Nucleation and growth mechanisms of γ" phase with single-unit-cell height in Mg-RE-Zn(Ag) series alloys: a first-principles study

The highly thermal stability precipitates strengthen creep resistance for alloys, which require precipitates to resist coarsen at elevated temperature. In the Mg-RE-Zn (Ag) series alloys, the key strengthening phase - gamma" phase, remains its single-unit-cell height throughout aging process. The origin of such extremely thermal stability of gamma" phase is still unclear. By using the first-principles calculations, it is found that the formation of gamma" phase introduces compressive strain to its surrounding alpha-Mg lattice and simultaneously alter charge distribution of alpha-Mg lattice near the alpha-Mg/gamma" hetero-interface. These two variations over alpha-Mg lattice lead to lower vacancy formation energies and migration energy barriers for solute atoms near the alpha-Mg/gamma" hetero-interface in comparison with that of bulk condition. Consequently, the variations facilitate rapid diffusion of solute atoms near the gamma" phase and promote nucleation rate of other gamma" plates around it. On the basis of ledge-thickening model, the origin of nucleation/growth of gamma" phase with single-unit-cell height was explained. Thermodynamically and kinetically, the solute atoms are not apt to migrate into the nearest basal plane adjacent to the gamma" phase. Moreover, the lower aging temperature (similar to 200 degrees C) and almost completely coherent alpha-Mg/gamma" hetero-interface lead to the ledges are extremely hard to nucleate on the hetero-interface. Therefore, gamma" plates maintain single-unit-cell height throughout aging process. (C) 2021 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.