Microstructure and properties of laser cladding and CoCr2.5FeNi2Tix high-entropy alloy composite coatings

Abstract High entropy alloys (HEAs) are promising materials with equiatomic or near-equiatomic multicomponent structures in the field of corrosion and abrasion resistance. In order to prolong the service life of corrosion and abrasion resistance for drilling pipes in offshore petroleum exploration, CoCr2.5FeNi2Tix (x = 0, 0.5, 1, 1.5) high-entropy alloy (HEA) coatings were successfully fabricated by a laser-cladding technique. The phase structures, microstructure, hardness, wear and corrosion resistance of HEA coatings were specifically investigated. The experimental results indicate that the CoCr2.5FeNi2Tix HEA exhibits an excellent bonding ability between the cladding layer and the substrate. The cladding zone is composed mainly of equiaxed grains and columnar crystal; the phase structures of the CoCr2.5FeNi2Tix coatings are composed of BCC (Ti = 0) and solid composition of BCC + FCC phases with the addition of Ti due to the high-entropy effect. Ti element promotes the formation of an FCC structure. During the solidification process of the HEA alloy, some Ti atoms were excluded from the solid solution. Based on Gibbs free energy, the repulsion of Ti atoms was analyzed under the condition of the equilibrium state. With the increasing Ti content, the corrosion resistance of CoCr2.5FeNi2Tix high-entropy alloy coatings are found enhanced in a simulated saturated salty water mud solution. The relative wear resistance of CoCr2.5FeNi2Tix high-entropy alloy coatings was found significantly improved almost 2 times than the Q235 steel. The highest average micro-hardness value (Ti = 1.5) of HEA coatings is 480 HV0.2, which is more than 2.4 times higher than that of the substrate. The HEA coating CoCr2.5FeNi2Ti exhibited the prominent corrosion resistance and excellent wear resistance indicated by lower volume wear rates and smoother worn surface under the same conditions among the four coatings.