Isothermal annealing of shocked zirconium: Stability of the two-phase α/ω microstructure. (1st June 2015) - Low, T.S.E.
Abstract: Under high pressure conditions, Zr undergoes a phase transformation from its ambient equilibrium hexagonal close packed α phase to hexagonal ω phase. Upon returning to ambient conditions, the material displays hysteretic behavior, retaining a significant amount of metastable ω 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 α 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 ω phase, leading to the postulation that transformation from ω → α is restricted by the pinning effect of dislocation structures within the ω phase. Electron backscatter diffraction analysis affirmed that the expected ( 0 0 0 1 ) α ‖ ( 1 0 1 ‾ 1 ) ω and [ 1 0 1 ‾ 0 ] α ‖ [ 1 1 2 ‾ 3 ‾ ] ω orientation relationship is maintained during nucleation and growth of the α phase during the annealing.