Effect of recovery treatment on the wear resistance of surface hammered AZ31 Mg alloy

Surface severe plastic deformation (SPD) is an approach used to nanocrystallize metallic surfaces for enhanced resistance to wear. However, SPD alone does not generate well-defined nanocrystalline structure but one consisting of nano-sized dislocation cells with diffuse boundaries. For materials with limited slip systems such as Mg, SPD may harden the materials but deteriorate their fracture toughness. As a result, the benefit of SPD to the wear resistance of materials may not be appreciable. In this study, we nanocrystallized the surface layer of AZ31 Mg alloy by repeated hammering, followed by recovery treatment (HR). The microstructure, wear behavior, grain structure and wear debris of differently treated samples, including hammering (HM), hammering plus recovery treatment (HM-R), and as-received, were investigated and compared. Results of the study show that the SPD-HR sample had a superior nanocrystalline surface exhibiting the highest wear resistance. The hammered sample (HM) was harder than the HM-HR one but its wear resistance is lower than that of the latter, though it still performed better than the as-received one. The wear debris analysis showed that the surface of the HM-R sample was tougher than that of the HM one. The observed phenomena indicate that the wear resistance of AZ31 Mg alloys depends on both hardness and toughness to which the ductility is of importance. The recovery treatment after HM certainly helped achieving well-defined nanocrystallization for improved wear resistance of the Mg alloy.