Influence of ultrasonic shot peening on the microstructure and impression creep performance of squeeze-cast AZ91 alloy reinforced with graphene nanoplatelets

Abstract

The present investigation explores the influence of graphene nanoplatelets (GNPs) (0.5, 1.0, and 2.0 wt %) addition as well as ultrasonic shot peening (USP) for 10, 20, and 30 s on the microstructure, hardness, and impression creep response of AZ91 alloy fabricated by squeeze-casting for transmission housing applications. The as-cast AZ91-xGNPs nanocomposites displays lower grain size, lesser Mg17Al12 phase fraction, and higher dislocation density. The USP technique induces compressive residual stress (CRS) and high strain hardening at the surface followed by high dislocation density that increases with time. Both surface roughness as well as the dimple size and thickness after USP treatment are lower for the nanocomposites than for the AZ91. The hardness and creep resistance of the as-cast AZ91-xGNPs nanocomposites are superior to the monolithic alloy, which further improved after USP treatment. In contrast to the base alloy, the GNC3 nanocomposite displays the best hardness and impression creep resistance both in the as-cast and USP-treated condition. Dislocation climb regulated by pipe diffusion is the dominating creep mechanism in the as-cast and USP-treated materials. The improvement of impression creep performance for as-cast nanocomposites is attributed to the reduced Mg17Al12 phase content, high dislocation density, and strengthening from the GNPs. In contrast, the superior creep performance of USP-treated nanocomposites is attributed to the presence of CRS along with high dislocation density and work hardening on the surface.