Modeling the Thermal Stability of the $\alpha/\omega$ Microstructure in Shocked Zr: Coupling between defect state and phase transformation

TL;DR

This paper develops a temperature-dependent model linking defect reduction and phase transformation in shocked Zr, revealing how dislocation dynamics influence the stability of the $ ext{omega}$ phase during annealing.

Contribution

It introduces a novel coupled model that connects dislocation removal mechanisms with phase transformation kinetics in shocked Zr.

Findings

Dislocation densities decrease prior to reverse transformation.

The model captures transient and asymptotic transformation behaviors.

Dislocation removal occurs via glide and climb processes.

Abstract

Under high pressure, Zr undergoes a transformation from its ambient equilibrium hexagonal close packed $\alpha$ phase to a simple hexagonal $\omega$ phase. Subsequent unloading to ambient conditions does not see a full reversal to the $\alpha$ phase, but rather a retainment of significant $\omega$. Previously, the thermal stability of the $\omega$ phase was investigated via in-situ synchrotron X-ray diffraction analysis of the isothermal annealing of Zr samples shocked to 8 and 10.5 GPa at temperatures 443, 463, 483, and 503 K. The phase volume fractions were tracked quantitatively and the dislocation densities were tracked semi-quantitatively. Trends included a rapid initial (transient) transformation rate from $\omega\to\alpha$ followed by a plateau to a new metastable state with lesser retained $\omega$ (asymptotic). A significant reduction in dislocation densities in the $\omega$ phase was observed prior to initiation of an earnest reverse transformation, leading to the hypothesis that the $\omega\to\alpha$ transformation from is being hindered by defects in the $\omega$ phase. As a continuation of this work, we present a temperature dependent model that couples the removal of dislocations in the $\omega$ phase and the reverse transformation via a barrier energy that is associated with the free energy of remaining dislocations. The reduction of dislocations in the $\omega$ phase occur as a sum of glide and climb controlled processes, both of which dictate the transient and asymptotic behavior of the annealing process respectively.