Abnormal strain-hardening in Co-rich CoNiV medium-entropy alloys

Two new nonequiatomic Co₇₀Ni₂₀V₁₀ and Co₆₅Ni₂₀V₁₅ medium entropy alloys were designed to investigate the synergistic strengthening effects of the dislocation slip, hexagonal close-packed (HCP) martensitic transformation, chemical short-range order (CSRO), and chemical medium-range order (CMRO), using X-ray diffraction, backscattered electron, electron channeling contrast imaging, electron backscattered diffraction, and transmission electron microscope. The results showed the Co₆₅Ni₂₀V₁₅ alloy presented an abnormally higher strain-hardening rate than the Co₇₀Ni₂₀V₁₀ alloy in the range of 5∼30 % strain, although the Co₇₀Ni₂₀V₁₀ alloy possessed higher dislocation density with extra HCP martensitic transformation strengthening. This abnormality could be attributed to the profound CSRO and CMRO strengthening in the Co₆₅Ni₂₀V₁₅ alloy with more large-sized V atoms, which was evidently observed by a high-resolution transmission electron microscope. It is expected that the strength of the CoNiV medium entropy alloys could be further enhanced via synthetically tuning the CSRO and CMRO effects, together with other strengthening mechanisms.