| Summary: | The effect of a boronizing and siliciding process on CoCrFeNiHf<sub>0.1–0.42</sub> high entropy alloys was examined in this study. When increasing the amount of added Hf in CoCrFeNiHf<sub>x</sub>, the structure of the alloys gradually transformed from single-phase FCC to firstly Ni<sub>7</sub>Hf<sub>2</sub> + FCC, and finally to C15 Laves and FCC phases. The boronizing/siliciding process resulted in the formation of a silicon-rich layer and a boride layer (BL). Increasing the amount of Hf in the alloys resulted in a decrease in the combined layer thickness, which was measured for CoCrFeNi, CoCrFeNiHf<sub>0.1</sub>, CoCrFeNiHf<sub>0.2</sub>, and CoCrFeNiHf<sub>0.42</sub> to be 70 µm, 63 µm, 20 µm, and 15 µm, respectively. In contrast, the thickness of the transition zone/diffusion zone increased with more Hf in the alloys. While silicon atoms were gathered close to the BL, they were not transferred into the CoCrFeNi substrate. In contrast to the observation for CoCrFeNi, Si atoms penetrated through the Ni-rich phase (Ni<sub>7</sub>Hf<sub>2</sub>) in the CoCrFeNiHf<sub>x</sub> alloys. Furthermore, the Cr-B rich area (Cr<sub>5</sub>B<sub>3</sub>) in the coating limited the transport of Si into the CoCrFeNiHf<sub>x</sub> substrates. XRD analysis showed that the BL contained Ni<sub>2</sub>Si, FeB, Fe<sub>2</sub>B, Co<sub>2</sub>B, and Cr<sub>5</sub>B<sub>3</sub> phases.
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