Cryogenic mechanical behavior and FCC → HCP phase transformation mechanism in a Si-added CrCoNi medium-entropy alloy under quasi-static and dynamic tension

The CrCoNiSi_0.3 MEA exhibits excellent cryogenic mechanical properties upon both quasi-static and dynamic tension. Under quasi-static tension at cryogenic temperature, the engineering yield and ultimate tensile strengths (UTS) reach 980 MPa and 1800 MPa, respectively, with notable ductility (62 %). The product of UTS and total elongation (TE) is 111.6 GPa %, surpassing most cryogenic high strength-ductility alloys. The significant mechanical strength enhancement is attributed to the denser deformation twins (DTs), multiple twinning, and extensive face-centered-cubic to hexagonal-close-packed (HCP) phase transitions, resulting in a high work hardening capacity. Upon dynamic tension at cryogenic temperature, the strength and strain hardening are further improved, which originates from the thickening DTs and HCP sequence and localized plastic deformation. The effects of temperature and strain rate on phase transition are studied. It is proposed that there is a competing relationship between high strain rate and increased stacking fault energy (SFE) due to temperature rise. The coupling effect of cryogenic temperature and high strain rate inhibits phase transition due to the deformation inhomogeneity in CrCoNiSi_0.3 MEA. The findings make a valuable contribution to understand the influence of temperature and strain rate on the mechanism of FCC-to-HCP phase transition.