Phase field study of the grain size and temperature dependent mechanical responses of tetragonal zirconia polycrystals: A discussion of tension-compression asymmetry

In this paper, the asymmetric mechanical responses of tetragonal zirconia polycrystals (TZPs) under uniaxial tension and compression are investigated by phase field (PF) modelling. The previous PF model for the tetragonal to monoclinic (t-m) phase transformation (Mamivand et al., 2013) is modified to characterize the grain size effect and the influences of ambient temperature. A modified chemical free energy functional based on the fourth-order Landau polynomial is proposed to properly describe the energy barrier of the t-m phase transformation at different ambient temperatures. In addition, a thermodynamic approach is proposed to describe the suppression effect of the grain boundaries on the t-m transformation. The interfacial energy between the martensitic twins is also differentiated from that at the austenite-martensite interface. PF simulations are then carried out to relate the microstructure evolution of TZPs with the mechanical responses during a loading cycle. In particular, the asymmetric mechanical responses of TZPs under tensile and compressive loading cycles are compared and discussed. It is demonstrated that the modified PF model can well characterize the microstructural evolution process as well as the grain size and temperature dependent stress-strain responses of TZPs. For both types of loadings, TZPs exhibit shape memory effect (SME) below the equilibrium temperature and pseudoelasticity above the austenite transition temperature. The grain size effect and the influences of ambient temperature on the microstructural evolution process and the characteristics of the stress-strain responses of TZPs under uniaxial tension and compression are systematically discussed. It is revealed that the tension-compression asymmetry of TZPs is mainly attributed to the dilatational transformation strain accompanying the t-m phase transformation.