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

• Published in: Surface & coatings technology 2024-10, Vol.494, p.131369, Article 131369
• Main Authors: Chen, Huitao, Sun, Shouyi, Tian, Fuzheng, Dou, Min, Liu, Lu, Li, Lei
• Format: Article
• Language: English
• Subjects: DD6; Local coarsening; Shear step length; Slip behavior; Thermal exposure; Ultrasonic surface rolling
• ISSN: 0257-8972
• DOI: 10.1016/j.surfcoat.2024.131369

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} slips transform to a mixture of octahedron {111} and dodecahedron {111} 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. •USR induces CRS and nano-hardness gradients on the DD6 surface via slip activation.•Raised shear step length form secondary γ′/γ interface to promote solute diffusion.•Matrix dislocations invade into γ′ particle preferentially across newborn interface.•Thermal exposure degrades surface integrities by slip elimination and γ′ coarsening.•Local γ′ coarsening are manipulated by the orientation transformation of SESF/SISF.