Low-cycle fatigue behavior of a solution-treated and HIPped nickel-based single-crystal superalloy at 760 °C

Micropores have a substantial impact on the fatigue behaviors of nickel-based single-crystal (SX) superalloys. Nonetheless, Hot Isostatic Pressing (HIP) treatment is an available approach to effectively reduce defects and enhance fatigue lives. In this work, the HIP treatment was applied to a solution-treated second-generation nickel-based SX superalloy and then subjected to low-cycle fatigue (LCF) tests at 760 °C. The effect of HIP treatment on fatigue crack initiation and propagation at intermediate-temperature LCF were studied by analyzing the development of micropores and fatigue behaviors. The results show that HIP treatment can remarkably reduce the micropores and residual eutectics in the interdendritic regions of the solution-treated alloy, changing the crack initiation site from the sub-surface micropore to the surface microcrack and transforming the crack propagation from mode I (i.e., perpendicular to the load direction) to mode II (i.e., parallel to the crystallographic {111} planes). Consequently, HIP treatment can increase the fatigue lives of this SX superalloy by about three times. Due to the reduction of micropores and residual eutectics by HIP treatment, on the one hand, the surface microcrack can propagate through the oxide layer and recrystallization zone to form the crack initiation, effectively prolonging the time of the crack initiation stage. On the other hand, the elimination of stress concentration areas inhibits rapid crack propagation in the direction perpendicular to the stress axis, extending the life of the crack propagation. This research provides a method for controlling micropores and residual eutectics to improve the intermediate-temperature fatigue properties of the second-generation nickel-based SX superalloys.