w.Z. Zhu dislocation during propagation, implying that the ture between c and t phases, and the larger the stress field of dislocation counteracts that of trans- effective driving force for precipitation;(2)nuclea formation tion of t-phase is kinetically favoured at regio Diffusional phase scparation to producc precipi- with relatively low yttria content because the con tates of t-phase has obviously occurred within centration of yttria is reduced in forming nuclei some regions of the c-phase grains during sintering, Comprehensively speaking, the phase constitu as shown ig. (a)and (b), respectively. tent in ZrO (3 mol%Y,O3)ceramics the Morphologies of the m-phase are quite different, in complicated and is composed of cubic, tetragonal that"N" typed m-phase exists in Fig 9(a) with and monoclinic phases. The t-phase can be either microcracks at the grain boundary, while m-phase sintered phase or precipitate depending on sinter- appears in the form of parallel laths in Fig 9(b) ing temperature and cooling conditions. The grain without microcracks at the grain boundary. a dif- size of the sintered t-phase is relatively small, while fraction pattern along the [lll] direction is shown precipitates of t-phase are produced in the form of in Fig. 9(c), in which three(112)type forbidden a"colony" microstructure through diffusional spots appear. Figure 9(d) shows the dark field phase separation within large c-phase grains during image taken by using a(112)forbidden spot, in high temperature sintering. The m-phase can either colony "microstructure- which is a occupy the whole grain or co-exist with the t-phase characteristic of product of the c-t diffusional within the original t-phase grains phase separation-is clearly observed Every(1 12 reflection corresponds to one kind of"colony" 3. 4 Mechanism of the isothermal t-m transition variant, each of which is composed of two groups of twinned precipitating t-phase with planes deter In tetragonal ZrOz, zirconium ions occupy sites of mined to be(101)m. It is found that regions with a the face-centred tetragonal lattice, where the dis lower yttria content in the c-phase grain are more tribution of oxygen ions deviates slightly from the favourable for the precipitation of t-phase, which (001)plane, leading to the appearance of certain can be accounted for as follows:()the lower the tetragonality. When Y2O3 is solid-solutioned into yttria content, the higher the equilibrium tempera- the Zro2 lattice, some of the zirconium ions are 0.2m 0.2um 0.2pm Fig.8. TEM photographs showing the microstructure of Zro2 (3 mol%Y203)specimens:(a)butterfly-like m-phase within a t phase grain and distribution of yttria content within different grains;(b) morphologies of m-phase within different grains; (c)tri- angular domains in twinned m-phase (d )interaction between the dislocations and the m-phase40 dislocation during propagation, implying that the stress field of dislocation counteracts that of trans￾formation. Diffusional phase separation to produce precipi￾tates of t-phase has obviously occurred within some regions of the c-phase grains during sintering, as shown in Fig. 9(a) and (b), respectively. Morphologies of the m-phase are quite different, in that “N” typed m-phase exists in Fig. 9(a) with microcracks at the grain boundary, while m-phase appears in the form of parallel laths in Fig. 9(b) without microcracks at the grain boundary. A dif￾fraction pattern along the [l 1 l] direction is shown in Fig. 9(c), in which three (112) type forbidden spots appear. Figure 9(d) shows the dark field image taken by using a (112) forbidden spot, in which a “colony” microstructure - which is a characteristic of product of the c-+t diffusional phase separation - is clearly observed. Every (112) reflection corresponds to one kind of “colony” variant, each of which is composed of two groups of twinned precipitating t-phase with planes deter￾mined to be { lOl},. It is found that regions with a lower yttria content in the c-phase grain are more favourable for the precipitation of t-phase, which can be accounted for as follows: (1) the lower the yttria content, the higher the equilibrium tempera￾W. Z. Zhu ture between c and t phases, and the larger the effective driving force for precipitation; (2) nuclea￾tion of t-phase is kinetical1.y favoured at regions with relatively low yttria content because the con￾centration of yttria is reduce’d in forming nuclei. Comprehensively speaking, the phase constitu￾tent in Zr02(3 mol% Y203) ceramics is rather complicated and is composed of cubic, tetragonal and monoclinic phases. The t-phase can be either sintered phase or precipitate depending on sinter￾ing temperature and cooling conditions. The grain size of the sintered t-phase is relatively small, while precipitates of t-phase are produced in the form of a “colony” microstructure through diffusional phase separation within large c-phase grains during high temperature sintering. The m-phase can either occupy the whole grain or co-exist with the t-phase within the original t-phase grains. 3.4 Mechanism of the isothermal t-m transition In tetragonal Zr02, zirconium ions occupy sites of the face-centred tetragonal lattice, where the dis￾tribution of oxygen ions deviates slightly from the (001) plane, leading to the appearance of certain tetragonality. When Y203 is solid-solutioned into the ZrO* lattice, some of the zirconium ions are Fig. phar 8. TEM photographs showing the microstructure of Zr02(3mol% YzOs) specimens: (a) butterfly-like m-phase within a t- ;e grain and distribution of yttria content within different grains; (b) morphologies of m-phase within different grains; (c) tri￾angular domains in twinned m-phase; (d) interaction between the dislocations and the m-phase