Investigation on the robust boronizing strategy for the surface strengthening of CoCrNi medium-entropy alloy

Boronizing has been considered as a promising strategy to enhance the surface hardness and corrosion resistance of the new emerged alloys, like CoCrNi medium-entropy alloy (MEA). However, recent studies showed that a brittle silicide layer was generated on the surface for Ni-containing alloys boronizied with the SiC-containing powder (SiC+KBF₄+B₄C). In this study, Si-free powder packing (900 °C 4 h) and electrochemical boronizing (900 °C 30 min) methods were explored to obtain the boronized layer for CoCrNi MEA. The morphology, element distributions and phases of the boronized layer were characterized by Scanning Electron Microscopy (SEM) and Energy Dispersive Spectrometer (EDS) and X-ray diffraction (XRD). The results show that the powder-pack boronizing method with 90 wt% B₄C + 10 wt% NaBF₄ (B₄C) and electrochemical (EC) methods have the thickest boronized layer (~70 μm) with hardness of about 22 GPa. The boronized layer is dominated by columnar CrB and Co₂B with obvious preferred orientation and strong texture. In addition, CoB, Ni₂B and Ni₃B also exists in the boronized layer, and these two optimal boronizing processes avoid the influence of Si. This study shows that the B₄C and EC boronizing methods can be considered as the viable alternative strategy for the preparation of strengthened layers.