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The first study of the effect of the initial microstructure on its evolutionunder hydrostatic compression before, during, and after the irreversible$\alpha\rightarrow\omega$ phase transformation and during pressure release inZr using in situ x-ray diffraction is presented. Two samples were studied: oneis plastically pre-deformed Zr with saturated hardness and the other isannealed. Phase transformation $\alpha\rightarrow\omega$ initiates at lowerpressure 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. Thisimplies that the general theory based on the proportionality between theathermal resistance to the transformation and the yield strength must beessentially advanced. The crystal domain size significantly reduces, andmicrostrain 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 anddislocation densities are much higher. Despite the generally accepted conceptthat hydrostatic pressure does not cause plastic straining, it does and isestimated. The microstructure is not inherited during phase transformation. Thesignificant evolution of the microstructure and its effect on phasetransformation demonstrates that their postmortem evaluation does not representthe actual conditions during loading. A simple model for the initiation of thephase transformation involving microstrain is suggested. The results suggestthat an extended experimental basis is required for the predictive models forthe combined pressure-induced phase transformations and microstructureevolutions.

Effect of initial microstructure on its evolution and $\alpha \rightarrow \omega$ phase transition in Zr under hydrostatic loading