Effects of heterogeneous ultrafine grain and strain rate on mechanical properties of CoCrNi medium entropy alloy

A thermo-mechanical process, including of cold rolling and short-time annealing, was applied for the formation of a heterogeneous ultrafine grain CrCoNi medium entropy alloy. Comparing with coarse grain CrCoNi medium entropy alloy, heterogeneous ultrafine grain CrCoNi one exhibited an excellent strength–ductility combination. It was found that the strain rate affected significantly deformation characteristics of heterogeneous ultrafine grain CrCoNi MEA based on the current study. In addition, the lower strain rate improved its strength and plasticity at the same time. Low strain rate induced lower dislocation density and lower intragranular dislocation storage ability, therefore, the grain boundary strengthening in lower strain rate was overwhelming grain intragranular strengthening in higher strain rate. Moreover, the lower strain rate also promoted the formation of more twins, which also significantly improved the tensile strength and the ductility of heterogeneous ultrafine grain CrCoNi medium entropy alloy.