Effect of γ to ε martensitic transformation on low-cycle fatigue behaviour and fatigue microstructure of Fe-15Mn-10Cr-8Ni-xSi austenitic alloys

The low-cycle fatigue (LCF) behaviour and microstructure of metastable Fe-15Mn-10Cr-8Ni-xSi (x = 0, 2, 4, 6 wt.%) austenitic alloys were examined. The alloys were fully austenitic prior to deformation and underwent strain-induced ε martensitic transformation during LCF. The increase in Si changes the cyclic hardening behaviour and the corresponding post-fatigue microstructures, which can be categorized into two groups. Initial cyclic hardening followed by near saturated cyclic deformation behaviour led to the final microstructure containing moderate fractions of ε-martensite (56 and 68% for 0 and 2 wt.% of Si, respectively) that coexisted with an austenitic dislocation cell structure in the alloys with Si ≤2 wt.%. Alloying with Si ≥4 wt.% leads to continuous but weak cyclic hardening, resulting in fractions of martensite above 75% and a planar arrangement of the dislocations. The longest fatigue life of 8500 cycles was observed for the alloy with 4 wt.% of Si, which is characterized by the lowest cyclic hardening response, and the highest fraction of ε-martensite of approximately 80% in fractured microstructure. We show that strain-induced ε martensitic transformation and planar dislocation glide are important for achieving a high fatigue resistance in the studied alloys. The role of Si on the fatigue resistance and strain-induced martensitic transformation is discussed in terms of the extension width of the dislocations, which was controlled by the stacking fault energy, and possible effect of the short range ordering and solution strengthening.