Isothermal Annealing of Shocked Zirconium: Stability of the Two-phase $\alpha /\omega$ Microstructure

Abstract

Under high pressure conditions, Zr undergoes a phase transformation from its ambient equilibrium hexagonal close packed $\alpha$ phase to hexagonal $\omega$ phase. Upon returning to ambient conditions, the material displays hysteretic behavior, retaining a significant amount of metastable $\omega$ phase. This study presents an in-situ synchrotron X-ray diffraction analysis of Zr samples shock-loaded to compressive peak stresses of 8 and 10.5 GPa and then annealed at temperatures of 443, 463, 483, and 503K. The evolution of the $\alpha$ phase volume fraction was tracked quantitatively, and the dislocation densities in both phases were tracked qualitatively during annealing. Upon heating, the reverse transformation of $\omega \to \alpha$ does not go to completion, but instead reaches a new metastable state. The initial rate of transformation is faster at higher temperatures. Samples shock-loaded to higher peak pressures experienced higher initial transformation rates and more extensive transformation. Dislocation content in both phases was observed to be high in the as-shocked samples. Annealing the samples reduces the dislocation content in both phases, with the reduction being lesser in the $\omega$ phase, leading to the postulation that transformation from $\omega \to \alpha$ is restricted by the pinning effect of dislocation structures within the $\omega$ phase. Electron backscatter diffraction analysis affirmed that the expected $(0001{)}_{\alpha }\parallel (10\overline{1}1{)}_{\omega }$ and $[10\overline{1}0{]}_{\alpha }\parallel [11\overline{2}\overline{3}{]}_{\omega }$ orientation relationship is maintained during nucleation and growth of the $\alpha$ phase during the annealing. \end{abstract}

26 Pages, 8 Figures, Accepted to Acta Materialia March 2015