Microstructure and mechanical properties of novel Si-added CrFeNi medium-entropy alloy prepared via vacuum arc-melting

• A series of novel Co-free CrFeNiSi x MEAs was prepared via vacuum arc-melting. • Si induced the formation of (Cr, Si)-rich σ phase in CrFeNiSi x MEAs. • The growth accident model was the main formation mechanism of Σ 3 boundaries. • The increasing σ phase resulted in the transformation of fracture mechanisms. • There is a good trade-off between strength and ductility in CrFeNiSi 0.15 alloy. In this paper, a series of novel Co-free CrFeNiSi x ( x = 0.05, 0.15, 0.25) MEAs was prepared via vacuum arc-melting. The addition of Si induced the formation of (Cr, Si)-rich σ phase in CrFeNiSi x MEAs, which effectively hindered the migration of dislocations and grain boundaries, resulting in the grain refinement. Meanwhile, the existence of σ phase inhibited the formation of twinning, leading to the decreasing fractions of low- Σ CSL boundaries with the increasing Si addition. Compared with CrFeNi alloy, both the strength and microhardness of CrFeNiSi x MEAs were significantly improved, but the plasticity had deteriorated. Overall, there was a good trade-off between strength and ductility in CrFeNiSi 0.15 MEA, including the yield strength of 550 ± 5.5 MPa, ultimate tensile strength of 795 ± 7.1 MPa, and elongation of 20 ± 0.4%. In addition, the experimental results demonstrated that the grain boundary strengthening and precipitation strengthening played a dominant role for the strengthening of CrFeNiSi x MEAs. The decreased plasticity was mainly attributed to the formation of σ phase and the reduced twinning. Furthermore, the increasing σ phase also resulted in the transformation of fracture mechanisms from ductile fracture to the mixed fracture. Our work provides significant reference for the further development of high performance Co-free MEAs/HEAs.