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

作者：Low T S E; Niezgoda S R^*

来源：Acta Materialia, 2018, 156: 104-115.

DOI：10.1016/j.actamat.2018.06.004

• Zirconium
• Omega phase
• High pressure
• Phase transformation
• Model
• Annealing

Under high pressure, Zr undergoes a transformation from its ambient equilibrium hexagonal close packed alpha phase to a simple hexagonal omega phase. Subsequent unloading to ambient conditions does not see a full reversal to the alpha phase, but rather a retainment of significant omega. Previously, the thermal stability of the omega phase was investigated via in-situ synchrotron X-ray diffraction analysis of the isothermal annealing of Zr samples shocked to 8 and 10.5 GPa at temperatures 443, 463, 483, and 503 K [25]. The phase volume fractions were tracked quantitatively and the dislocation densities were tracked semi quantitatively. Trends included a rapid initial (transient) transformation rate from omega -> alpha followed by a plateau to a new metastable state with lesser retained omega (asymptotic). A significant reduction in dislocation densities in the omega phase was observed prior to initiation of an earnest reverse transformation, leading to the hypothesis that the omega -> alpha transformation from is being hindered by defects in the omega phase. As a continuation of this work, we present a temperature dependent model that couples the removal of dislocations in the omega phase and the reverse transformation via a barrier energy that is associated with the free energy of remaining dislocations. The reduction of dislocations in the omega phase occurs as a sum of glide and climb controlled processes, both of which dictate the transient and asymptotic behavior of the annealing process respectively.

• 出版日期2018-9-1