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Abnormal grain growth in sintering powder compacts
Scripta METALLURGICA
Vol. 22, pp. 9-11, 1988 Printed in the U.S.A.
Pergamon Journals, Ltd. All rights reserved
ABNORMAL GRAIN GROWTH IN SlNTERING POWDER COMPACTS Tsang-Tse Fang Department of Materials Engineering National Cheng Kung University Tainan, Taiwan 70101 Republic of China (Received July 6, 1987) (Revised October 26, 1987) Introduction A main difficulty in the interpretation of the microstructures in sintered compacts is because of the complexity of grain growth phenomena in sintering processes. The reasons for the initiation of discontinuous grain growth are ambiguous. A classic phenomenological consideration (i) is that pore-boundary separation creates the condition suitable for the onset of abnormal grain growth. More extensive studies (2-6) have been made not only to conduct a more rigorous analysis of the breakaway problem but also to work out the microstructural evolution. While most people accept pore-boundary separation as the reason for the initiation of discontinuous grain growth, the recent theoretical considerations (7) and experimental observations (8,9) show that it is not the primary reason. The objectives of this paper are (i) to discuss the existing qualitative model for the initiation of discontinuous grain growth and (ii) to propose possible reasons for interpreting the development of faceted abnormal grains. Experimental
results and discussion
In this study two different alumina powders have been prepared but have the same purity (99.98%). One is heavily agglomerated through shock conditioning and the other is highly dispersed. Although those compacts have nearly the same green density (= 64% th.), they display two quite different microstructures (Fig. i): highly agglomerated powder compacts develop a duplex structure with faceted abnormal grains and the abnormal grains occur very early, even in the intermediate stage of sintering (sintered @ 20 K/min to 1600 oC; final fractional density: 0.9741); for highly dispersed powder compacts, they develop very uniform microstructures with a few abnormal grains without facets in general and with the abnormal grains occurring very late, above 98Z theoretical density (sintered @ 2 K/min to 1550 oc; final fractional density: 0.9926). According to the c l a s s i c v i e w p o i n t (I) of discontinuous grain growth, we might consider that the shock-conditioned powders have aggregates as the nuclei of abnormal grains which would grow unimpeded through a fine-grained matrix in which normal growth has been inhibited by pores. However, this viewpoint could not explain the faceted abnormal grains developed in the shock-conditioned powder compacts and the highly dispersed powder compacts with few abnormal grains without faceted shape, which develop when the pores nearly are eliminated (Fig.l). Moreover, the experimental observations (8,9) have shown that the inhibiting porosity combined with grain size variation is not a primary reason for discontinuous grain growth. Furthermore, Srolovitz et al. (7) have recently reported the results of a simulation indicating that the inhibition of normal growth and the introduction of abnormally large grains is not a sufficient condition for the instigation of
9 0 0 5 6 - 9 7 4 8 / 8 8 $5.00 + .00 C o p y r i g h t [c) 1988 P e r g a m o n J o u r n a ] s
Ltd.
