Investigation of uniaxial ratcheting fatigue behaviours and fracture mechanism of GH742 superalloy at 923 K

In this study, the ratcheting fatigue behaviours of GH742 superalloy at 923 K were investigated through a series of symmetric and asymmetric stress-controlled tests. The effects of stress amplitude and mean stress on the fatigue behaviours were analysed through scanning electron microscopy, transmission electron microscopy and electron backscatter diffraction. The results highlight that the fatigue life significantly decreases with increasing mean stress or stress amplitude. Tensile ratcheting strain occurs and increases with the increasing number of cycles under both symmetric and asymmetric cyclic loading. The alloy initially exhibits a cyclic hardening response, followed by cyclic softening until fracture failure under symmetric and asymmetric cyclic loading. The increase in mean stress promotes the cyclic softening in the tensile strain direction to a certain extent. The fracture mode does not change with increasing mean stress. Multiple fatigue cracks initiate from the surface inclusions, crystallographic facets and slip bands, and propagate in mixed transgranular and intergranular modes. The modified Basquin equation, which considers the mean stress and stress amplitude, can accurately predict the fatigue life, and all the fatigue data are located within the 1.5X scatter band.