Novel Si-added CrCoNi medium entropy alloys achieving the breakthrough of strength-ductility trade-off

A novel CrCoNiSix (x = 0.1, 0.2, 0.3) medium entropy alloys (MEAs) system was designed to achieve enhanced strength and ductility. The CrCoNiSix MEAs are all single-phase face-centered cubic (FCC) structure. The recrystallization rate and average grain size obviously increase with the increase of Si. Compared to CrCoNi MEA, the ultimate tensile strength (UTS) and uniform elongation (UE) of CrCoNiSi0.3 MEA were increased from 790 MPa to 960 MPa and 58% to 92%, respectively, and the product of UTS and total elongation (TE) increased up to 88 GPa% (46 GPa% for CoCrNi MEA), superior to most high strength-ductility alloys. The stacking-fault energy (SFE) in the Si-added MEAs decreases with Si addition, as proved by thermodynamic model. Therefore, the underlying strengthening mechanism is the reduction of SFE and increase in the lattice distortion via Si addition. More concentrated and thinner deformation twins and multiple twinning structures were observed in Si-added MEAs. Furthermore, a nanoscale diffusionless transformation from the FCC to the hexagonal close-packed (HCP) phase occurred at room-temperature tension in Si-added MEAs, which further increased the work hardening and uniform elongation. This work provides a new and significant approach to break through the strength-ductility trade-off in FCC MEAs/HEAs, especially serving as energy-absorbing materials.