High energy laser-shock induced phase transformation and micro-spallation on surface of stainless steels: The effect of stacking fault energy and deformation mechanisms

Nanosecond laser-shock peening (LSP) was applied at single spots with up to 20 shots on stainless steels (SS304L and SS316L). Interestingly, it was found that the depth of the craters increased almost linearly with the number of LSP shots, and the rates were -6.6 and -10.8 mu m/shot on SS304L and SS316L, respectively. Micro surface spallation surrounded by outward extrusions was observed at the crater center. Chemical and Raman analyses revealed a presence of carbon oxides and an absence of any LSP-induced metal oxidation at the crater center. However, LSP-induced gamma-to-e transformation occurred along with the presence of alpha'-martensite in the crater on SS304L rather than on SS316L, providing direct evidence for e-mediated martensite transformation. The LSP-induced e/alpha' ratio was significantly higher than that induced by robotic hammer peening (RHP) at similar depth. These differences are attributed to stacking fault energy and different plastic deformation mechanisms associated with LSP and RHP.