Effect of Fe content on microstructure and mechanical properties of Fex(CoCrNi)100-x medium-entropy alloys

The high cost of high- and medium-entropy alloys (H/MEAs) has limited their structural applications, leading to increased interest in cost-effective ferrous MEAs (Fe-MEAs), where microstructure and mechanical properties are strongly influenced by Fe content. In this study, we systematically investigated the microstructural evolution, mechanical properties and deformation mechanisms of Fex(CoCrNi)100-x (x = 25, 40, 50, 62, 64, 66, and71, at.%) MEAs. The results revealed that the constituent phases evolved from a single face-centered cubic (FCC) phase (Fe25, Fe40, and Fe50) to a mixture of FCC and body-centered cubic (BCC) phases (Fe62, Fe64, and Fe66), eventually forming a single BCC phase (Fe71). As the Fe content increased, the yield strength (YS) gradually decreased from -394.8 MPa for Fe25 to -206.0 MPa for Fe62, while the uniform elongation (UE) remained above 40 % for all alloys. Further increases in Fe content significantly enhanced strength, though at the cost of a substantial reduction in elongation. Fe71 exhibited a high YS of -831.0 MPa but with a UE of only -1.5 %. In addition to dislocation slip, deformation nanotwins (DTs) were observed in deformed Fe50, and deformationinduced martensitic transformation (DIMT) was detected in deformed Fe64.