Stable and metastable phase equilibria in binary Mg-Gd system: A comprehensive understanding aided by CALPHAD modeling

In this paper, a comprehensive understanding of stable and metastable phase equilibria in binary Mg-Gd system was conducted with an aid of the CALculation of PHAse Diagram (CALPHAD) modeling. Firstly, thermodynamic descriptions of all the stable phases in the Mg-Gd system were re-assessed by considering all the experimental data in the literature. The discrepancy between the phase equilibria and thermochemical properties existing in the previous assessments was eliminated, and the better agreement with the experimental data was achieved in the present assessment. Secondly, the Gibbs energies for metastable β''-Mg3Gd and β'-Mg7Gd were constructed based on the first-principles and CALPHAD computed results as well as their correlation, and then incorporated into the CALPHAD descriptions. The model-predicted solvuses of (Mg) in equilibrium with the metastable β''-Mg3Gd and β'-Mg7Gd compounds showed very good agreement with the limited experimental data. Finally, the presently obtained thermodynamic descriptions of both stable and metastable phases in the binary Mg-Gd system were further validated by realizing the quantitative Scheil-Gulliver solidification simulations of 5 as-cast Mg-Gd alloys, and the successful prediction of the precipitation sequences in Mg-15Gd and Mg-12Gd alloys during the aging process.