Effects of oxidation on fatigue crack initiation and propafation in an advanced disk alloy

Understanding the fatigue performance of aeroengine disk alloys is important for both alloy development and life prediction of disk components. In this study, fatigue crack initiation and propagation in coarse grained (CG) and fine grained (FG) Low Solvus, High Refractory (LSHR) alloy developed for disk applications have been assessed at 650 and 725 oC by carrying out three-point bend tests in air and vacuum under a 1-1-1-1 trapezoidal waveform in combination with a surface replication procedure. Optical microscopy, scanning electron microscopy, focussed ion beam (FIB), transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) spectroscopy have been employed to reveal the underlying mechanisms of fatigue crack initiation and early propagation under these fatigue-oxidation conditions. The results show that FG LSHR possesses a better fatigue life although it exhibits more severe grain boundary oxidation as indicated by more intergranular fracture surfaces. Cracks mainly initiate from bulged grain boundary oxides which consist of outermost Co-rich and Ni-rich oxide complex at the surface and underneath Cr/Ti/Al oxide intrusions along the grain boundaries. Once initiated, cracks propagate mostly by the coalescence of neighboring grain boundary cracks as observed on the replicas, especially in the FG variant. Formation of these grain boundary oxides is closely related to the applied stress and the strain localization at the grain boundary, and is accompanied by ?? dissolution. The distribution of oxides examined by TEM-EDX suggests fatigue failure processes involve repeated cycles of oxide formation ? oxide cracking ? oxide formation at the crack tip ? oxide cracking at the crack tip