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

Abstract

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 - γ’’ phase, remains its single-unit-cell height throughout aging process. The origin of such extremely thermal stability of γ’’ phase is still unclear. By using the first-principles calculations, it is found that the formation of γ’’ phase introduces compressive strain to its surrounding α-Mg lattice and simultaneously alter charge distribution of α-Mg lattice near the α-Mg/γ’’ hetero-interface. These two variations over α-Mg lattice lead to lower vacancy formation energies and migration energy barriers for solute atoms near the α-Mg/γ’’ hetero-interface in comparison with that of bulk condition. Consequently, the variations facilitate rapid diffusion of solute atoms near the γ’’ phase and promote nucleation rate of other γ’’ plates around it. On the basis of ledge-thickening model, the origin of nucleation/growth of γ’’ 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 γ’’ phase. Moreover, the lower aging temperature (∼200 °C) and almost completely coherent α-Mg/γ’’ hetero-interface lead to the ledges are extremely hard to nucleate on the hetero-interface. Therefore, γ’’ plates maintain single-unit-cell height throughout aging process.