The role of Ag, Ca, Zr and Al in strengthening effects of ZK series alloys by altering G.P. zones stability

Abstract The density of precipitates is the most important factor in the strengthening effects of alloys by aging. We here use first-principles calculation to reveal the mechanism of increasing density of precipitates by adding alloying elements for Mg-Zn alloys. The Guinier-Preston (G.P.) zones, which begin to occur during the early stage of precipitation, play an important role in the strengthening of the Mg-Zn alloys. To reveal the mechanism of formation of G.P. zones in Mg-Zn alloys with different alloying elements, we have undertaken systematic calculations to investigate the influence of Ag, Ca, Zr and Al on the stability of G.P. zones in Mg-Zn alloys. We find that the formation energies of Zn G.P. zones with Ag and Ca are much lower than that of pure Zn G.P. zones, while the formation energies of Zn G.P. zones with Zr and Al are slightly higher than that of pure Zn G.P. zones. It indicates that the effects of Ag and Ca to enhance the stability of G.P. zones are much stronger than these of Zr and Al, which is consistent with the experimental observation that Ag and Ca can promote efficiently the density of precipitation of G.P. zones in Mg-Zn alloys, but Zr and Al cannot. We also find that the combined addition of Ag and Ca not only enhances the stability of G.P. zones but also decreases the difference of formation energies of G.P. zones parallel to different habit planes, which provides conditions for increasing nucleation sites for G.P. zones that corresponds better strengthening effect. Furthermore, we analyze electronic structures to understand the different roles of these alloy elements in alteration of stability of G.P. zones.