Density-functional study of the U–Zr system

Density-functional formalism is applied to study the phase equilibria in the U–Zr system. The obtained ground-state properties of the γ (bcc) and δ (C32) phases are in good agreement with experimental data. The decomposition curve for the γ-based U–Zr solutions is calculated. Our calculations confirm that experimentally observed “partial” ordering of the alloy components in the δ-UZr2 (AlB2) phase, in which Zr atoms occupy the “Al” position and the two “B” sites are randomly shared by the U and Zr atoms, is the most energetically favorable within the C32 structure. We argue that stabilization of the δ-UZr2 phase relative to the α-Zr (hcp) structure is due to an increase of the Zr d-band occupancy that occurs when U is alloyed with Zr. A comparison with stabilization of the ω-phase (also C32) in Zr under compression is made.