Thermodynamics of the tetragonal to monoclinic phase transformation in constrained zirconia microcrystals

Abstract

End-point thermodynamic analyses were made of the tetragonal to monoclinic transformation (t→m) occurring in ZrO₂ precipitates in a Ca-PSZ alloy and particles in Al₂O₃-ZrO₂ composites. Calculated plots of the reciprocal critical size for transformation temperature were in excellent agreement with experimental data for both systems. Contributions to the total free energy change included bulk chemical, dilatational and residual shear strain energies and also interfacial energies. The latter term consisted of contributions from the change in the chemical surface free energy, the presence of twin boundaries in the precipitate (particle)-matrix interfacial energy. The major impediment to the transformation was the shear strain energy which could not be reduced sufficiently by twinning alone. The t → m reaction proceeded spontaneously when the energy barrier was reduced by the response of the particle-matrix interface. The response comprised loss of coherency and grain boundary microcracking for the Ca-PSZ and Al₂O₃-ZrO₂ alloys, respectively. These results are in accord with recent suggestions that either a stress-free strain or a free surface is a necessary conditions for the initiation of a martensitic transformation.