Alpha - omega and omega - alpha phase transformations in zirconium under hydrostatic pressure: A 3D mesoscale study

A three dimensional (3D) elastoplastic phase-field model is developed for modeling the hydrostatic pressure-induced alpha - omega phase transformation and the reverse phase transformation, i.e. omega - alpha, in zirconium (Zr). Plastic deformation and strain hardening of the material are also considered in the model. The microstructure evolution during both phase transformations is studied. The transformation start pressures at different temperatures are predicted and are plotted as a phase diagram. The effect of phase transformations on the mechanical properties of the material is also studied. The input data corresponding to pure Zr are acquired from experimental studies as well as by using the CALPHAD method. Our simulations show that three different omega variants form as laths. On release of pressure, reverse phase transformation initiates at lath boundaries. We observe that both phase transformations are martensitic in nature and also occur at the same pressure, i.e. little hysteresis. The transformation start pressures and the kinetics of the transformation predicted by our model are in good agreement with experimental results. (C) 2015 Acts Materialia Inc. Published by Elsevier Ltd. All rights reserved.