w.Z. Zh Here y is the volume fraction of oxygen vacancies 4 CONCLUSIONS and M is the concentration of Y, O in mol%. It can be seen that the concentration of oxygen (a) The time-temperature-transformation TTT) vacancies incrcascs with increasing Y2 O3 concen curve for the t→→ m transformation in Thus, it is proposed that in ZrO2(Y2O3) ZrO2(3 mol% Y2O3)ceramics is C-shaped, containing a large amount of vacancies, the trans- with a"nose"temperature determined to be formation behaviour is quite different from that in 300C. The TTt curve of Zro2 (3mo- pure zirconia > For pure zirconia, when the t-m Io Y2O3)ceramics lies to the down-left side transformation occurs, the lattice composed of zir of that of Zro2 (2 mol%Y2O3)ceramics conium ions shears in a coordinating and military (b) Preferential growth of manner, concurrent with obvious volume expan- ZrO2 (3 mol%Y203) ceramics effectively sion as well as shape change. As a result, this dif- refines the grain size of the t-phase fusionless shear process becomes a predominant (c) The microstructure of Zro2 (3 mol%Y2O3) factor in controlling the kinetics of transformation, ceramics is very complicated, in that it con- which appears to be athermal. Because ceramic tains tetragonal phase (which can either be a materials possess high strength and elastic modu sintered phase or precipitates), monoclinic lus, the strain energy caused by a shear-like trans phase(which can either occupy the whole formation is large and morphologies of the final grain or co-exist with the tetragonal phase products, which are always twinned, are deter and cubic phase mined by the strain energy For ZrO2(Y203)cera (d)The mechanism of the t-m transition in mics. when the t-m transformation occurs, not Zro2(Y2O3)ceramics is different from that in only a coordinating shift of zirconium ions is pure ZrO2. The nucleation and longitudinal ded but short-range diffusion of oxygen ions also growth of m-phase plates in ZrO2(Y2O3 takes place, with the implication that this kind of transformation possesses both displacive and non growth thereafter is controlled by short displacive features. TEM photographs of the in- range diffusion of oxygen ions and, in this situ growth of m-phase induced by the irradiation the t-m transformation of an electron heam are shown in Fig. 10, from ZrO2(Y2O3)ceramics possesses both displa which direct proof of heterogeneous nucleation of cive and non-displacive features m-plates at the grain boundaries can be obtained It is observed that the nucleation process is rather rapid and once nucleation completes, growth in the REFERENCES longitudinal direction is much faster than that in the tranverse direction. It is roughly estimated th CHEN, I w.& CHIAO, Y H, Acta Metall, 33(10) the growing velocity along the longitudinal direc- 2 HEuEr. AH. clausSEN N. KRIVEN w.M.& tion is 30 times faster than that along the tranverse RUHLE. M.J. Am Ceram Soc., 65(12)(1982)642 direction. It is thus proposed that the nucleation 3. SUBBARAO, E. C, Advances in and longitudinal growth of m-phase plates in Ceramics Society, Columbus, OH, 1981, p. I ZrO2(Y2O3)ceramics is believed to be displacive, 4. SATO, T, OHTEKL, S, ENDO, T& SHIMADA, M while the sidewise growth thereafter is proved to be short-range diffusion controlled. The short-range 5. NAKANISHI, N &SHIGEMATSU,T,Mater.Trans diffusion of oxygen ions becomes a predominant 6. LEL, T. C, ZhU, w.z.& ZhoU, Y, Mater. Chem factor in controlling the kinetics of the t-+m Phys.34(4)(1993)317 transformation in ZrO2(Y2O3)ceramics. The 7. ZHU, W.Z., LEl, T C.& ZHOU, Y, J. Mater. Sci release of the large transformation strain incurred 8(12)(1993)6479 8. NAKANISHi, N. SHIGEMATSU, T Muter. Truns. by the expansion of monoclinic phase through the JM,32(8)(1991)778. short-range diffusion of oxygen ions might be 9. nakanishi n.& shigematsu. t. zirconia responsible for the diversity of m-phase morphol Ceram,8(1)(1986)71 10. BEHRENS. D. DRANSMANNG w& heuer. a ogies observed in ZrO2(Y2O3)ceramics. In view of J.Am. ceran.Soc,76(4)(1993)1025 the fact that the transformation activation energ 11. GARVIE, R. C, J. Am. Ceram. Soc., 55(6)(1972) calculated using the kinetics data is in the range of Advances in Ceramics, Science 20-40 kJ/mol, which is far less than the activation IL, Vol. 12. American Ceramics Society, C energy for self-diffusion of oxygen ions (96 kJj OH,1984,pp.352 mol), it is speculated that the shifting distance of 13. WOLTEN, G. M, J. Am. Ceram. Soc., 46(10)(1963) the oxygen ions during transition is less than the 14. INGEL R. P.