Theoretical investigation on aging precipitation mechanisms and precipitation strengthening effect of Mg-Gd alloy

Magnesium gadolinium (Mg-Gd) alloys have received increasing attention as the lightest structural materials. Although a large number of experimental results have been reported, the aging precipitation mechanism of Mg-Gd alloys is still unclear. In this work, the elastic strain energy, formation energy, and interface energy of existed precipitates in Mg-Gd alloy is studied, and their underlying mechanism is analyzed. It is found that the habit planes of β′, β_F′ and β_T are the (100) plane (or the (11 $\stackrel{-}{2}$ 0) plane in the hexagonal system), but β″ cannot form long-range ordered structures. The phase transition resistance of β_F′ is greater than that of β′, causing β_F′ to precipitate after β′, while the negative interface energy and larger strain energy lead to the precipitation of β_T after β′. Moreover, the influence of the size and volume fraction of precipitates on the strengthening effect caused by precipitates is also calculated. The strengthening effect of β′ is predicted to be the best among the four coherent precipitates. These findings offer guidelines for the design of high-performance Mg-Gd alloys.