Isothermal tetragonal to monoclinic transformation ceramics. This is because the "nose"'temperature of the TTt curve is largely determined by the starting temperature of the t-m transformation 3.3 Microstructure of ZrO2 3 mo/%Y203)ceramics Microstructure and morphologies of the m-phas are shown in Fig. 8, among which Fig 8(a) indi cates that the grain with dark contrast is monocli- nic phase, while the grain with relatively bright m dual phase. Figure 8(b)shows the different morphologies of m-phase within two irre l/Tx10+3 gular t-phase grains. In one grain the m-phase takes the form of parallel laths, which is similar to Fig. 6. Functional relationship between In(nk)and 1/T for the morphology of low-carbon martensite. Lath ZrO2(3 mol%Y2O3)specimens shaped m-phase is one of the features of the microstructure in ZrO2(Y2O3)ceramics, which is different from those in pure zirconia. In another the grain boundaries t-phase. 12 As a result, grain, the m-phase appears in the form of an"N the size of c-phase grai lways larger than that shape, large"N"typed m-phase passes across the of t-phase grains. SEM photographs of ZrO2 grain and the un-transformed part of the grain (2 mol%Y203) and Zro2 (3 mol% Y2O3)ceramics intercepted by a smaller one. " N"typed re simultaneously shown in Fig. 7, which indicates morphology is similar to that of high-carbon mar- that the average grain size of t-phase in the former tensite. The different morphology of the m-phase is Is larger than that in the latter with a small amount probably associated with different mechanisms of f larger c-phase grains. Therefore, formation, in that"N"typed m-phase is indicative preferential growth of c-phase grains inhibits the of auto-catalytic nucleation and coordinating growth of t-phase grains in ZrO2(3 mol%Y2O3) growth and the whole process is much faster, while ceramics and the existence of a small amount of c- the formation of lath-shaped m-phase is relatively phase grains refines the t-phase grains. The fact slower. Small triangular domains formed at inter chat the incubation periods of the ttt curves of sections between lattice invariant shear(Lis)twins ZrO2 (3 mol%Y2O3)ceramics are shorter than and deformation twins are shown in Fig 8(c).It those of the Ttt curves of Zro2(2 mol%Y203) significant that microcracks caused by collisions ceramics can be attributed to the refinement of the bctween the two kinds of twins appear at areas phase grains through the existence of c-phase without triangular domains and vice versa, indi grains. The lower"nose"temperature of the TTt cating that the triangular domain is essentially a curve of ZrO2 (3 mol% Y203)ceramics as com- type of coordinating twin, among which the LIS pared to that of zrO2(2 mol%Y2O3)ceramics can twin plane is(001)m and the twin plane of the be explained by the lower starting temperature of triangular domain is(011)m 13 Figure 8(d)illus- the t-m transformation of zro2(2 mol% Y203) trates the case in which the m- plate is deflected by a Fig. 7. SEM photographs showing the natural surfaces of (a)ZrO2(2 mol%Y203)and(b)ZrO2(3 mol%Y203)specimensIsothermal tetragonal to monoclinic transformation l/T x 1O‘4 (K-l) Fig. 6. Functional relationship between In(&) and l/T for ZrOz(3 mol% YzO3) specimens. the grain boundaries of the t-phase.12 As a result, the size of c-phase grains is always larger than that of t-phase grains. SEM photographs of Zr02 (2 mol% Y203) and ZrOz(3 mol% Y203) ceramics are simultaneously shown in Fig. 7, which indicates that the average grain size of t-phase in the former is larger than that in the latter with a small amount of larger c-phase grains. Therefore, in some sense, preferential growth of c-phase grains inhibits the growth of t-phase grains in Zr02(3mol% Y203) ceramics and the existence of a small amount of c￾phase grains refines the t-phase grains. The fact that the incubation periods of the TTT curves of Zr02(3 mol% YzOs) ceramics are shorter than those of the TTT curves of Zr02(2mol% Y203) ceramics can be attributed to the refinement of the t-phase grains through the existence of c-phase grains. The lower “nose” temperature of the TTT curve of Zr02(3mol% Y203) ceramics as com￾pared to that of Zr02(2 mol% Y203) ceramics can be explained by the lower starting temperature of the t--+m transformation of Zr02(2 mol% Y203) 39 ceramics. This is because the “nose” temperature of the TTT curve is largely determined by the starting temperature of the t-+m transformation. 3.3 Microstructure of ZrO2(3mo/% Y2Os) ceramics Microstructure and morphologies of the m-phase are shown in Fig. 8, among which Fig. 8(a) indi￾cates that the grain with dark contrast is monocli￾nic phase, while the grain with relatively bright contrast is t + m dual phase. Figure 8(b) shows the different morphologies of m-phase within two irre￾gular t-phase grains. In one grain the m-phase takes the form of parallel laths, which is similar to the morphology of low-carbon martensite. Lath￾shaped m-phase is one of the features of the microstructure in ZrOz(Y20s) ceramics, which is different from those in pure zirconia. In another grain, the m-phase appears in the form of an “N” shape, large “N” typed m-phase passes across the grain and the untransformed part of the grain is intercepted by a smaller one. “N” typed m-phase morphology is similar to that of high-carbon mar￾tensite. The different morphology of the m-phase is probably associated with different mechanisms of formation, in that “N” typed m-phase is indicative of auto-catalytic nucleation and coordinating growth and the whole process is much faster, while the formation of lath-shaped m-phase is relatively slower. Small triangular domains formed at inter￾sections between lattice invariant shear (LIS) twins and deformation twins are shown in Fig. 8(c). It is significant that microcracks caused by collisions between the two kinds of twins appear at areas without triangular domains and vice versa, indi￾cating that the triangular domain is essentially a type of coordinating twin, among which the LIS twin plane is (OOl), and the twin plane of the triangular domain is (01 1),.13 Figure 8(d) illus￾trates the case in which the m-plate is deflected by a Fig. 7. SEM photographs showing the natural surfaces of (a) Zr02(2mol% Y203) and (b) ZrOz(3 mol% Y203) specimens