A comprehensive investigation on thermomechanical fatigue failure mechanism and remaining properties of 316L stainless steel

Thermomechanical fatigue (TMF) is a critical failure mode in high-temperature components, as it not only accelerates microstructural degradation but also significantly impacts the remaining deformation resistance at elevated temperatures during prolonged operation. In this study, the microstructural failure mechanisms of 316L during the TMF process are investigated using interrupted tests and multiscale characterization techniques, followed by an evaluation of the remaining tensile and creep properties. Electron backscatter diffraction (EBSD) analysis reveals that prolonged TMF exposure leads to increases in kernel average misorientation, geometrically necessary dislocations, and low-angle grain boundaries. Interestingly, these microstructural degradations are partially alleviated after fatigue fracture due to strain relaxation. Additionally, transmission electron microscopy (TEM) analysis shows that secondary slip activity, including cross-slip, intensifies with increasing TMF cycles and strain amplitude, contributing to the evolution of dislocations into equiaxed and elongated dislocation cells. Subsequent tensile tests indicate that TMF loading enhances yield strength but reduces elongation. Creep tests further reveal that creep resistance improves after TMF at lower fatigue cycles and a strain amplitude of 0.5 %. However, at higher fatigue cycles and strain amplitudes, creep life decreases significantly, with reductions of up to 50 %. To address the effects of TMF on remaining life, a fatigue damage parameter based on tensile elongation and plastic strain energy is proposed and incorporated into the Ostergren fatigue life and Wilshire creep life prediction models. The strong agreement between experimental results and model predictions demonstrates that the proposed models with the defined fatigue damage parameter is effective in accurately predicting the remaining life under varying TMF loading conditions.