Gradient age-precipitation behavior induced by gradient nano-grain in Mg–Gd–Ag–Zr alloy

Gradient nano-grained structure is frequently engineered into metallic materials, including Mg alloys, to achieve superior combination of strength and ductility. Nevertheless, the influence of this microstructural feature on aging precipitation behavior remains inadequately understood. In this study, the age-precipitation behavior of a gradient nano-grained Mg–Gd–Ag–Zr alloy prepared via ultrasonic severe surface rolling was investigated. The result indicates that the aging precipitation behavior exhibits a depth-dependent variation within this alloy. In the nano-grained surface layer, hierarchical β nano-precipitates are predominant, while at greater depths, the precipitates consist of β nanoparticles located at grain boundaries, along with intragranular β′ and γ″ nano-plates. The formation of β nano-precipitates deviates from the conventional precipitation behavior observed in Mg–Gd–Ag alloys, and is attributed to the high density of defects induced by the surface nano-grained structure. This finding advances our understanding of the precipitation behavior in the alloys with heterogeneous microstructure.