Microstructural origins of cycle hardening behaviors and fracture mechanisms of 304L stainless steel during low-cycle fatigue

Low-cycle fatigue behaviors of 304L stainless steel were investigated under different strain amplitudes (0.25 %, 0.3 %, 0.4 %, 0.5 %) and number of cycles to establish the relationship between macro-properties and micro- mechanisms. In all cases of strain amplitude, the 304L stainless steel displays a slight degree of cycle softening subsequent to the initial hardening in the cyclic stress-strain response. In the final stages of fatigue, the 304L stainless steel once again exhibits intense cycle hardening, contingent on the strain amplitude. In the two internal stress components of flow stress, the back stresses consistently exceed the effective stresses at varying strain amplitudes, demonstrating that the long-range resistance stresses for dislocation slip are larger than the short-range obstacles. Detailed microstructural investigation reveals that dislocations and stacking faults are the predominant microstructures observed at a low strain amplitude of 0.25 %. A phase transformation from FCC to HCP leads to cycle hardening at 0.3 % strain amplitude. At 0.4 % strain amplitude and above, the formation of dislocation substructures (veins, walls and cells) and BCC structural alpha,-martensite results in a more pronounced hardening effect, albeit at the cost of premature fracture. The present study offers a fundamental insight into the deformation mechanisms of 304L stainless steel during cyclic loading.