Microstructure evolution and residual stress distribution of nanostructured Mg-8Gd-3Y alloy induced by severe shot peening

A gradient nanostructured layer was prepared on the surface of Mg-8Gd-3Y alloys by severe shot peening (SSP). The microstructure evolution, residual stress distribution and mechanical property of the gradient nanostructured layer were systematically investigated by Rietveld refinement, transmission electron microscopy, and X-ray stress and hardness analyses. The results show grain sizes of the peened samples were 80-100 nm at the top surface and increased gradually with depth. The refinement process of Mg-8Gd-3Y alloys to nano size during SSP mainly involved three steps: i) introduction of deformation twinning and dislocations in grains with a large size, ii) subdivision of large grains into substructures by twin-twin interaction and dislocation cells interacting twins, iii) evolution of subgrains to nanograins by the dislocation-assisted lattice rotation. Simultaneously, high-level compressive residual stresses were induced with a maximum magnitude of - 206 MPa. Besides, the surface roughness values obviously increased and several surface cracks were detected at the near surface, which might have detrimental effects on surface behavior. Owing to the microstructural improvement, the top surface of the peened Mg-8Gd-3Y alloy attained the maximum microhardness of 155 +/- 5 HV, exhibiting approximately 2 times higher than the pristine alloy.