w.Z. Zhu 673 Fig. 4. Time-temperature-transformation (TTT)curves for the(a)ZrO2(2 mol%Y2O3) and (b)ZrO2(3 mol%Y2O3)specimens curve consists of t and m phases, and part of the (c) The functional relationship between In(nk phase transforms isothermally to m-phase during and 1/T, shown in Fig. 6, reveals the activa- holding. Change of volume fraction of m-phase tion energy for the transformation to be obtained during holding with time is shown in 2274 kJ/ mol, irrespective of the holding Fig. 3, and the corresponding time-temperature- transformation (TTt) curve turns out to be C (d)The isothermal transformation cannot pro- shaped with a"nose"temperature determined to ceed to completion with part of the con be300° hown in Fig. 4. For comparison, the strained t-phase remaining in the final TTT curve of ZrO=(2 mol%Y2O3)ceramics is also microstructure shown in Fig. 4. The kinetics of the isothermal t-m transformation possesses the following fea As stated above, ZrO2 (3 mol%Y2O3)ceramics were fabricated in the c+t dual phase region at a sintering temperature of 1600oC, according to the (a)The existence of an incubation period which binary ZrO2-Y2O3 phase diagram '2 Morphologies initially becomes shorter with a decrease in of the c-phase grains are comparatively easier to holding temperature, then becomes longer distinguish from those of the t-phase grains. It was when the holding temperature is further previously reported that c-phase grains are rela decreased,resulting in the appearance of a tively difficult to nucleate during sintering and once nose"temperature. The existence of an they nucleate, growing speed is much higher than incubation period is one of the features of a that of t-phase grains until the phase equilibrium diffusional transformation whose isothermal state, which can be ascribed to the fact that a small kinetics can be expressed in terms of the amount of c-phase grains are primarily located at Johson-Mehl-Avrami (JMA)equation, 1.e f=1-exp(kr), where f is the volume frac- tion of transformed phase, k is a variable 1623K associated with the energy barrier for critical 2573K nucleation and growth, and n is a constant 4473K depending on the nucleation sites. The TTT 5423K curve of ZrO2 (3 mol%Y2O3)ceramics lies to 2 he down-left side of that of Zro2 (2 mo- 1%Y2O3)ceramics, indicating that at the same holding temperature, the incubation period of the former is shorter than that of he lat (b)The value of n, the exponent in the JMA equation, is somewhat different at different holding temperatures, implying slight varia tions of nucleating sites with holding tem- Fig. 5. Relationship between InIn 1/(1-) and Int for Zro2(3 perature(Fig. 5) mol% Y2O3)specimens.38 , W. Z. Zhu (a) 723- 523- 1% 10% I I I I 10 102 IO3 Time (s) 623- (b) 5 513 - 0 2 I & 523- F + 473- 423- I- IO 102 I03 IO“ Time (s) Fig. 4. Time-temperature-transformation (TTT) curves for the (a) ZrOz(2 mol% YzO3) and (b) ZrOT(3 mol% YzO3) specimens. curve consists of t and m phases, and part of the t￾phase transforms isothermally to m-phase during holding. Change of volume fraction of m-phase obtained during holding with time is shown in Fig. 3, and the corresponding time-temperature￾transformation (TTT) curve turns out to be C￾shaped with a “nose” temperature determined to be 300°C as shown in Fig. 4. For comparison, the TTT curve of Zr02(2 mol% Y203) ceramics is also shown in Fig. 4. The kinetics of the isothermal t+m transformation possesses the following fea￾tures: (a> co The existence of an incubation period which initially becomes shorter with a decrease in holding temperature, then becomes longer when the holding temperature is further decreased, resulting in the appearance of a “nose” temperature. The existence of an incubation period is one of the features of a diffusional transformation whose isothermal kinetics can be expressed in terms of the Johson-Mehl-Avrami (JMA) equation, i.e. f= 1 -exp(-kt”), where f is the volume frac￾tion of transformed 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. The TTT curve of Zr02(3 mol% Yz03) ceramics lies to the down-left side of that of Zr02(2mo- 1% Y203) ceramics, indicating that at the same holding temperature, the incubation period of the former is shorter than that of the latter; The value of n, the exponent in the JMA equation, is somewhat different at different holding temperatures, implying slight varia￾tions of nucleating sites with holding tem￾perature (Fig. 5). (c) The functional relationship between ln(nk) and l/T, shown in Fig. 6, reveals the activa￾tion energy for the transformation to be 22.74 kJ/mol, irrespective of the holding temperature. (d) The isothermal transformation cannot pro￾ceed to completion with part of the con￾strained t-phase remaining in the final microstructure. As stated above, Zr02(3 mol% Y203) ceramics were fabricated in the c + t d.ual phase region at a sintering temperature of 16OOC, according to the binary ZrO2-Y203 phase diagram.12 Morphologies of the c-phase grains are co’mparatively easier to distinguish from those of the t-phase grains. It was previously reported that c-phase grains are rela￾tively difficult to nucleate during sintering and once they nucleate, growing speed is much higher than that of t-phase grains until the phase equilibrium state, which can be ascribed to the fact that a small amount of c-phase grains are primarily located at 1 623K 2 573K -I- 3523K 4 473K Int Fig. 5. Relationship between lnln 1/(1-f) and In t for Zr02(3 - mol% Y204 specimens