Corrosion-erosion behavior and mechanism of Cu[sbnd]Mo co-doped CoCrFeNi high-entropy alloy coating prepared by directed energy deposition

Abstract

Developing cost-effective and high-performance high-entropy alloy (HEA) coatings on demand is imperative. This study designed a class of HEAs on-demand by synergistic doping of Cu and Mo elements based on the first-principles and multi-principal alloy design concept. A CoCrFeNiCu0.25Mo0.75 HEA coating with outstanding corrosion and slurry erosion wear performances was fabricated on the duplex stainless steel using the directed energy deposition technique. The corrosion mechanism of the HEA coating in 3.5 % NaCl and 0.5 M H2SO4 solutions and the slurry erosion wear mechanism under a slurry concentration of 5 wt% were analyzed in combination with the density functional theory. The results indicate that the SQS crystal model is a better predictor of phases and properties of HEA than the VCA model. The HEA coating is a cellular structure that consists mainly of a face-centered cubic phase structure and contains a small amount of homogeneous sigma nanocrystalline secondary phase precipitation. Meanwhile, the passive film of the HEA coating is mainly composed of metal oxides and hydroxides. Cu is not oxidized during the corrosion process and always exists in the metallic form, which serves as a barrier layer to prevent the transfer of point defects. Mo with a larger atomic size can induce lattice distortion, enhance the coating strength and hardness, and reduce the density of point defects. Further, the slurry erosion wear rate of the HEA coating is lower at a 30° impingement angle than 90° for brittle erosion mode. At the 30° impingement angle, micro-cutting and ploughing are the main wear mechanisms of the HEA coating, while at the 90° impingement angle, micro-indents and craters are the main wear mechanisms. In addition, the corrosion and slurry erosion wear performances of CoCrFeNiCu0.25Mo0.75 HEA coating are better than the most reported once-formed HEAs.