Ordering in liquid and its heredity impact on phase transformation of Mg-Al-Ca alloys

It is a long-sought goal to achieve desired mechanical properties through tailoring phase formation in alloys, especially for complicated multi-phase alloys. In fact, unveiling nucleation of competitive crystalline phases during solidification hinges on the nature of liquid. Here we employ ab initio molecular dynamics simulations (AIMD) to reveal liquid configuration of the Mg-Al-Ca alloys and explore its effect on the transformation of Ca-containing Laves phase from Al₂Ca to Mg₂Ca with increasing Ca/Al ratio (r_Ca/Al). There is structural similarity between liquid and crystalline phase in terms of the local arrangement environment, and the connection schemes of polyhedras. The forming signature of Mg₂Ca, as hinted by the topological and chemical short-range order originating from liquid, ascends monotonically with increasing r_Ca/Al. However, Al₂Ca crystal-like order increase at first and then decrease at the crossover of r_Ca/Al = 0.74, corresponding to experimental composition of phase transition from Al₂Ca to Mg₂Ca. The origin of phase transformation across different compositions lies in the dense packing of atomic configurations and preferential bonding of chemical species in both liquid and solid. The present finding provides a feasible scenario for manipulating phase formation to achieve high performance alloys by tailoring the crystal-like order in liquid.