Evolution of gradient structured layer on AZ91D magnesium alloy and its corrosion behaviour

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 101¯2 〈101¯1〉 dense twins and 101¯1 – 101¯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.