Printed in Grcat Britain. All righ PII:S02728842(96)00074-0 Effect of cubic Phase on the Kinetics of the Isothermal Tetragonal to Monoclinic Transformation in ZrO2 3 mol%Y203 Ceramics W.Z. Zhu Institute for Nonmetallic Materials, Swiss Federal Institute of Technology(ETH), CH-8092, Zurich, Switzerland (Received 12 December 1995; accepted 23 July 1996) Abstract: In this paper, the effect of cubic phase on the kinetics of the isothermal gonal (t) to monoclinic(m) transformation in ZrO2(3 mol%Y2O3)ceramics was studied by means of thermal expansion analysis (TEA), scanning electron microscopy(SEM), transmission electron microscopy (TEM)and X-ray diffrac- (XRD). Experiment t-m transformation could be expressed using the Johson-Mehh-Avrami equa tion. 1.e where f refers to the volume fraction of transformed m-phase, k is a variable associated with the energy barrier for critical nucleation and growth, and n is a constant depending on the nucleation sites. It was found that both the"nose empcrature and the incubation periods of the tTt curve of the Zro2( mo 1%Y2O3)ceramics were decreased in comparison with those of the TTt curve of the ZrO2(2 mol%Y2O3) ceramics. The time-temperature-transformation (Ttt) curve is C-shaped, with the"nose"temperature determined to be 300 C. Activa tion energy for transformation was regressed to be kJ/mol, which didn't cubic phase grains refined t-phase grains during hot-pressing. It is proposed that the nucleation and longitudinal growth of m-phase plates in Zro2(Y2O3)cera mics are displacive, while the sidewise growth thereafter is controlled by short range diffusion of oxygen ions and, in this sense, the t-m transformation in ZrO2(Y2O3) ceramics possesses both displacive and non- displacive features. 997 Elsevier Science Limited and Techna S.r.I 1 INTRODUCTION of stabilizer, grain size, shape and site of the t phase grains, as well as constraints imposed by the Transformation toughening through the tetragonal neighbouring matrix, -3many problems still (t)to monoclinic(m) transformation in Zro2 alloys remain unsolved. The stress-triggered t-m transi- is believed to be one of the most effective ways of tion occurring in Zro2 alloys is usually considered improving the toughness and reliability of struc- to be athermal, but may also show some isothermal tural ceramics. Although our knowledge of this character 4-7 This isothermal component of the aspect has advanced considerably, e.g. it is gener transformation has received relatively little atten ally agreed that control of the metastability of the tion. There are divided views on the nature of the t-phase is the key to obtaining optimum toughen- kinetics of the isothermal t-m transformation in ing effect and is affected by such factors as content ZrO2(,) ceramics. Results by Nakanishi and Shi su have shown that the kinetics of the Present address: Department of Materials Science and Engi- isothermal t-m transformation is controlled by neering, Zhejiang University, Hangzhou 310027, P R China. the diffusion of oxygen ions with the implication ofCeramics Iniernational24 (1998) 3543 0 1997 Elsevier Science Limited and Techna S.r.1. Printed in Great Britain. All rights reserved PII:SO272-8842(96)00074-O 0272-8842/97 $17.00+.00 Effect of Cubic Phase on the Kinetics of the Isothermal Tetragonal to Monoclinic Transformation in Zr02(3 mol%Y203) Ceramics W. 2. Zhu” Institute for Nonmetallic Materials, Swiss Federal Institute of Technology (ETH), CH-8092, Zurich, Switzerland (Received 12 December 1995; accepted 23 July 1996) Abstract: In this paper, the effect of cubic phase on the kinetics of the isothermal tetragonal (t) to monoclinic (m) transformation in ZrOz(3 mol% YzOs) ceramics was studied by means of thermal expansion analysis (TEA), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffrac￾tometry (XRD). Experimental results showed that the kinetics of the isothermal t--tm transformation could be expressed using the Johson-Mehl-Avrami equa￾tion, i.e. f = 1 - exp(-kt”) where f refers to the volume fraction of transformed m-phase, k is a variable associated with the energy barrier for critical nucleation and growth, and n is a constant depending on the nucleation sites. It was found that both the “nose” temperature and the incubation periods of the TTT curve of the Zr02(3mo- 1% Yz03) ceramics were decreased in comparison with those of the TTT curve of the Zr02(2 mol% YzOs) ceramics. The tim&emperature-transformation (TTT) curve is C-shaped, with the “nose” temperature determined to be 300°C. Activa￾tion energy for transformation was regressed to be 22.74 kJ/mol, which didn’t change with temperature. TEM observation revealed that preferential growth of cubic phase grains refined t-phase grains during hot-pressing. It is proposed that the nucleation and longitudinal growth of m-phase plates in ZrOz(YzOs) cera￾mics are displacive, while the sidewise growth thereafter is controlled by short￾range diffusion of oxygen ions and, in this sense, the t--+m transformation in ZrOz(YzOs) ceramics possesses both displacive and non-displacive features. 0 1997 Elsevier Science Limited and Techna S.r.1. 1 INTRODUCTION Transformation toughening through the tetragonal (t) to monoclinic (m) transformation in Zr02 alloys is believed to be one of the most effective ways of improving the toughness and reliability of struc￾tural ceramics. Although our knowledge of this aspect has advanced considerably, e.g. it is gener￾ally agreed that control of the metastability of the t-phase is the key to obtaining optimum toughen￾ing effect and is affected by such factors as content *Present address: Department of Materials Science and Engi￾neering, Zhejiang University, Hangzhou 310027, P. R. China. of stabilizer, grain size, shape and site of the t￾phase grains, as well as constraints imposed by the neighbouring matrix,1-3 many problems still remain unsolved. The stress-triggered t+m transi￾tion occurring in ZrOz alloys is usually considered to be athermal, but may also show some isothermal character.47 This isothermal component of the transformation has received relatively little atten￾tion. There are divided views on the nature of the kinetics of the isothermal t+m transformation in Zr02(Y20s) ceramics. Results by Nakanishi and Shigematsus have shown that the kinetics of the isothermal t-m transformation is controlled by the diffusion of oxygen ions with the implication of 35