  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
$(0\;0\;0\;1)_\alpha\parallel(1\;0\;\overline{1}\;1)_\omega$ and
$[1\;0\;\overline{1}\;0]_\alpha\parallel[1\;1\;\overline{2}\;\overline{3}]_\omega$
orientation relationship is maintained during nucleation and growth of the
$\alpha$ phase during the annealing. \end{abstract}
