Evolution of microstructure and grain refinement mechanism of pure nickel induced by laser shock peening

This paper investigates microstructure evolution in laser shock peened pure nickel. The microstructure of the deformed layer produced by laser shock peening (LSP) was systematically characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). Results indicated that the amplitude and depth of micro-hardness in the surface layer increased with the number of laser impacts. A nanocrystalline layer was prepared in pure Ni after LSP and the LSP induced microstructures included dislocations (Ds), ultra-fine laminates (UFLs), ultra-fine grains (UFGs), nano-laminates (NLs), and nano-grains (NGs). Based on the in-depth microstructure observations, a grain refinement mechanism induced by LSP in pure Ni was proposed. The strengthening mechanism of micro-hardness induced by LSP could be attributed to the barriers of dislocation motion, including low-angle grain boundaries, large-angle grain boundaries and dislocation multiplication.