Superior fretting fatigue resistance of Ni-based single crystal superalloy from wave texture and heterostructure induced by ultrasonic surface rolling

Ni-based single crystal (NBSC) superalloys employed as turbine blades are generally vulnerable to elevated-temperature fretting fatigue (FF) damages during services. To prolong lifetime, ultrasonic surface rolling (USR) was adopted to NBSC in this study, with its FF performance investigated at 650 °C. Results reveal that USR induced a wave texture with reduced roughness and a heterostructure layer characterized by gradient slip trace (ST) network on DD6 surface. These modifications enhance the surface microhardness by 68.6% and yield compressive residual stress (CRS) with amplitude of 941.25 MPa, thus raising FF lifetime by 210.6% compared to as-received sample. During FF test, the wave texture considerably mitigates surface stress concentration and protects the sample bulk from the direct contact of the pad, while CRS effectively reduces the actual stress and compels the inward migration of crack source. These alleviate the synergistic damages from the fretting and fatigue components. Besides, the octahedron-dominated ST networks facilitate dodecahedral slip activations to accommodate cyclic plastic strains, and serve as skeletons to collect free dislocations to fulfill recrystallizations. Slip mode transitions and intense recrystallizations in USR sample require additional dissipated energies. Unlike common rafting in FF-failed as-received sample surfaces, the gradient ST networks manipulate crack propagations through the formations of inclined γ’-raft domains. Frequent crack deflections elevate the critical fracture thresholds and consequently extend FF lifetimes. The findings provide new insights to understand the deformation mechanisms of DD6 during FF and guide their anti-FF designs.