Initiation of microcracks at internal micropores during thermal fatigue of a second-generation nickel-based single crystal superalloy

Thermal fatigue seriously threatens the service safety of single crystal (SX) guiding vanes, and crack initiation is a key factor affecting the thermal fatigue behavior of nickel-based SX superalloys. However, the factors affecting the internal crack initiation during thermal fatigue remain unclear. This work investigated the evolution of micropores and microstructure as well as the crack initiation of a second-generation nickel-based SX superalloy during thermal fatigue under 25 °C ↔ 1100 °C. The significant changes in micropores and the formation of recrystallization (RX) at the micropores were discovered during thermal fatigue. The results indicated that the size and volume fraction of micropores were increased while their sphericity was decreased with increasing fatigue cycles. Meanwhile, the stress concentration at the micropores intensified local plastic deformation and promoted the multiplication and movement of dislocations. Dislocations accumulated at the γ/γ′ interfaces, sheared into the γ′ phase, and contributed to the formation of low-angle grain boundaries (LAGBs). These LAGBs were subsequently transformed into high-angle grain boundaries through continuous absorption of dislocations and crystal rotation, resulting in the development of RX at the micropores. The size and quantity of recrystallized grains were enlarged with increasing fatigue cycles, thereby promoting the initiation of microcracks from the RX zone at the micropores after 1000 cycles. Based on these findings, this work will be helpful for enhancing the understanding of the microcrack initiation in nickel-based SX superalloys during thermal fatigue and providing a reference for improving the service safety of guiding vanes.