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abnormal growth. Moreover, experimental observations in thin sheet materials (i0) show that when abnormal grain growth occurs, it is often a s s o c i a t e d with grain boundary motion in a direction opposed to that of the grain boundary curvature. This is a clear indication that growth of secondary grains is not solely caused by capillarity. They also suggest (7) that if abnormal grain growth resulting in the development of a bimodal d i s t r i b u t i o n is observed, it could be the result of factors other than grain boundary energy alone, such as the anisotropy of i n t e r f a c i a l energy. As far as a n i s o t r o p i c grain boundary energy is concerned, Yan (ii) has shown in a computer simulation that when different m i s o r i e n t a t i o n s have been introduced into the grain boundary mobility, discontinuous grain growth occurs after many iterations. Based on the arguments m e n t i o n e d above, the anisotropic grain boundary energy might play an important role in d i s c o n t i n u o u s grain growth. However, the development of faceted abnormal grains still remains unresolved. One might attribute it to a strong anisotropy of grain boundary energy with orientation, such as accounts for the appearance of straight twin boundaries (12); however, this argument is e l i m i n a t e d because faceted abnormal grains are not observed in most specimens of u l t r a h i g h - p u r i t y alumina (8,13) and in Fig. i (b). It is reported (8) that in tests on > 99.99% pure alumina powders with a g g r e g a t e s and seed grains, the faceted abnormal grains are not observed and the grain growth rate of abnormal grains is not sufficient to account for the extremely large grains frequently found in sintering undoped, c o m m e r c i a l alumina. It is obvious that anisotropic grain growth and a g g l o m e r a t i o n s alone are not s u f f i c i e n t to i n t e r p r e t the faceted abnormal grains with duplex structure. Kooy (14) and Kaysser et al. (15) suggest that when facetting occurs, it is more likely to have a continuous liquid phase along the facet with grain boundaries of the matrix i n t e r s e c t i n g the liquid along the faceted plane. If this is true, the reasons why the u n i n t e n t i o n a l liquid phase would develop in the a g g l o m e r a t e d powder compacts rather than in uniform compacts should be interpreted. Kaysser et al. (15) have proposed that the liquid phase need not be present initially, but that, at some critical grain size, the capacity of the grain boundary will be exceeded and a liquid phase will appear. This concept seems promising as a basis for the i n t e r p r e t a t i o n of the development of faceted abnormal grains, but it is still not sufficient to explain the results shown in Fig. I. It is quite important to recognize that the so-called "critical grain size" is p r o c e s s i n g - d e p e n d e n t because the temperature for forming a liquid phase varies with c o m p o s i t i o n and the grain growth rate increases with temperatures. Hence, purity and temperature both have a great influence on the critical grain size. Moreover, impurities could be r e d i s t r i b u t e d because of the agglomeration, as d e s c r i b e d as follows. The grain b o u n d a r i e s are well d e v e l o p e d in the local dense areas even though the matrix is still in the i n t e r m e d i a t e or initial stage. Because impurity ions such as Ca+2or Si+4 , which could form an unintentional liquid phase, have a tendency to segregate to grain boundaries, it is reasonable to believe that these i m p u r i t i e s in the n e i g h b o r i n g grains of local dense areas could segregate and diffuse in these areas owing to surface and grain boundary diffusion. When the r e d i s t r i b u t i o n of these i m p u r i t i e s occurs, the critical grain size should be different between the local dense areas and matrix, as mentioned above. Thus the development of a liquid phase in the local dense areas of agglomerated powder compacts would occur earlier and at lower t e m p e r a t u r e s than in u n i f o r m powder compacts, and this is a possible basis for the interpretation of the results shown in Fig.l. Once the liquid phase is formed, the boundaries wetted by the liquid phase will become faceted (15,16). The facetting will become more p r o n o u n c e d arising from the fact that nonbasal l i q u i d - s o l i d interfaces can move rapidly while basal facets wetted by the liquid phase and twin boundaries are relatively immobile (15). Based on this p h e n o m e n o l o g i c a l description, the contradictions existing in the recent reports (13,17) can be solved. However, further studies on developing liquid phases in the local dense areas are recommended.
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Inferring from the arguments above, we can explain why low purity alumina powder compacts that are undoped and pressureless-sintered, including those wellmade specimens that are uniformly compacted and have high green densities, have difficulty in attaining full density and uniform microstructures because of the presence of anisotropic grain growth and liquid phases. Conclusion The phenomenon of abnormal grain growth in sintering powder compacts is quite complicated. The presence of abnormal large grains or pore-bOundary separation is not the primary reason for the initiation of discontinuous grain growth, but the correlation of the anisotropic grain growth, the purity level, the degree of agglomeration, and temperature should be considered. The facetting of grains is associated with the liquid phases. References i. 2.
3. 4. 5. 6. 7. 8. 9. I0. Ii. 12. 13. 14. 15. 16.
17.
