The Martensitic Transformation and Strain-Hardening Behavior of Austenitic Steels During Fatigue and Tensile Loading

The tensile and low-cycle fatigue deformation and α′-martensitic transformation behavior of three austenitic steels with varied silicon, aluminum, and nickel levels were characterized using mechanical testing and transmission electron microscopy. Silicon alloying promoted deformation twinning and high work-hardening rates in tension by lowering the stacking fault energy (SFE). Deformation twins and their intersections served as martensite nucleation sites in tension. Martensitic transformation was maximized in the alloy with a low SFE, which increased the alloy capacity to form strain-induced nucleation sites, and low nickel content, which increased the thermodynamic driving force for martensite formation. In fatigue loading, martensite nucleation occurred on localized austenite shear bands composed of dissociated dislocations that form in the cyclically stabilized portion of the fatigue life. The shear bands occurred in all materials irrespective of the SFE. The extent of martensitic transformation in fatigue is apparently dictated more by thermodynamic driving force for transformation and not by SFE. In both tension and fatigue, martensite formation led to strain hardening.