Phase evolution and cavitation erosion-corrosion behavior of FeCoCrAlNiTi x high entropy alloy coatings on 304 stainless steel by laser surface alloying

Abstract FeCoCrAlNiTix (x: molar ratio, x = 0.5, 1.0, 1.5 and 2.0) high entropy alloy coatings were prepared on 304 stainless steel by reaction synthesis of Fe with Co-Cr-Al-Ni-Tix powder, aiming at investigating the effect of Ti elements on their phase evolution, microstructure, microhardness and cavitation erosion-corrosion behavior. Experimental results showed that the crystal structures of FeCoCrAlNiTix high entropy alloy coatings evolved from FCC + BCC structures for x = 0.5 and 1.0 to FCC + BCC + Ti2Ni structures for x = 1.5, and then to FCC + BCC + Ti2Ni + ordered BCC structures for x = 2.0. The phase formation of FeCoCrAlNiTix coatings was analyzed based on rules of high entropy alloys. It was found that the formation of simple solid solution was governed by the competition outcome of mixing entropy (ΔS), mixing enthalpy (ΔH), atom-size difference (δ), valence electron concentration (VEC). The microhardness of FeCoCrAlNiTix coatings increased obviously from 615 HV to 730 HV with the addition of Ti element, which was at least 3.6 times that of 304 stainless steel substrate (170 HV). Accompanying the increase in hardness, the alloy for x = 2.0 exhibited the best cavitation erosion resistance among the tested samples in distilled water, but the worst in 3.5% NaCl solution, which was attributed to the fact that corrosion played an important role among the tested samples.