R.L. Coble and J.E. Burke, Progress in Ceramic Science, Vol.3, p. 197, Macmillan Co., New York, (1963). R.J. Brook, J. Am. Ceram. Soc., 52, 56 (1969). F.M.A. Carpay, J. Am. Ceram. Soc., 60, 82 (1977). K. Uematsu, R.M. Cannon, R.D. Bagley, M.F. Yan, U. Chowdhry and H. K. Bowen Proceedings of International Symposium of Factors in Densification and Sintering of Oxide and Nonoxide Ceramic, p. 190, Japan. (1978). C.H. Hsueh, A.G. Evans and R.L. Coble, Acta Metall. 30, 1269 (1982). M.A. Spears and A.G. Evans, Acta Metall. 30, 1281 (1982). D.J. Srolovitz, G.S. Grest and M.P. Anderson, Acta Metall., 33, 2233 (1985). W.T. Patrick and I.B. Cutler, J. Am. Ceram. Soc., 48, 541 (1965). S.J. Bennison and M.P. Harmer, Ceramic Powders, p. 929, Elsevier, Amsterdam, The Netherlands, (1983). J.L. Walter and C.G. Dunn, Trans. Am. Inst. Min. Engrs 218, 1033 (1960). M.F. Yan, Mater. Sci. and Eng., 48, 53 (1981). H. Schmelz, Sci. of Ceram., 12, 341 (1983). S.J. Bennison and M.P. Harmer, J. Am. Ceram. Soc., 68, C22 (1985). C. Kooy, Sci. of Ceram., I, 21 (1962). W. A. Kaysser, M. Sprissler, C. A. Handwerker and j. E. Blendell, J. Am. Ceram. Soc., 70, 339 (1987). D. S. Philips and Y. R. Shiue, Adv. in Ceram., I0, 357 (1983). T. Kimura, Y. Matsuda, M. Oda and T. Yamaguchi, Ceramics International, 13, 27 (1987).
(a)
FIG. I. SEM photomicrograghs of alumina, showing comparisons resulting from different powder powders and (b) well-dispersed,
(b)
polished and thermally etched sections of undoped of two different patterns of abnormal grains processing: (a) shock-conditioned, agglomerated deagglomerated powders.
Vol. 22, pp. 9-11, 1988 Printed in the U.S.A.
Pergamon Journals, Ltd. All rights reserved
ABNORMAL GRAIN GROWTH IN SlNTERING POWDER COMPACTS Tsang-Tse Fang Department of Materials Engineering National Cheng Kung University Tainan, Taiwan 70101 Republic of China (Received July 6, 1987) (Revised October 26, 1987) Introduction A main difficulty in the interpretation of the microstructures in sintered compacts is because of the complexity of grain growth phenomena in sintering processes. The reasons for the initiation of discontinuous grain growth are ambiguous. A classic phenomenological consideration (i) is that pore-boundary separation creates the condition suitable for the onset of abnormal grain growth. More extensive studies (2-6) have been made not only to conduct a more rigorous analysis of the breakaway problem but also to work out the microstructural evolution. While most people accept pore-boundary separation as the reason for the initiation of discontinuous grain growth, the recent theoretical considerations (7) and experimental observations (8,9) show that it is not the primary reason. The objectives of this paper are (i) to discuss the existing qualitative model for the initiation of discontinuous grain growth and (ii) to propose possible reasons for interpreting the development of faceted abnormal grains. Experimental
results and discussion
In this study two different alumina powders have been prepared but have the same purity (99.98%). One is heavily agglomerated through shock conditioning and the other is highly dispersed. Although those compacts have nearly the same green density (= 64% th.), they display two quite different microstructures (Fig. i): highly agglomerated powder compacts develop a duplex structure with faceted abnormal grains and the abnormal grains occur very early, even in the intermediate stage of sintering (sintered @ 20 K/min to 1600 oC; final fractional density: 0.9741); for highly dispersed powder compacts, they develop very uniform microstructures with a few abnormal grains without facets in general and with the abnormal grains occurring very late, above 98Z theoretical density (sintered @ 2 K/min to 1550 oc; final fractional density: 0.9926). According to the c l a s s i c v i e w p o i n t (I) of discontinuous grain growth, we might consider that the shock-conditioned powders have aggregates as the nuclei of abnormal grains which would grow unimpeded through a fine-grained matrix in which normal growth has been inhibited by pores. However, this viewpoint could not explain the faceted abnormal grains developed in the shock-conditioned powder compacts and the highly dispersed powder compacts with few abnormal grains without faceted shape, which develop when the pores nearly are eliminated (Fig.l). Moreover, the experimental observations (8,9) have shown that the inhibiting porosity combined with grain size variation is not a primary reason for discontinuous grain growth. Furthermore, Srolovitz et al. (7) have recently reported the results of a simulation indicating that the inhibition of normal growth and the introduction of abnormally large grains is not a sufficient condition for the instigation of
9 0 0 5 6 - 9 7 4 8 / 8 8 $5.00 + .00 C o p y r i g h t [c) 1988 P e r g a m o n J o u r n a ] s
Ltd.
