Isothermal annealing of shocked zirconium: Stability of the two-phase α/ωα/ω microstructure

Under high pressure conditions, Zr undergoes a phase transformation from its ambient equilibrium hexagonal close packed αα phase to hexagonal ωω phase. Upon returning to ambient conditions, the material displays hysteretic behavior, retaining a significant amount of metastable ωω 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 503 K. The evolution of the αα phase volume fraction was tracked quantitatively, and the dislocation densities in both phases were tracked qualitatively during annealing. Upon heating, the reverse transformation of ω→α 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 ωω phase, leading to the postulation that transformation from ω→α is restricted by the pinning effect of dislocation structures within the ωω phase. Electron backscatter diffraction analysis affirmed that the expected (0001)α‖(101‾1)ω and [101‾0]α‖[112‾3‾]ω orientation relationship is maintained during nucleation and growth of the αα phase during the annealing.