Microstructural evolution and fatigue mechanism of 316LN austenitic stainless steel welded joint at 4.2 K

The large-scale superconducting magnets used in the International Thermonuclear Experimental Reactor (ITER), demand structural materials that can endure long-term cyclic electromagnetic loading at 4.2 K. In this study, the microstructural evolution and plasticity mechanisms of a 316LN stainless steel welded joint subjected to fatigue at 4.2 K were systematically investigated for the first time. A combination of nanoindentation, electron backscatter diffraction, energy dispersive spectroscopy, electron channeling contrast imaging and transmission electron microscopy was employed for comprehensive characterization. The results reveal distinct fatigue mechanisms between the heat-affected zone (HAZ) and the fusion zone (FZ). The HAZ primarily accommodates deformation through wavy dislocation slip, while the FZ is dominated by planar slip. Notably, the FZ exhibits enhanced formation of stacking faults and nanotwins. These features are attributed to the elemental segregation in the interdendritic regions, which locally reduces the stacking fault energy and lowers the critical stress required for stacking fault formation via dislocation dissociation. Besides, a strong solidification-induced texture is formed in the FZ, which increases the local Schmid factor and thereby the resolved shear stress for promoting the nucleation of stacking faults and nanotwins. Additionally, Lomer–Cottrell locks are observed in the FZ as a result of dominant planar slip, contributing to dislocation multiplication. These findings offer important insights into the cryogenic fatigue behavior of austenitic stainless steel welds and contribute to the design of reliable structural components for fusion energy applications.