i0
ABNORMAL
GRAIN
GROWTH
Vol.
22, No.
i
abnormal growth. Moreover, experimental observations in thin sheet materials (i0) show that when abnormal grain growth occurs, it is often a s s o c i a t e d with grain boundary motion in a direction opposed to that of the grain boundary curvature. This is a clear indication that growth of secondary grains is not solely caused by capillarity. They also suggest (7) that if abnormal grain growth resulting in the development of a bimodal d i s t r i b u t i o n is observed, it could be the result of factors other than grain boundary energy alone, such as the anisotropy of i n t e r f a c i a l energy. As far as a n i s o t r o p i c grain boundary energy is concerned, Yan (ii) has shown in a computer simulation that when different m i s o r i e n t a t i o n s have been introduced into the grain boundary mobility, discontinuous grain growth occurs after many iterations. Based on the arguments m e n t i o n e d above, the anisotropic grain boundary energy might play an important role in d i s c o n t i n u o u s grain growth. However, the development of faceted abnormal grains still remains unresolved. One might attribute it to a strong anisotropy of grain boundary energy with orientation, such as accounts for the appearance of straight twin boundaries (12); however, this argument is e l i m i n a t e d because faceted abnormal grains are not observed in most specimens of u l t r a h i g h - p u r i t y alumina (8,13) and in Fig. i (b). It is reported (8) that in tests on > 99.99% pure alumina powders with a g g r e g a t e s and seed grains, the faceted abnormal grains are not observed and the grain growth rate of abnormal grains is not sufficient to account for the extremely large grains frequently found in sintering undoped, c o m m e r c i a l alumina. It is obvious that anisotropic grain growth and a g g l o m e r a t i o n s alone are not s u f f i c i e n t to i n t e r p r e t the faceted abnormal grains with duplex structure. Kooy (14) and Kaysser et al. (15) suggest that when facetting occurs, it is more likely to have a continuous liquid phase along the facet with grain boundaries of the matrix i n t e r s e c t i n g the liquid along the faceted plane. If this is true, the reasons why the u n i n t e n t i o n a l liquid phase would develop in the a g g l o m e r a t e d powder compacts rather than in uniform compacts should be interpreted. Kaysser et al. (15) have proposed that the liquid phase need not be present initially, but that, at some critical grain size, the capacity of the grain boundary will be exceeded and a liquid phase will appear. This concept seems promising as a basis for the i n t e r p r e t a t i o n of the development of faceted abnormal grains, but it is still not sufficient to explain the results shown in Fig. I. It is quite important to recognize that the so-called "critical grain size" is p r o c e s s i n g - d e p e n d e n t because the temperature for forming a liquid phase varies with c o m p o s i t i o n and the grain growth rate increases with temperatures. Hence, purity and temperature both have a great influence on the critical grain size. Moreover, impurities could be r e d i s t r i b u t e d because of the agglomeration, as d e s c r i b e d as follows. The grain b o u n d a r i e s are well d e v e l o p e d in the local dense areas even though the matrix is still in the i n t e r m e d i a t e or initial stage. Because impurity ions such as Ca+2or Si+4 , which could form an unintentional liquid phase, have a tendency to segregate to grain boundaries, it is reasonable to believe that these i m p u r i t i e s in the n e i g h b o r i n g grains of local dense areas could segregate and diffuse in these areas owing to surface and grain boundary diffusion. When the r e d i s t r i b u t i o n of these i m p u r i t i e s occurs, the critical grain size should be different between the local dense areas and matrix, as mentioned above. Thus the development of a liquid phase in the local dense areas of agglomerated powder compacts would occur earlier and at lower t e m p e r a t u r e s than in u n i f o r m powder compacts, and this is a possible basis for the interpretation of the results shown in Fig.l. Once the liquid phase is formed, the boundaries wetted by the liquid phase will become faceted (15,16). The facetting will become more p r o n o u n c e d arising from the fact that nonbasal l i q u i d - s o l i d interfaces can move rapidly while basal facets wetted by the liquid phase and twin boundaries are relatively immobile (15). Based on this p h e n o m e n o l o g i c a l description, the contradictions existing in the recent reports (13,17) can be solved. However, further studies on developing liquid phases in the local dense areas are recommended.
