Abstract
Recently, remarkable experimental progress has been made toward understanding the metastable phases in Mg-Sn alloys, a promising candidate for rare-earth free Mg alloys. However, the detailed structures, stability, and formation mechanisms of these metastable phases at early stages of precipitation remain unclear. Here, we report a first-principles study of the ground-state and finite-temperature phase stability of hexagonal closest packed (HCP)- and face-centered cubic (FCC)-based phases in Mg-Sn alloys. The ground-state phases are searched by the cluster expansion approach coupled with energy input from first-principles calculations. The Monte Carlo simulations with Hamiltonian from the cluster expansion are performed to calculate the configurational free energies and the corresponding metastable phase diagrams are constructed. Our calculations reveal rich varieties of orderings on HCP and FCC lattices, metastable phases with and structures, and possible precursors for Guinier-Preston (G.P.) zones. Further analysis of the vibrational free energy indicates that is stabilized by vibrational entropy at high temperatures. A new precipitation sequence for Mg-Sn alloys is suggested, i.e., supersaturated solid solution → G.P. zone → .
8 More- Received 29 August 2019
DOI:https://doi.org/10.1103/PhysRevMaterials.4.013606
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