In-situ SEM investigation of low-cycle fatigue behavior and microstructure evolution of CoCrNi medium entropy alloy

We performed an in-situ fatigue test by in-situ scanning electron microscope (SEM) to elucidate the deformation and failure behavior of equiatomic CoCrNi medium entropy alloy under cyclic loading. The SEM results indicate that the crack growth speed is slow at the initial stage, and the growth direction is perpendicular to the loading direction. As the cyclic loading continues, small cracks are generated in the material, which changes the direction of the main cracks and accelerates the growth of the main cracks. The deformation twins and elongated grains induced by cold rolling are frequently observed in samples before cyclic deformation. These grains are changed from nearly parallel elongated grains to irregular during cyclic deformation. However, deformation twins disappeared or became thinner after cyclic deformation. Molecular dynamics simulations are carried out to reveal the microstructure evolution mechanisms during cyclic deformation. The results show that the grain growth occurred after cyclic deformation in the model without deformation twins. However, the de-twinning, re-twinning and grain growth are observed in the models containing deformations twins. Molecular dynamic simulations also indicated that twin boundaries lead to higher strain energy density and significantly improve the material's fatigue resistance.