Chemical ordering as a precursor to formation of ordered δ-UZr2 phase: a theoretical and experimental study

Abstract

A combination of special quasi-random structure (SQS) analysis, density functional theory (DFT) based simulations and experimental techniques are employed in determining the transformation pathway for the disordered γ-(U, Zr) phase (bcc structure) to transform into the chemically ordered δ-UZr2 phase (C32, AlB2 type structure). A novel Monte-Carlo based strategy is developed to generate SQS structures to study the β → ω displacive phase transformation in A1−x B x binary random alloy. Structures generated with this strategy and using DFT calculations, it is determined that (222)bcc plane collapse mechanism is energetically unfavorable in chemically disordered environment at UZr2 composition. A mechanically and dynamically stable 24 atom SQS structure is derived which serves as a structural model of chemically ordered δ-UZr2 structure. Finally, a thermodynamic basis for the mechanism of the γ to δ transformation has been established which ensures chemical ordering is a precursor to the subsequent displacive transformation to form chemically ordered δ-UZr2 structure.