Deformation behavior and failure mechanism during in-phase and out-of-phase thermomechanical fatigue in directionally solidified CM247LC superalloy

This work investigates thermomechanical fatigue (TMF) behavior in directionally solidified CM247LC superalloy at the strain amplitude of ± 0.5 % and ± 0.8 % under in-phase (IP) and out-of-phase (OP) conditions in the temperature interval of 573 K ↔ 1143 K. TMF test results reveal that fatigue life significantly reduces under IP condition due to accumulation of greater strain localization attributed to synergistic effect of creep-fatigue-oxidation damage, especially at high strain amplitude. Conversely, under OP condition, fatigue life degrades due to oxidation-fatigue damage and formation of micro-twins, specifically at high strain amplitude. The alloy exhibits hardening followed by softening behavior under IP condition, whereas it shows continuous hardening under OP condition. Initial hardening in all the TMF conditions is associated with dislocation–dislocation and dislocation-precipitate interactions. The softening phenomena under IP condition is attributed to shearing of γ′ precipitates by stacking faults and anti-phase boundary, whereas the hardening phenomena under OP condition is associated with micro-twins, especially at high strain amplitude. Commonly, strain accumulation is found near the MC carbides in all conditions. Fractography analysis substantiates this fact and shows micro-cracks near MC carbides. Additionally, fractography analysis reveals oxide spike under OP condition, where strain localization is significantly high, as evidenced by electron backscatter diffraction analysis.