Effects of Noncompositional Inhomogeneity on t→m Phase Transformation During Grinding of Various Rare‐Earth‐Doped Zirconias

The phase transformation from metastable tetragonal (t) into monoclinic (m) phase must be studied using samples that are homogeneous in their compositions and microstructures. The present study investigated the transformability by grinding for compositionally homogeneous samples prepared by rapid quenching of melts in the system ZrO₂–RO_1.5 (R: Nd, Sm, Y, Er, Yb). The volume fraction of m‐phase V_m(t_g) against grinding time t_g was investigated using powder X‐ray diffraction for the rapidly quenched sample. The V_m(t_g) value increased with grinding time t_g, then saturated to a certain value V_m(∞) of between 1 and 0 when it was ground and crushed for a long time. Both the transformation rate (dV_m/dt_g)_{tg = 0} and the saturated value V_m(∞) decreased with dopant content X. The species of dopants had less influence on the transformability. The experimental V_m(∞) and V_m(0) data could be explained using a model which has a normal distribution of effective free energy change between t and m phases, ΔG^t‐m, among hypothetical parts of the sample. This distribution occurred because of nonchemical energies such as strain, interfacial, and surface energies, which associated with the nucleation of m phase. The average effective free energy change ΔG¯^t‐m/σ was found to be a linear function of ErO_1.5 content X as 0.65X+ 3.00, where σ is a standard deviation of the normal distribution. The transformation rate dV_m/dt_g at t_g= 0 decreased with RO_1.5 content. The axial ratio c/a of the t phase was an important factor in the transformability, the V_m(0), V_m(∞), and the dV_m/dt_g. The validity and limitation that the c/a is a factor of the transformability were discussed.