Interfacial dislocation induced precipitation in a Mg−Gd−Zn−Zr alloy

Heterogenous precipitation of the γ′ phase on different types of interfacial defects on {10 1 ¯ 2} twin boundaries in a Mg-Gd-Zn-Zr alloy has been systematically investigated using atomic-resolution high-angle annular dark-field scanning transmission electron microscopy, molecular dynamics simulations and first-principles calculations. Prior to precipitation, twin boundaries comprise a coherent twin boundary and various basal-prismatic/prismatic-basal interfacial defects. These defects include those associated with an I 1 stacking fault, dislocations with an extra half atomic plane oriented either normal or parallel to the interface, and defects without stacking faults or dislocations. During ageing, Gd and Zn atoms segregate to the I 1 stacking fault, the coherent twin boundary, interfacial defects lacking stacking faults or dislocations, and interfacial defects with stacking faults. Heterogeneous precipitation of the γ′ phase occurs specifically at the interfacial defects associated with dislocations. This preference arises from two factors: the easy emission of Shockley partial dislocations from these interfaces, which facilitate the formation of the γ′ structure, and elemental segregation, which creates a favourable chemical environment for γ′ precipitation. During the lengthening of γ′, its compositional transformation closely follows its structural transition due to the attraction of Gd and Zn atoms into the γ′ structure. The γ′ phase form as either variant I or II, depending on the extra half atomic plane of the associated dislocation is normal or parallel to the interface. This variant selection is influenced by the type of partial dislocations emitted from the interfacial defects.