Comprehensive damage and deformation mechanisms during out-of-phase thermal mechanical fatigue of the fourth-generation single crystal superalloy

To shed light on the stability and reliability of the fourth-generation single crystal (SX) superalloy during in-service operation, out-of-phase (OP)- thermal mechanical fatigue (TMF) experiments were systematically carried out. The TMF behavior, deformation and damage mechanisms were thoroughly investigated. The results showed that at comparatively low strain ranges (0.5% and 0.6%), the dislocation movements were basically concentrated in γ matrix and the TMF life was basically governed by high-temperature oxidation. More locally, the fatigue cracks initiated from the spalling of surface oxides, while the discontinuous Al2O3 layer could only provide limited protective resistance to crack propagation. Additionally, deformation twins could nucleate in the vicinity of micropores or crack tips, which further induced more complex defects and accelerated the fatigue fracture of the alloy. Nevertheless, as the strain range reached 0.8% and 0.9%, the nucleation and extension of micro-twins were independent on the defects, the mechanisms of twinning-shearing and anti-phase boundary (APB)-shearing were observed throughout the specimen. Under these conditions, the fatigue cracks basically initiated from the severe plastic deformation, which resulted in the much-reduced TMF life of the specimen. Ultimately, salutary guidance for the further application of fourth-generation SX superalloys was rationally summarized.