& Ill. D. L.J. Am. Ceram. Soc., 69(4) lattice constant of the tetragonal phase(0.50 nm) (1986)32542 Here V is the volume fraction of oxygen vacancies and M is the concentration of Y203 in mol%. It can be seen that the concentration of oxygen vacancies increases with increasing Y203 concen￾tration. Thus, it is proposed that in Zr02(Y203) containing a large amount of vacancies, the trans￾formation behaviour is quite different from that in pure zirconia. is For pure zirconia, when the t-+m transformation occurs, the lattice composed of zir￾conium ions shears in a coordinating and military manner, concurrent with obvious volume expan￾sion as well as shape change. As a result, this dif￾fusionless shear process becomes a predominant factor in controlling the kinetics of transformation, which appears to be athermal. Because ceramic materials possess high strength and elastic modu￾lus, the strain energy caused by a shear-like trans￾formation is large and morphololgies of the final products, which are always twinned, are deter￾mined by the strain energy. For Zr02(Y203) cera￾mics, when the t--+m transformation occurs, not only a coordinating shift of zirconium ions is nee￾ded, but short-range diffusion of oxygen ions also takes place, with the implication that this kind of transformation possesses both displacive and non￾displacive features. TEM photographs of the in￾situ growth of m-phase induced by the irradiation of an electron beam are shown in Fig. 10, from which direct proof of heterogeneous nucleation of m-plates at the grain boundaries can be obtained. It is observed that the nucleation process is rather rapid and once nucleation completes, growth in the longitudinal direction is much faster than that in the tranverse direction. It is roughly estimated that the growing velocity along the longitudinal direc￾tion is 30 times faster than that along the tranverse direction. It is thus proposed that the nucleation and longitudinal growth of m-phase plates in Zr02(Y203) ceramics is believed to be displacive, while the sidewise growth thereafter is proved to be short-range diffusion controlled. The short-range diffusion of oxygen ions becomes a predominant factor in controlling the kinetics of the t+m transformation in ZrOz(YzO3) ceramics. The release of the large transformation strain incurred by the expansion of monoclinic phase through the short-range diffusion of oxygen ions might be responsible for the diversity of m-phase morphol￾ogies observed in ZrOz(Y203) ceramics. In view of the fact that the transformation activation energy calculated using the kinetics data is in the range of 2640 kJ/mol, which is far less than the activation energy for self-diffusion of oxygen ions (96 kJ/ mol), l6 it is speculated that the shifting distance of the oxygen ions during transition is less than the lattice constant of the tetragonal phase (0.50 nm). W. Z. Zhu 4 CONCLUSIONS (4 cc> (4 The time-temperature--transformation (TTT) curve for the t-+m transformation in Zr02(3 mol% Y203) c,eramics is C-shaped, with a “nose” temperature determined to be 300°C. The TTT curve of Zr02(3mo- 1% Y203) ceramics lies to the down-left side of that of Zr02(2 mol% Y20s)ceramics. Preferential growth of c-phase grains in Zr02(3 mol% Y20~) ceramics effectively refines the grain size of the t-phase. The microstructure of Zr02(3 mol% Y203) ceramics is very complicated, in that it con￾tains tetragonal phase (which can either be a sintered phase or precipitates), monoclinic phase (which can either occupy the whole grain or co-exist with the tetragonal phase) and cubic phase. The mechanism of the t+m transition in Zr02(Y203) ceramics is different from that in pure Zr02. The nucleation and longitudinal growth of m-phase plates in Zr02(Y203) ceramics are displacive, while the sidewise growth thereafter is controlled by short￾range diffusion of oxygen ions and, in this sense, the t--tm transformation in ZrOz(Yz03) ceramics possesses both displa￾cive and non-displacive features. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. CHEN, I. W. & CHIAO, Y H., Acta Mefall., 33(10) (1985) 1827. HEUER, A. H., CLAUSSEN, N., KRIVEN, W. M. & RUHLE, M., J. Am. Ceram. Sot., 65(12) (1982) 642. SUBBARAO, E. C., Advance.x in Ceramics, Science and Technology of Zircon& Vol. 3. American Ceramics Society, Columbus, OH, 1981, p. 1. SATO, T., OHTEKI, S., ENDO, T. & SHIMADA, M., J. Am. Gram. SOL, 68(10) (1985) C-320. NAKANISHI, N. & SHIGEMATSU, T., Mater. Trans. JIM, 33(3) (1992) 318. LEI, T. C., ZHU, W. Z. & ZHOU, Y., Mater. Chem. Phys., 3q4) (1993) 317. ZHU, W. Z., LEI, T. C. & ZHOU, Y., J. Mater. Sci., 28(12) (1993) 6479. NAKANISHI, N. & SHIGEMATSU, T., Mater. Trans. JIM, 32(8) (1991) 778. NAKANISHI, N. & SHIGEMATSU, T., Zirconiu Gram., 8(l) (1986) 71. BEHRENS, D., DRANSMANN, G. W. & HEUER, A. H., J. Am. Ceram. Sot., 76(4) (1993) 1025. GARVIE, R. C., J. Am. Ceram. Sot., 55(6) (1972) 303. RUHLE, M., CLAUSSEN, N. & HEUER, A. H., Advances in Ceramics, Science and Technology of Zirco￾niu ZI, Vol. 12. American Ceramics Society, Columbus, OH, 1984, pp. 352. WOLTEN, G. M., J. Am. Ceram. Sot., 46( 10) (1963) 418. INGEL, R. P. & III, D. L., .I. Am. Ceram. Sot., 69(4) (1986) 325