Isothermal tetragonal to monoclinic transformation Table 1. Starting and ending temperatures of the t-+m and m-t transitions for ZrO2(3 mol%Y2O3) and zrO2(2 mol%Y2O3) specimens sintered at 1600c M A ZrOz (3 mol%Y2O3) 387°C 142°C 374°C 542°C ZrO2(2 mol%Y2O3 459C 322°C 567°C 602C 0.8(a) 08(b) 85945四 Fig. 2. Thermal expansion curves obtained at a heating and cooling rate 100 C/min for ZrO2 (3 mol%Y2O3) ceramics held at Table 2. Results of thermal expansion and XRD measurements for ZrO2(3 mol%Y2O3)specimens held at differ ent temperatures for 4 h Temperature(°C) 200 e-m phase 13.4% sothermal m-phase 52.47% Athermal m-phase Overall m-phase Expansion before hol 3532% 0.24335 Expansion during holding n after holding 24 00589%04098% Overall expansion 1.3990% 1.1510% 15120% diagram. 2 The microstructure of the t+m dual is evident that a cooling rate of 100 C/min is not phasc obtaincd at room tempcrature implies that high enough to inhibit the t-m transformation part of the sintered t-phase has transformed to m- before g, and phase during cooling. However, ZrO2 (3 mo- perature, the more pre-Im phase produced prior Lo 1%Y2O3) ceramics were sintered in the c+t dual holding. Therefore, the initial phase constitutent phase region and the microstructure obtained at for the measurement of the isothermal kinetics room temperature is composed of c, t and m-pha ses. Table 1 lists the transformation temperatures of ZrO2(2 mol% Y2O3)and ZrO2 (3 mol% Y2O3) 1623 ceramics, which reveals that the appearance of c- phase results in the lowering of the transformation 4473K temperature 5423K 3.2 Kinetics of the isothermal/t-m transformation in ZrO2 3mo/%Y2O3) ceramics v8z Thermal expansion curves obtaincd at a hcating and cooling rate of 100C/min for ZrO2(3 mo 1%Y2O3)ceramics held at 200 C for 4 h are shown in Fig. 2:(a) refers to the curve of expansion vs temperature;()refers to the curve of expansion Time(s) time. Results of thermal expansion and XRD measurements for specimens held at difierent tem Fig. 3. Chan the fraction of m-phase with holding time for Zro2(3 mol%Y2O3) specimens held at different tempera peratures for 4 h are listed in Table 2, from which it Ires for 4 hIsothermal tetragonal to monoclinic transformation 31 Table 1. Starting and ending temperatures of the t-+m and m+t transitions for Zr02(3 mol% Y20J) and Zr02(2 - mol%Y203) specimens sintered at 1600°C Specimens Zr0,(3mol%Y203) Zr02(2 mol%Y203) MS Mf AS Ar 387°C 142°C 374°C 542°C 459°C 322°C 567°C 602” C 0.2 - I I I I I I I 0 100 200 300 400 500 600 700 Temperature (“C) 0.8 -(b) 0.6 - 0.4 - 0.2 0 Time (ks) Fig. 2. Thermal expansion curves obtained at a heating and cooling rate lOO”C/min for ZrOz(3 mol% Y&I,) ceramics held at 200°C for 4 h: (a) curve of expansion vs temperature; (b) curve of expansion vs time. Table 2. Results of thermal expansion and XRD measurements for Zr02(3mol%Yz03) specimens held at differ￾ent temperatures for 4 h Temperature (“C) Pre-m phase Isothermal m-phase Athermal m-phase Overall m-phase Expansion before holding Expansion during holding Expansion after holding Overall expansion 150 15.53% 16.40% 31 .Q& 0.3532% 1.0460% 0 1.3990% 200 13.4% 52.47% 0 65.80% 0.2344% 0.9214% 0 1.1510% 250 10.93% 49.68% 2.69% 63.30% 0.1995% 0.9069% 0.04919% 1.1510% 300 13.23% 48.19% 3.20% 64.40% 0.24335% 0.8864% 0.0589% 1 .I 880% 350 3.60% 15.70% 7.20% 26.50% 0.2061% 0.4098% 0.4098% 1.5120% diagram. l2 The microstructure of the t + m dual phase obtained at room temperature implies that part of the sintered t-phase has transformed to m￾phase during cooling. However, ZrOz(3 mo- 1% Y203) ceramics were sintered in the c + t dual phase region and the microstructure obtained at room temperature is composed of c, t and m-pha￾ses. Table 1 lists the transformation temperatures of Zr02(2 mol% Y203) and Zr02(3 mol% Y203) ceramics, which reveals that the appearance of c￾phase results in the lowering of the transformation temperature. 3.2 Kinetics of the isothermal t-m transformation in Zr02(3 mol% YzOs) ceramics Thermal expansion curves obtained at a heating and cooling rate of lOO”C/min for Zr02(3mo- 1% Y203) ceramics held at 200°C for 4 h are shown in Fig. 2: (a) refers to the curve of expansion vs temperature; (b) refers to the curve of expansion vs time. Results of thermal expansion and XRD measurements for specimens held at different tem￾peratures for 4 h are listed in Table 2, from which it is evident that a cooling rate of lOO”C/min is not high enough to inhibit the t-m transformation before holding, and the lower the holding tem￾perature, the more pre-m phase produced prior to holding. Therefore, the initial phase constitutent for the measurement of the isothermal kinetics I 623K 2 573K 3 523K 4 473K 5 423K Time (s) Fig. 3. Changes of the fraction of m-phase with holding time for Zr02(3 mol% Y203) specimens held at different tempera￾tures for 4 h