Deformation mechanism of Ni-based single crystal superalloy under ultrasonic surface rolling and subsequent thermal exposure

In present work, ultrasonic surface rolling (USR) is applied to DD6 Ni-based single crystal (NBSC) superalloys, and the thermal relaxation behavior of the pre-treated alloys are investigated at 650 °C. Combined with mechanical properties, the plastic deformation and recovery mechanisms of the hardened layers are revealed in detail. Results show that compressive residual stress amplitude of 1163 MPa, 75.1 % improvement of surface nano-hardness and 83.02 % reduction on surface roughness are achieved by USR. Both deformed layer thickness and slip density progressively increases with USR cycles, where initial octahedron {111}<110> slips transform to a mixture of octahedron {111}<110> and dodecahedron {111}<112> slips. Meanwhile, shear step length expansions form γ′ segment/γ terminal interface with superlattice extrinsic stacking fault (SESF) on 1¯11, which may have inferior energy threshold for γ dislocation invasions than primary ones through shortening mean free path. Also, this drives γ′/γ solute mingling to induce tortuous interfaces. While dislocation proliferation dominates γ channel hardening, the γ′ hardening derives from planar fault strengthening and net system energy augmentation. During thermal exposure, surface integrities of USRed DD6 are strongly degraded, attributable to slip trace elimination and local γ′ coarsening. Thermal exposure further compels the γ dislocation to migrate towards γ′ segment across γ′ segment/γ terminal interface. Then, they are probably consumed to motivate solutes to phase boundary to reshape the γ′ morphologies through interface rearrangements under the assistance of γ dislocation network. The orientation transformation of SESF on 1¯11 to 010 and the mixture of SESF and superlattice intrinsic stacking fault on 1¯11 are thought to be possible manipulated mechanisms for γ′ coarsening, significant impairing the work hardening of alloy due to interfacial coherency deterioration. This paper could provide guidance for USR strengthening designs of NBSC.