Thermodynamics-based design strategy for optimizing strength and ductility of Cr-Ni-Mn-Fe medium-entropy alloys

•First ever preparation of stacking fault energy maps (SFE maps) for the Cr-Ni-Mn-Fe MEA alloy system, applicable to various temperatures.•The most balanced tensile properties can be achieved by designing alloys with an SFE value adjacent to the TWIP-TRIP boundary line in the SFE maps.•Cr12Ni12Mn16Fe60 MEA possesses a combination of superior strength and good ductility at cryogenic temperatures, owing to the TWIP-TRIP effects. Single-phase high-entropy alloys and medium-entropy alloys (MEAs) with face-centered-cubic (FCC) structures have attracted considerable attention owing to their exceptional strength and ductility at cryogenic temperatures by performing twinning and martensitic transformations. In this study, a series of novel MEAs were developed (Cr12Ni12MnxFe76−x, x = 12, 16, 20) based on a thermodynamic approach. The Cr12Ni12Mn16Fe60 MEA was discovered to possess a unique combination of excellent strength (~ 1050 MPa) and ductility (~ 86%) at cryogenic temperatures. Through experimental assessment and thermodynamic modeling, the strength–ductility synergy was quantitatively demonstrated to be derived from the interplay among a series of plasticity mechanisms. In addition, a model was proposed for evaluating the stacking fault energy of Cr-Ni-Mn-Fe MEAs at different temperatures. This work demonstrates an efficient and novel strategy for developing high-strength and high-ductility alloys with single-phase FCC structures.