A new approach of a gradient nanograined surface layer for Mg-3Al-1Zn alloy induced by SMRGT

In order to improve the overall properties and performance, a gradient nanostructured surface layer on AZ31B magnesium alloy was generated by newly developed surface nanocrystallization (SNC) and hardening process. In this paper, the surface mechanical rolling grinding treatment (SMRGT) was carried out at room temperature for the first time. The grain size refinement and working hardening were confirmed by the microstructure pictures, XRD results, and microhardness test; the reason of which is supposed to be the strain-induced plastic deformation during the SMRGT process. An average grain size of ~ 100 nm nanograins were observed on the uppermost surface, and grain size of the cross section increased gradually. The deformation twining was observed in the metallograph of local area of Mg alloy. The average microhardness of gradient nanograin (GNG) surface layer was 113 HV, which elevated by ~ 60% compared with the as-received substrate (average 66.1 HV). The mechanism of grain refinement is supposed to be the compressive strain-induced deformation twining, dislocation slip systems movement, the refinement effect of subgrain boundaries, and preferred orientation of local grains and boundaries.