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This paper investigates the thermo-mechanical constitutive relationships of a CoCrFeNi high-entropy alloy (HEA) from quasi-static 10−2/s, medium strain rate of 101/s to high strain rates of 5 × 102/s-1.1 × 104/s. A series of tests, with the use of cylindrical specimen and shear compression specimen (SCS), were performed on a hydraulic Instron machine and a split Hopkinson pressure bar (SHPB) synchronized with a high-speed camera. The temperature rise (self-heating) during medium strain rate deformation is evaluated experimentally. Effects of the environmental temperatures on the dynamic flow response are studied using the SHPB and an environmental chamber. The constitutive relationships of CoCrFeNi HEA present noticeable strain rate and strain sensitivity. This HEA shows exceptional resistance to dynamic shear failure, with an equivalent shear failure strain of about 173%. Microstructural examinations illustrate that the CoCrFeNi HEA fails by adiabatic shear banding. A comparison of dynamic shear flow responses of the CoCrFeNi HEA and eight different metals and alloys is made. This reveals the potential use of HEA in impact resistant applications in aerospace engineering and armour design. [ABSTRACT FROM AUTHOR]