Abstract:

We study the local atomic structure evolution of UZr and UZr₂ alloy melts upon solidification through ab initio molecular dynamics simulations. This is achieved by analyzing in detail the temperature dependence of structure factors, pair correlation functions, the bond angle distributions, Honeycutt-Anderson index and Voronoi tessellation analysis as well as local bond orientation order parameters. We observe that as the temperature decreases the pair correlation functions and structure factors become more structured with clear distinctions at the liquid-solid phase transition temperature. The Honeycutt-Anderson indices and Voronoi tessellation analysis indicate that the liquid phase is predominantly comprised of the icosahedra-like local structures, whose fraction increases with decreasing temperature up to the transition temperature and then abruptly drops at the transition temperature, whereas the bcc-like local atomic structures dominate during the solidification process. Furthermore, the bond orientation order analyses with \({\overline{w} }_{6}\)-\({\overline{q} }_{6}\) correlation map and bond angle distribution imply that the local structures mainly consist of the bcc-type during the solidification below the transition temperature. All the analyses are consistent with each other, showing a first-order liquid to solid phase transition for both UZr and UZr₂ solid solutions, which only differ in different predicted transition temperatures. This work provides a comprehensive insight into the detailed local structure evolution during the solidification of the U-Zr alloy melts at the atomic level. Similar strategies used here can be extended to studying the liquid-solid phase transition in other alloy systems.

Key words: Local atomic structure, Molecular dynamics, Solidification, U-Zr alloy melts