VHCF life evolution after microstructure degradation of a Ni-based single crystal superalloy

Very high cycle fatigue (VHCF) properties of a Ni-based single crystal superalloy (CMSX-4) have been studied at 20 kHz and temperature of 1000 °C between 106 and 1010 cycles. Tests have been performed using 4 microstructural states of CMSX-4 (as-received, thermally aged at 1100 °C/300 h and creep deformed in tension at 1100 °C/120 MPa for 24 h and 100 h) to evaluate the impact of microstructure degradation on VHCF fracture mechanisms and lifetime. At ratio of R = −1, crack initiation at internal casting pore and subsequent planar slip propagation is systematically observed for all investigated microstructure. At ratio of R = 0.8, crack initiation occurs from multiple internal or subsurface pores. Mode I micro-propagation is observed and associated with the addition of a large mean stress. In specimens prior crept-deformed in tension, the density of micro-cracks is higher due to the precipitation coarsening. Moreover, the crack propagation path is affected by the morphology of the γ/γ′ microstructure. Both phenomena result in reduced VHCF lifetimes in comparison to the reference state.