Abstract: Normal strengthening methods through precipitations and deformation obviously enhance the strength of metallic materials while resulting in the sacrifice of ductility, and synergistic improvement of strength and ductility is currently an urgent requirement. Herein we developed a cryogenic deformation combined with an annealing method to fabricate CoCrNiMo_0.2 medium entropy alloy, which achieved an ultrahigh strength of 1.8 GPa with synergistic improvement in strength and ductility. Microstructure, mechanical performance, and strengthening mechanisms of the developed alloys were investigated compared with that prepared by the regular room temperature deformation method. It was found that high-density nanotwins were produced in CoCrNiMo_0.2 MEA via cryogenic deformation. Fine grains, hard precipitations, and high volume fraction of nanotwins greatly strengthened the alloy, obtaining a yield and ultimate tensile strength of 1400 MPa and 1800 MPa. Ductility improvement of the developed alloy was mainly attributed to the production of deformation nanotwins due to the lower stacking fault energy, which greatly increases the dislocation storage ability, and thus, the ductility of the alloy was enhanced.

Key words: Nanotwins, Mechanical properties, Deformation behavior, Strengthening mechanisms