Effect of hydrostatic pressure on the kinetics of alpha-omega phase transformation in zirconium

A three dimensional (3D) elastoplastic phase-field model, including strain hardening, is employed to study the effect of hydrostatic pressure in the range of 4-9 GPa on the kinetics of alpha-omega phase transformation in zirconium (Zr). The input data corresponding to pure Zr are acquired from experimental studies as well as by using the CALPHAD method. A decreasing incubation time, for the formation of omega variants, with increasing pressure is observed. Avrami (JMAK) equation is used to study the transformation kinetics by analysing the phase fraction plots predicted by the phase-field simulations. The estimated activation energy is in the range of 54-59 kJ mol(-1) and decreases at an average rate of 992 J mol(-1) per 1 GPa increase in pressure. The analysis of Avrami exponents, based on Cahn's approach, show that the transformation region can be divided into two distinct regions with a change in slope, which is attributed to the site saturation. It is concluded that in the first region where the exponents are above 3, the transformation proceeds by nucleation and growth. In the second region where the exponents are sub-unity, the transformation proceeds by growth of the existing variants.