Effect of carbon on the low-cycle fatigue resistance and microstructure of the Fe-15Mn-10Cr-8Ni-4Si seismic damping alloy

Fe-15Mn-10Cr-8Ni-4Si has been developed as a novel seismic damping alloy that absorbs seismic energy through elasto-plastic deformation and has an extraordinarily long low-cycle fatigue (LCF) life, N-f, owing to reversible dislocation motions in a dual austenite/epsilon-martensite phase developed under cyclic deformation. As carbon is an important interstitial element affecting mechanical properties of iron-based steels and alloys, the effect of the carbon concentration on the LCF properties and microstructures of Fe-15Mn-10Cr-8Ni-4Si-xC (x = 0, 0.1, 0.2, and 0.3 wt%) alloys was examined in the present study at total strain amplitudes, Delta epsilon(t)/2, of 0.005, 0.01, 0.02, and 0.03. Up to 0.1 wt% carbon, the N-f of the alloys was an order of magnitude greater than that of conventional steels, whereas carbon noticeably reduced the N-f at higher strain amplitudes and higher carbon concentrations (Delta epsilon(t)/2 >= 0.03 at 0.2 wt% and Delta epsilon(t)/2 >= 0.02 at 0.3 wt%). Microstructural observations revealed that the increase in carbon concentration decreased the epsilon-martensite fraction in fatigue-fractured specimens owing to an increase in the stacking fault energy of the austenite phase. Such an increase usually reduces the degree of slip reversibility, but the gamma-slip mode in the carbon-added alloys exhibited planar characteristics, probably because the slip plane softened in the short-range ordering/softening zones to maintain a high N-f, unless the carbon content or strain amplitude exceeded certain levels.