Martensitic Transformations in Zirconia Ceramics

Martensitic (shear displacive) phase transformations of the tetragonal to monoclinic and the reverse transformations in Y2O3 doped tetragonal ZrO2 ceramics, which are thought to be included in the displacive phase transformations, were found to propagate isothermally, while in contrast, in CeO2 doped ZrO2 a burst-like (athermal type) propagation was observed.One of the reasons for the isothermal nature, which occurred in the tetragonal to monoclinic transformation and also upon thermal cycles in a few mol% Y2O3 doped ZrO2, is believed to be due to a fairly large amount of oxygen vacancies produced by ionic replacement of Zr4+ by Y3+ ions. In other words, a high diffusibility of oxygen ions through their vacancies controls the propagation rate of the shear transformation in locally distorted tetragonal lattice. In the CeO2 doped ZrO2, on the other hand, a typical athermal nature is expected because of a small amount of oxygen vacancies which exists thermodynamically in the tetragonal lattice.