LAUSR

Abstract This article investigates the microstructure evolution and corrosion response of surface mechanical attrition treated (SMAT) AZ91D magnesium alloy. In-depth transmission electron microscopy and combined isothermal calorimetry and pressure measurement technique, a novel and powerful tool for in situ monitoring of the magnesium corrosion process, are explored in the present study. A gradient structured layer of ~ 500 µm thickness with improved (~2.5 times) surface hardness is successfully obtained on the AZ91D alloy surface. SMAT introduces compressive residual stress in the treated layer. TEM results confirmed nanoscale grains of ~125 nm in topmost region and multiple deformation twin-modes, including 10 1 ¯ 2 〈 10 1 ¯ 1 〉 dense twins and 10 1 ¯ 1 – 10 1 ¯ 2 double twinning in SMATed layer. Twining of secondary twins is established in the TEM analysis. Moreover, a twin density gradient is evident within the treated layer, where it decreases with an increase in depth. After 24 h of immersion in 0.9% NaCl solution, the average corrosion rate of SMATed and non-SMATed specimens is ~11.0 and ~3.8 mm/year, respectively. The corrosion product on non-SMATed specimens has densely packed nano-flakes morphology; however, the SMATed surface shows two different morphologies: sparse nanowires and porous honeycomb-like structure. The SMATed specimen's lower corrosion resistance is attributed to the combined effect of the high density of defects, rougher surface, and smaller volume fraction of β phase at the surface.