Microstructure Evolution Associated with a Superior Low-Cycle Fatigue Resistance of the Fe-30Mn-4Si-2Al Alloy

The microstructure evolution responsible for the superior low-cycle fatigue (LCF) resistance (N_f > 8000 cycles at a total strain range of 2 pct) was studied in the Fe-30Mn-4Si-2Al alloy susceptible to strain-induced martensitic transformation. To investigate the microstructure effect on the LCF behaviors of the alloy, a series of interrupted fatigue tests at total strain range of 2 pct were carried out. A characteristic softening stage followed by the secondary hardening was observed during cyclic loading of the studied alloy. This softening is associated with the strain localization caused by persistent Lüders bands formation and the transformation of Lüders bands into strain-induced ε-martensite is found to have a key role in the delayed fatigue fracture of the alloy being studied. Therefore, the continuous transformation process involving Lüders bands and ε-martensite formation associated with intermediate stacking fault energy (SFE) (γ_SF of 14 mJ/m²) is necessary to prevent the rearrangement of dislocations into walls/channels and substructures inherent to high-SFE (γ_SF higher 20 mJ/m²) alloys capable to accelerated fatigue damage. However, sluggish martensite transformation kinetics is necessary to delay the formation of the ε-martensite associated with the development and propagation of fatigue crack in alloys with very low SFE.