Mechanical Properties of a CrCoNi Medium Entropy Alloy in a Gradient Twin Structure

In order to improve its yield strength, an equi-atomic CrCoNi medium entropy alloy (MEA) has undergone a sliding friction treatment (SFT), i.e., a severe plastic deformation, on its surface to form a gradient twin structure. Such a formed microstructure was evaluated and analyzed using different techniques, such as the optical microscope, electron backscattered diffraction, and transmission electron microscope, while its mechanical properties were measured by performing micro-tensile tests before and after the SFT process. After the SFT procedure, not only are a large number of deformation twins found to gradually form in the CrCoNi MEA, but also they show a gradient structure, that is, the fraction of deformation twins is higher near the surface but lower near the center. Compared to those before the SFT process, both the surface microhardness and yield strength of such a CrCoNi MEA are significantly improved, but its elongation and quasi-static toughness do not decrease appreciably. By analyzing the strain hardening behavior through a ln(dσ/dε)-lnσ plot, the CrCoNi MEA in a homogenization annealing state and after the SFT process can be divided into five stages, each of which is closely related to the corresponding twin deformation mechanisms, further confirming that the CrCoNi MEA with a gradient twin structure has an excellent ability for strain hardening.