Chemical ordering as a precursor to formation of orderedδ-UZr2phase: a theoretical and experimental study.

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 disorderedγ-(U, Zr) phase (bcc structure) to transform into the chemically orderedδ-UZr2phase (C32, AlB2type structure). A novel Monte-Carlo based strategy is developed to generate SQS structures to study theβ→ωdisplacive phase transformation in A1-xBxbinary random alloy. Structures generated with this strategy and using DFT calculations, it is determined that (222)bccplane collapse mechanism is energetically unfavorable in chemically disordered environment at UZr2composition. A mechanically and dynamically stable 24 atom SQS structure is derived which serves as a structural model of chemically orderedδ-UZr2structure. Finally, a thermodynamic basis for the mechanism of theγtoδtransformation has been established which ensures chemical ordering is a precursor to the subsequent displacive transformation to form chemically orderedδ-UZr2structure.