In situ microscopic observations of low-cycle fatigue-crack propagation in high-Mn austenitic alloys with deformation-induced ϵ-martensitic transformation

In this study, the microstructural changes in Fe-30Mn-6Al, Fe-30Mn-4Si-2Al, and Fe-30Mn-6Si alloys that were subjected to bending fatigue tests with a total strain amplitude of 0.7% were observed in situ. The Fe-30Mn-4Si-2Al and Fe-30Mn-6Si alloys exhibited deformation-induced ϵ-martensitic transformation, but the Fe-30Mn-6Al alloy did not. The resistance of the Fe-30Mn-4Si-2Al alloy against fatigue-crack growth was superior to that of the other alloys, which is attributed to the effects of the ϵ-martensitic transformation. The ϵ-martensitic transformation in the alloy has three positive effects on crack growth: I) the suppression of strain localization; II) zigzag crack propagation, which enhances roughness-induced crack closure; and III) subcrack formation, which induces crack toughening, such as stress redistribution. On the other hand, the ϵ-martensitic transformation has a negative effect on crack growth, i.e., it causes subcrack initiation, which leads to the subcracks coalescing with the main crack. However, the ϵ-martensitic transformation in the Fe-30Mn-4Si-2Al alloy is optimized so that the positive effects are maximized and the negative effect is minimized, which results in the superior resistance of the alloy against low-cycle fatigue.