Effect of solid carburization on the surface microstructure and mechanical properties of the equiatomic CoCrFeNi high-entropy alloy

The surface modification of the equiatomic CoCrFeNi high-entropy alloy has been studied via solid carburization at 920 °C for 10 h. Microstructure, hardness and wear resistance were investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), microhardness and nanoindentation tester. Microstructure and phase analysis indicated that two types of nano-size carbide precipitates (M7C3 and M23C6) formed in the surface. The orientation relationship between the M7C3 precipitate and FCC matrix were (1(_)100)C1//(11(_)1)M and [0001]C1//[1(_)12]M. The M23C6 precipitate possessed a cube-cube orientation relationship with the FCC matrix. Vickers hardness and nanoindentation testing results indicated that the carburized surface performed significant improvement of about 100% in hardness compared with the matrix. The strengthening mechanisms including solid-solution strengthening and precipitate strengthening were studied. Moreover, nano-scratch experiments have been conducted on the cross section of the carburized sample under a constant load mode to investigate the friction and wear behavior. The surface profile and depth of scratched tracks were examined using scanning probe microscopy (SPM) technology. The wear and deformation mechanisms at different distances from the surface have been discussed. The calculated values of the wear volume, wear rate, and wear resistance coefficient reveal that the carbide precipitates can improve the wear resistance of the carburized surface.