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We investigated the effects of the addition of Co and carbon on the deformation behavior of new medium-entropy alloys (MEAs) designed by increasing the entropy of the conventional NiCrFe-type Alloy 600. The strength/ductility combination of carbon-free (NiCo)75Cr17Fe8 MEA was found to be 729 MPa/81% at 298 K and it increased to a remarkable 1212 MPa/106% at 77 K. The excellent strength and ductility of (NiCo)75Cr17Fe8 at cryogenic temperature is attributed to the increased strain hardening rate caused by the interaction between dislocation slip and deformation twins. Strength/ductility combinations of carbon-doped (NiCo)75Cr17Fe8C0.34 and (NiCo)75Cr17Fe8C0.83 at cryogenic temperature were observed to be 1321 MPa/96% and 1398 MPa/66%, respectively, both of which are superior to those of other high-entropy alloys (HEAs). Strength/ductility combinations of (NiCo)75Cr17Fe8C0.34 and (NiCo)75Cr17Fe8C0.83 at room temperature were found to be 831 MPa/72% and 942 MPa/55%, respectively and both are far superior to 676 MPa/41% of the commercial Alloy 600. Yield strengths of carbon-free and carbon-doped alloys comprised strengthening components from the friction stress, grain size strengthening, carbide strengthening and interstitial strengthening and excellent agreement between the predictions and the experiments was obtained. A design strategy to develop new MEAs by increasing the entropy of the conventional alloys was found to be effective in enhancing the mechanical performance.

High temperature deformability and fracture behavior of deformation-processed high nitrogen high carbon Fe-Cr-Mn-Ni stainless steel rods were studied. The effective fracture elongation increased rapidly from 1000 °C, and reached high values (>45%) at 1100–1200 °C, accompanied by strain softening and stress serrations, supporting periodic dynamic recrystallization (DRX). Dynamically recrystallized grains were observed close to the fracture surface, suggesting that active DRX worked until its fracture. Pre-deformation-annealing of Fe-Cr-Mn-Ni stainless steel rods at 1200 °C was found to deteriorate in deformability above 1000 °C, while it enhanced ductility below 950 °C. Pre-deformation annealing had a negative effect on the deformability above 1000 °C due to the reduction of driving forces for DRX, but it exhibited a beneficial effect on the ductility at lower temperatures because of the ease of slip in large-grained structures. The fracture surface at 1250 °C exhibited intergranular fractures due to partial melting at grain boundaries, supported by the thermodynamic calculation of the solidus temperature of Fe-Cr-Mn-Ni austenite stainless steel. In this study, effective fracture elongation, defined based on the assumption that the effective gage length decreases with straining, was found to be an accurate measure of hot deformability.