Surface strengthening and fatigue life improvement of single crystal Ni-based superalloys via laser shock peening without coating

Improving fatigue properties of engineering alloys via conventional laser shock peening (LSP) with protective coating frequently causes surface grain subdivision, which is undesirable for single crystal (SX) superalloys. Herein, an alternative approach, i.e., LSP without coating (LSPwC), is proven to be feasible in the surface strengthening of SX superalloys with preserving its original SX nature. The thermal effect of LSPwC imposes a layered heterogeneous near-surface microstructure, comprising a recast layer (∼0.8 μm thick) and a severely deformed layer. Upon rapid remelting process, the recast layer converts to a γ solid solution with its outer layer (∼0.4 μm thick) decorated by densely dispersed Al-rich oxides. Such a particulate-reinforced composite possesses a nanohardness 55% higher than the base metal. The severely deformed layer with denser dislocations in the γ matrix than in the γ′ interior forms a 400 μm-deep work-hardened layer. Under cyclic loads, the near-surface structure still preserves the SX structure with increased stacking faults density in the γ matrix. Despite increased surface roughness, the synergistic action of compressive residual stress, work hardening, and heterogeneous near-surface microstructure prolongs the fatigue life of SX superalloys by ∼36.5%. This work sheds light on the development of novel surface-strengthening techniques for engineering applications.