Research on Fatigue Crack Initiation Mechanism of Gh4169 During In-Situ Low-Cycle Fatigue Testing

Abstract

The relationship between the crack initiation mechanism and the microstructure of GH4169 during in-situ low-cycle fatigue with 1% strain rate and -1 strain ratio were investigated through Scanning Electron Microscope as well as Electron Back Scatter Diffraction. The results show that the main mode of fatigue crack initiation is intergranular cracking during high strain-controlled fatigue. Specifically, the initiation of fatigue cracks is related to the twist angle and SF difference between grains. In details, during the fatigue testing, slip traces appear on the surface in sequence according to the Schmid factor value of the grains. The preferred orientation of grains rotated from <111> parallel to transverse direction to <111> parallel to loading direction with increasing of the fatigue cycles. Based on the evolution of microstructural parameters during the LCF testing, the process of fatigue crack initiation can be given. Large twist angle between grains will lead to dislocation accumulation at the grain boundary, while the different difficulty of slip system activation between adjacent grains causes uneven deformation on both sides of the grain boundary, resulting in the widening gap between the SFs and OD of adjacent grains followed by the formation of micro-cracks.

Keywords: GH4169 alloy, Low-cycle fatigue (LCF), Electron Back Scatter Diffraction (EBSD), Initiation of fatigue crack, Schmid factor (SF)