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 ZrO 2 –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 (d V m /d t g ) t g = 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, Δ G 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 Δ G¯ t‐m /σ was found to be a linear function of ErO 1.5 content X as 0.65 X + 3.00, where σ is a standard deviation of the normal distribution. The transformation rate d V m /d t 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 d V m /d t g . The validity and limitation that the c/a is a factor of the transformability were discussed.