  The first study of the effect of the initial microstructure on its evolution
under hydrostatic compression before, during, and after the irreversible
$\alpha\rightarrow\omega$ phase transformation and during pressure release in
Zr using in situ x-ray diffraction is presented. Two samples were studied: one
is plastically pre-deformed Zr with saturated hardness and the other is
annealed. Phase transformation $\alpha\rightarrow\omega$ initiates at lower
pressure for the pre-deformed sample but above volume fraction of $\omega$ Zr
$c= 0.7$, a larger volume fraction is observed for the annealed sample. This
implies that the general theory based on the proportionality between the
athermal resistance to the transformation and the yield strength must be
essentially advanced. The crystal domain size significantly reduces, and
microstrain and dislocation density increase during loading for both $\alpha$
and $\omega$ phases in their single-phase regions. For the $\alpha$ phase,
domain sizes are much smaller for prestrained Zr, while microstrain and
dislocation densities are much higher. Despite the generally accepted concept
that hydrostatic pressure does not cause plastic straining, it does and is
estimated. The microstructure is not inherited during phase transformation. The
significant evolution of the microstructure and its effect on phase
transformation demonstrates that their postmortem evaluation does not represent
the actual conditions during loading. A simple model for the initiation of the
phase transformation involving microstrain is suggested. The results suggest
that an extended experimental basis is required for the predictive models for
the combined pressure-induced phase transformations and microstructure
evolutions.
