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

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 503K. 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 * Corresponding Author

Modeling the α/ω thermal stability in shocked Zr: A coupling between dislocation removal and phase transformation

“…α/ω interface, dislocations, grain boundaries, etc.). The molecular dynamics study of Zong suggested that the reverse transformation via interfacial growth of existing martensitic α laths was less likely to occur than the defect mediated heterogenous nucleation process at all temperatures, which was inconsistent with the ex-situ annealing experiments from Low et al [25]. This apparent disagreement between molecular dynamics predictions and experimental observation indicates that a more detailed description of the defect states and their role in the reverse transformation is warranted.…”

“…The data used to develop and validate this model came from annealing studies of shocked Zr performed by Low et al [25] and Brown et al [24]. The shocked material for those studies came from gas gun experiments performed by Ceretta and collaborators [4,6,9,26,27].…”

“…The experimental setup was typical for powder diffraction on a polycrystalline sample. A high energy monochromated incident beam (E = 86 keV) was used to illuminate the samples and the resulting diffraction images were collected on a a twodimensional detector placed at an appropriate distance to capture at least 5 diffraction rings from each phase [25]. The individual (α/ω) phase fractions were tracked quantitatively and the dislocation densities were tracked semi-quantitatively.…”

“…Significant ω → α transformation was observed within the temperature range of 475 < θ < 550 K [24]. Low et al further investigated the transformation under isothermal conditions at temperatures of 443, 463, 483 and 503 K in samples similarly shocked to 8 and 10.5 GPa [25] and observed a rapid initial transformation rate. At all temperatures there was a continuous deceleration of the transformation, with the transformation rate approaching zero with significant ω phase remaining.…”

“…However, the model over-predicts the reduction of microstrain for all temperatures except 443K. Low et al observed that the evolution of microstrain (and by extension dislocation density) of the 10.5 GPa samples was fundamentally different than the 8 GPa samples, leading to a speculation that the defect populations of the two samples were qualitatively different likely due to more extensive ω phase plasticity during the shock event [25]. The model predictions are consistent with this hypothesis and suggest that within the 10.5 GPa samples there is a population of defects that are not readily annealed out at these low homologous temperatures but is also not actively resisting the ω → α transformation.…”