Very High Cycle Fatigue Crack Initiation Mechanisms in Nickel-Based Superalloy at Elevated Temperatures: Competitive and Transition Roles of Twins vs. Inclusions

Abstract

Through high-temperature ultrasonic fatigue testing and microstructural characterization techniques, the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) behaviors of GH4169 superalloy under high-temperature conditions were investigated, along with the microstructural configurations related to fatigue crack initiation. The results were compared with the previous VHCF test data conducted at room temperature. It was found that the HCF and VHCF lives of GH4169 superalloy decreases significantly with increasing temperature, which is related to a transition in the crack initiation sites at high temperatures. Additionally, a competitive failure mode between internal and external crack initiation sites appears within the HCF range. Detailed investigation of the characteristics of preferred strain localization and microcrack initiation regions revealed that the crack initiation life fraction at high temperatures is shortened. Furthermore, at high temperatures, the activation of a new cubic slip system, along with thermal activation effects and the thermal expansion mismatch between non-metallic inclusions (NMIs) and the matrix, leads to a transition of VHCF crack initiation sites from twin boundaries (TBs) at room temperature to NMIs at high temperatures.

Keywords: GH4169 superalloy, Very high cycle fatigue, high temperature, Twin boundary, non-metallic inclusions