Surface morphology, plastic deformation and phase transformation in single crystal superalloys under laser shock peening without coating

Laser shock peening without coating (LSPwC) as a surface treatment has been demonstrated the ability to enhance the fatigue performance while maintaining the grain boundary-free nature of single crystal superalloys. The evolution of morphology and microstructure in the peened surface, however, still remains puzzled for the superalloys under varying technical parameters. This investigation explores the progression of surface topography, plastic deformation, phase transformation, as well as microhardness increment in the LSPwC affected layer. The laser pulses endue the target surface with a specific morphology and increase the roughness initially, while it diminishes gradually after multiple thermal-mechanical cycles. Experimental findings indicate that a stratified structure comprising a recast layer and a deformation layer is formed underneath the surface due to the thermal-mechanical effects during the peening process. The recast layer exhibiting an average depth of 1 μm, is composed of γ matrix and amorphous nano-particles rich in Al and O element, while these particles are capable of impeding dislocation glide in the subsequent peening. Additionally, the diffusion owing to high temperature and strain gradient causes elemental segregation and phase transformation of γ' → γ at the interface between the recast and the deformation layer. Accompanied by the severe plastic deformation, the crystalline orientation exhibits a noticeable local deviation across the slip bands in the deformed layer. Finally, as a result of γ' dissolution, nano-particles formation and dislocations multiplication, the LSPwC treated specimens exhibit a maximum hardness of approximately 600 HV at surface and a hardness gradient extending to around 350 μm at depth.