Fretting Fatigue Cracking Mechanisms of Additively Manufactured Superalloys with Gradient Grains Based on In Situ SEM

This study aims to elucidate the influence mechanisms of gradient grain structures on the fretting fatigue damage evolution of additively manufactured superalloys under high-temperature conditions. To this end, two types of specimens with and without grain gradients were designed and subjected to in situ scanning electron microscopy (SEM) fretting fatigue tests at 650 °C to capture the microscopic damage evolution processes under different microstructures. In situ SEM fretting fatigue tests were conducted at 650 °C to capture the microscopic damage evolution processes of the specimens under different microstructures. In situ observation results indicate that fretting fatigue cracks primarily initiate in the region of high stress and severe wear at the contact edge. Furthermore, the gradient grain structure can effectively induce the deflection and branching of microcracks, significantly improving the fretting fatigue life and reducing the crack propagation rate. Electron backscatter diffraction (EBSD) analysis reveals that compared to uniform grains, the gradient structure effectively suppresses local grain refinement and limits the extreme increase of geometrically necessary dislocation (GND) density and grain reference orientation deviation (GROD) during fretting damage. This indicates that the gradient grains can better coordinate local plastic deformation and relieve stress concentration, thereby demonstrating optimal resistance to fretting fatigue damage. Therefore, the grain gradient in the microstructure has a significant influence on the fretting damage mechanisms and the early evolution of cracks.