Thermodynamic modelling for the long-period stacking ordered phase in Mg-Y-Al system: Construction and application

An adequate thermodynamic model is required for describing the long-period stacking ordered (LPSO) phase because previously developed models cannot account for the accurate atomic occupancy of the LPSO structure. In this study, atomic structures of the equilibrated 18R and 10H phases in Mg-Y-Al alloys were systematically investigated via atomic-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-TEM) and first-principles calculations. Three structures (alpha-type, beta-type, and gamma-type) with different numbers of Mg layers (one, two, and three, respectively) sandwiched between L12 structures were present in the 18R phase. Meanwhile, delta-type (without Mg layers), epsilon-type (with one Mg layer), and zeta-type (with two Mg layers) structures were detected in the 10H phase. The presence of the interstitial atoms in the L12 structure was confirmed by TEM and theoretical calculations. Based on the obtained results, a new thermodynamic model for the LPSO phase was constructed and successfully applied to establish the thermodynamic description of the Mg-Y-Al system. The CALPHAD-type calculation data obtained using the proposed thermodynamic description were in good agreement with the experimental results. The findings of this study can help elucidate the formation mechanism of LPSO phases and guide Mg-Y-Al alloy design.