Scrlpta Met. vol 20, 961, (1986) D. WOLF Journal de Physique n" 4, Colloque C4, 197, ('1985) H.J. FROST, M.F. ASHBY, F. SPAEPEN Report of Division of Applied Sciences Harward University, June 1982 (4) L. BEAUNIER, D. BOUCHET, C. COLLIEX, P. TREBBIA, C. VIGNAUD Journal de Physique n*4, Colloque C4, 505, (1985) (5) D.G. BRANDON Acta Met, Vol 14, 1479, (1966) (6) A. LARERE, K.I. MASUDA-JINDO, R. YAMAMOTO, M. DOYAMA "Grain boundary related phenomena* Proceedings of JIMIS 4, 229, (1986) (7) V. VITEK, G.J. WANG, Journal de Physique C6, 43, 147 (1982) (8) V. PAIDAR Phys. Stat. S o l . ( a ) 92, 115 (1985).
structure and