Enhancing surface compressive residual stress and its depth in TB6 alloy via sequentially activated plasticity mechanisms for exceptional fretting fatigue life

Introducing high and deep compressive residual stress (CRS) through gradient plastic strain by surface strengthening technologies improves the fretting fatigue life of titanium alloys, but the effects of deformation mechanisms on CRS remain unclear. In this study, the high (-713 MPa) and deep (1148 μm) CRS was achieved in TB6 alloy by laser shock peening (LSP) and subsequent shot peening (SP) via sequential plasticity mechanisms in the primary α (α_p), secondary α (α_s), and β phases, indicating a superior CRS magnitude-depth balance over other surface strengthening technologies. The high CRS originates from dislocation multiplication in the α_p, α_s, and β phases, and {101¯1}<101¯2¯> twins in the α_s phase induced by LSP, as well as higher dislocation density in the α_p, α_s, and β phases, and {101¯2}<1¯011> twins in the α_p phase induced by SP. Meanwhile, the deep CRS results from large penetration depth of shock waves and plastic strain during LSP. Crystal plasticity simulations reveal that high strain rate and large strain under combined LSP and SP promote the activation of {101¯2}<1¯011> and {101¯1}<101¯2¯> twinning of α phase, enhancing the plastic strain and CRS. Besides, phase morphology and size govern twinning modes in the α_s and α_p phases, while stacking faults further facilitate {101¯2}<1¯011> twinning. Sequential plasticity mechanisms also induce surface-to-interior gradient microstructure, and the synergy of CRS and gradient microstructure increases the fretting fatigue life to beyond 2 × 10⁶ cycles under a maximum stress of 125 MPa and a stress ratio of 0.06, over 13-fold that of the untreated alloy.