Effects of Nb on Borosiliciding of CoCrFeNiNbx High-Entropy Alloy

Abstract

The effect of Nb addition (2, 5, 10 at. %) on the borosiliciding behavior of CoCrFeNiNbx high-entropy alloy was studied. Vacuum arc melted CoCrFeNiNbx alloys consisted of FCC and Laves-C14 phases. The increase of Nb in the alloys resulted in a decrease of the amount of FCC phase and an increase of the amount of Laves phase. A multiphase boride layer (BL) and a silicon-rich layer (SL) were evident with the formation of Ni2Si, FeB, Fe2B, Co2B, Cr5B3, Nb5Si3 and NbB2 phases after the thermochemical treatment. The Nb5Si3 and NbB2 phase formation was caused by the Nb addition into the CoCrFeNi alloy. The thickness of the SL + BL dramatically decreased, and the thickness of the transition zone/diffusion zone (DZ) decreased with increasing Nb content. The Nb content in the alloys hinders the inward diffusion of Si/B atoms and decreases the Si/B penetration depth. The total layer thicknesses were 45 μm, 27 μm and 12 μm for the alloys containing 2, 5 and 10 at. % Nb, respectively. The surface hardness values of alloys increased after surface treatment.