Ab initio investigation on the ordering mechanism of L12 phase in Al-Zr-Y (Sc) alloy

The L12-Al3Y(Sc) phase promotes the precipitation kinetics of Al3Zr. However, although Al3Y and Al3Sc are both with L12 structures, the mechanisms are not entirely the same due to different structures of the compound composites: Al3(Zr, Sc) has a core-shell structure, while Al3(Zr, Y) does not. Formation mechanisms of core-shell and non-core-shell structures were revealed in this work through analyzing surface energy, interface binding energy, and interface energy. The results showed that the interface energy and the lattice mismatch of Al/Al3Sc and Al/Al3Y are 0.011 J/m2,1.9% and 0.512 J/m2 and 5% respectively. During the growth of Al3(Zr, Sc) with Al3Sc as the heterogeneous core, the interface energy decreased linearly with increasing Zr contents at the interface and eventually a Zr rich shell formed. However, during the formation of Al3(Zr, Y), the interface energy fluctuated with changes in the composition and Zr(Y) content at the interface, eventually forming a composite phase shell of Al3Y(Zr). The diffusion migration energy for Zr atoms in pure Al was 1.102 eV, and it increased by at least 2% near the Al/Al3Sc interface, while it decreased by up to 46.3% near the Al/Al3Y interface. The impact of the L12-Al3Y(Sc) heterogeneous core on the diffusion ability of Zr atoms is crucial for its influence on the kinetics of Al3Zr precipitation.