Precipitation and heterogeneous strengthened CoCrNi-based medium entropy alloy with excellent strength-ductility combination from room to cryogenic temperatures

The improved understandings of the mechanical properties as well as deformation mechanisms at cryogenic temperatures are the prerequisite for realizing the application of any new engineering materials to cryogenic industries. Here, a（CoCrNi）94Al3Ti3 medium entropy alloy（MEA） with nanoscale L12 coherent precipitates and heterogeneous grain structures was prepared by codoping Al and Ti elements with subsequent cold rolling and heat treatment processes. The mechanical properties were evaluated at the temperature range of 293–113 K. The ultimate strength of the MEA increases almost linearly from 1326 to 1695 MPa as the temperature decreases from 293 to 113 K, while the total elongation remains approximately constant of ～35%. The underlying deformation and strengthening mechanisms were investigated using various characterization techniques. Due to the effect of co-doped Al/Ti on channel width of the matrix and the increasing critical twinning stress induced by heterogeneous ultrafine grain size, the formation of deformation twins is inhibited at all temperatures. Consequently, only a slight increase of the deformation twins and stacking faults in the deformed specimens with a decreasing temperature, which leads to the relative temperature-independence of the ductility. The dislocation cutting mechanism of L12 coherent precipitates and the heterodeformation induced（HDI） hardening both significantly contribute to the strain hardening so that an excellent combination of strength and ductility is obtained. Additionally, the evolution of lattice friction stress with deformation temperature is determined by quantitative analysis, indicating an approximately linear relationship between the lattice friction and temperature. The present work provides new insights into the strategy of achieving outstanding strength-ductility synergy of the MEA under the wide temperature range by coupling heterogeneous ultrafine-grained structure and coherent precipitation strategy.