Phase field modeling of stress-induced tetragonal-to-monoclinic transformation in zirconia and its effect on transformation toughening

This paper proposes a two-dimensional elastic phase field model for capturing the effect of external stress on the tetragonal-to-monoclinic (T → M) phase transformation in zirconia. The model was able to predict the sensitivity of the monoclinic microstructural formation and evolution to the external loading conditions. The effect of stress on the T → M phase transformation was captured by explicitly applying stresses on the computational domain by entering them in the mechanical equilibrium equations as boundary conditions. Simulation results showed that, regardless of the stress loading direction, the monoclinic twinning plane always corresponded to {1 0 0} _m. Results of simulations showed that external stress favors the production of monoclinic variants which exhibit transformation strains aligned with the applied stress direction. When applied to the transformation toughening phenomenon in zirconia, the model was able to elucidate the mechanisms of phase transformation ahead of a crack tip, including the generation of a compressive stress field responsible for the retardation of further crack growth. This work presents the first model capable of demonstrating the process of transformation toughening and crack closure in zirconia.