Computational Modeling of the Structure and Stability of GP Zones in Mg Alloys 15 views

Mg alloys are the lightest among all structural metals. However, currently Mg alloys are reviewed as either too soft, too brittle or both, which limits their broader application. GP zones are nanoscale, solute-enriched precipitate formed at the early stage of precipitation. Mg alloys with GP zone strengthening are reported to achieve a good balance between strength and ductility. In this dissertation, three computational methods, DFT, cluster expansion, and CALPHAD (CALculation of PHAse Diagram) modeling have been applied to understand thermodynamic behaviors of a series of Mg alloy systems. In the Mg-Ca-Al system, the atomic structures and thermodynamic stability of GP zones has been predicted firstly, followed by an analysis of generalized stacking fault energies (GSFE) to explain the similar strengthening effect of GP zones of Mg-Ca-Al and Mg-Ca-Zn alloys. Two methods - alias shear deformation and slab deformation – are employed to calculate the GSFEs, and the differences in GSFEs calculated by these two methods are discussed. In Mg-Al system with large lattice distortion, the traditional cluster expansion and mixed-spaced cluster expansion are applied to predict the ground state, and the prediction results are compared between the two methods. In the Mg-Ca-In system, a more accurate crystal structure representation of prismatic precipitate than the reported one has been determined, then the relation between the precipitate and D019 structure has been revealed. Furthermore, coherency strain energy is calculated in both basal and prismatic planes to analyze the key factor in controlling prismatic precipitate morphology. In the Mg-In system, thermodynamic modeling of the Mg-In system is performed by the CALPHAD (CALculation of PHAse Diagram) approach in combination with available experimental data in the literature and first-principles calculations from the present work. To find a rare-earth free Mg-Ca-X system capable of forming prismatic GP zones, we established four screening filters based on solute elements’ solubility in the Mg matrix, cost and safety consideration, thermodynamic stability of the precipitate phase, and the morphology of the precipitate phase. By applying these criteria through DFT, CALPHAD and reported experimental results, we find that Ga might be a promising replacement element.

PHD (Doctor of Philosophy)

English

2025-12-12