The effect of rafting on the fatigue strength of single crystal superalloy: Experimental investigation and failure mechanism

The fatigue strength of single crystal superalloys influenced by γ/γ′ rafting was experimentally investigated and the HCF failure mechanism was revealed. The step-loading method was employed to test the fatigue strengths at 10⁷, 10⁶, and 10⁵ after pre-rafting treatment. The effect of rafting states on fatigue strength was quantified using the dimensionless rafting factor. Scanning electron microscope (SEM) and transmission electron microscope (TEM) were utilized to observe the fracture morphology, crack propagation, and dislocation configuration. The results showed that the residual fatigue strengths decreased to 82.2 % of the standard value when the dimensionless rafting factor exceeded 0.43. Pre-rafting would induce crack initiation from γ phase and promote mixed mode fracture of mode-I opening along γ/γ′ interfaces and crystallographic shearing. Moreover, more slip systems were activated with the increase of rafting states, especially the {111} <112> slip system. TEM results illustrated that the decrease in γ/γ′ interface coherence, dislocation climbing and cross slip mechanism would reduce the fatigue strength of rafting specimens.