Vol.
22, No. i
ABNORMAL GRAIN GROWTH
ii
Inferring from the arguments above, we can explain why low purity alumina powder compacts that are undoped and pressureless-sintered, including those wellmade specimens that are uniformly compacted and have high green densities, have difficulty in attaining full density and uniform microstructures because of the presence of anisotropic grain growth and liquid phases. Conclusion The phenomenon of abnormal grain growth in sintering powder compacts is quite complicated. The presence of abnormal large grains or pore-bOundary separation is not the primary reason for the initiation of discontinuous grain growth, but the correlation of the anisotropic grain growth, the purity level, the degree of agglomeration, and temperature should be considered. The facetting of grains is associated with the liquid phases. References i. 2.
3. 4. 5. 6. 7. 8. 9. I0. Ii. 12. 13. 14. 15. 16.
17.
R.L. Coble and J.E. Burke, Progress in Ceramic Science, Vol.3, p. 197, Macmillan Co., New York, (1963). R.J. Brook, J. Am. Ceram. Soc., 52, 56 (1969). F.M.A. Carpay, J. Am. Ceram. Soc., 60, 82 (1977). K. Uematsu, R.M. Cannon, R.D. Bagley, M.F. Yan, U. Chowdhry and H. K. Bowen Proceedings of International Symposium of Factors in Densification and Sintering of Oxide and Nonoxide Ceramic, p. 190, Japan. (1978). C.H. Hsueh, A.G. Evans and R.L. Coble, Acta Metall. 30, 1269 (1982). M.A. Spears and A.G. Evans, Acta Metall. 30, 1281 (1982). D.J. Srolovitz, G.S. Grest and M.P. Anderson, Acta Metall., 33, 2233 (1985). W.T. Patrick and I.B. Cutler, J. Am. Ceram. Soc., 48, 541 (1965). S.J. Bennison and M.P. Harmer, Ceramic Powders, p. 929, Elsevier, Amsterdam, The Netherlands, (1983). J.L. Walter and C.G. Dunn, Trans. Am. Inst. Min. Engrs 218, 1033 (1960). M.F. Yan, Mater. Sci. and Eng., 48, 53 (1981). H. Schmelz, Sci. of Ceram., 12, 341 (1983). S.J. Bennison and M.P. Harmer, J. Am. Ceram. Soc., 68, C22 (1985). C. Kooy, Sci. of Ceram., I, 21 (1962). W. A. Kaysser, M. Sprissler, C. A. Handwerker and j. E. Blendell, J. Am. Ceram. Soc., 70, 339 (1987). D. S. Philips and Y. R. Shiue, Adv. in Ceram., I0, 357 (1983). T. Kimura, Y. Matsuda, M. Oda and T. Yamaguchi, Ceramics International, 13, 27 (1987).
(a)
FIG. I. SEM photomicrograghs of alumina, showing comparisons resulting from different powder powders and (b) well-dispersed,
(b)
polished and thermally etched sections of undoped of two different patterns of abnormal grains processing: (a) shock-conditioned, agglomerated deagglomerated powders.