Enhanced fatigue resistance from metastable phase transformation in cold drawn austenitic stainless steel 316L

Metastable phase transformations have shown significant benefits in overcoming the strength-ductility tradeoff during static loading for a range of alloys. However, the fatigue behavior of metastable materials has been rarely studied. To evaluate the impact of metastable phase transformation on cyclic behavior, stress-controlled fatigue tests were conducted on cold drawn 316L austenitic stainless steel bars. The material exhibits martensitic transformation and enhanced fatigue resistance under low-stress ranges. Microstructure and alpha '-martensite were characterized using backscattered electron (BSE), electron backscatter diffraction (EBSD), transmission Kikuchi diffraction (TKD), x-ray diffraction (XRD), and ferritescope. Dislocation evolution was investigated using scanning transmission electron microscopy (STEM). It was revealed that the cyclic strain response of 316L at certain stress ranges showed an initial stage of cyclic softening, followed by cyclic hardening. This mechanical response can be attributed to two competing mechanisms: dislocation density reduction due to rearrangement and the formation of dispersed fine alpha '-martensite particles in the dislocation-free regions. Fatigue resistance is significantly enhanced by the delay of crack initiation induced by phase transformation, as the dislocation-free regions are dispersion-strengthened by alpha '-martensite particles at the early stage of fatigue life. This study elucidates the benefits and mechanism of metastable phase transformation on the fatigue resistance of 316L, paving the way for the development of new, more fatigue-resistant alloys.