Study On Microstructural Evolution Mechanism And Wear-corrosion Resistance Of CoCrFeMnNi Based High-entropy Alloy Coatings

Developing wear-corrosion resistant materials for the essential moving parts of marine equipments are the key to ensuring the safe and reliable operation of marine equipments.High-entropy alloy(HEA)coatings with superior wear and corrosion resistance have showed great potential for the application in key moving parts of marine equipments.In this study,based on CoCrFeMnNi alloy that possesses superior corrosion resistance and promising mechanical properties at cryogenic temperatures,the components and proportions were optimized from the point of view of coating performance and cost,and the HEA coatings were prepared by self-developed plasma cladding device.The microstructural evolution mechanism,wear-corrosion resistance and wear-corrosion mechanism were studied.The research results will provide theoretic foundation and guidance for the composition,microstructure design of wear-corrosion resistant HEA coating materials under marine environment.(1)The effect of Ti on microstructures and properties of CoCrFeMnNi HEA coating was studied.With the increase of Ti,the phases of coatings were transformed from single FCC phase,to FCC+BCC phases,and to FCC+BCC+Laves phases.The hardness,wear and corrosion resistance of these coatings are better than those of Q235 substrate.The hardness of the coatings increases with the increase of Ti content,and however,the wear resistance increases first and then decreases with the increase of Ti content.The addition of high Ti content leads to the main phase of the coating transform to BCC+Laves,which results in the increase in brittleness,and the wear mechanism of coatings changed from adhesion wear at low Ti content to slight fatigue wear at medium Ti content,and then to severe brittle fatigue wear at high Ti content.The corrosion resistance of coatings with low Ti content is better than that of 304 stainless steel and most of the bulk HEAs reported in the literature,which is attributed to the "rapid quenching effect" of non-equilibrium plasma rapid heating and cooling.(2)According to the research results of CoCrFeMnNiTix coatings,replacing Mn and Co elements with Cu and Ni in CoCrFeMnNiTix system,and the CuCrFeNi2Tix system HEA coatings were obtained to reduce brittleness and cost of the coatings.With the increase of Ti,the phases of coatings were transformed from two FCC phases to FCC+BCC+? phases and to FCC+BCC+?+Laves phases.Compared with CoCrFeMnNiTix system,even when the ratio of Ti is high,the coating is compact and defect-free,which is attributed to the main phase of coating was transformed to eutectic FCC+Laves phases that possesses excellent strength and toughness,leading to the brittleness of the coating was inhibited effectively,and thus the hardness and wear resistance have been improved simultaneously.The wear mechanism of coatings changed from adhesion wear at low Ti content to abrasive wear at medium and high Ti content.The corrosion resistance increases first and then decreases with the increase of Ti content,and a small amount of Ti can improve the corrosion resistance of CuCrFeNi2Tix system coatings.(3)Based to the research results of CoCrFeMnNiTix and CuCrFeNi2Tix system coatings,in order to avoid the formation of bulk intermetallic compounds and to realize the precipitation strengthening of nanophases as the main strengthening mechanism,On the basis of CoCrFeMnNi system,replacing Mn with Cu and Fe with Al,and the addition ratio of Cu and Al was reduced to 0.5,the CoCrAl0.5NiCu0.5 system HEA coating was prepared by plasma cladding and spark plasma sintering(SPS),respectively.The results show that the CoCrAl0.5NiCu0.5 HEA coating was mainly consisted of solid solutions and nano-precipitates,and there is no bulk intermetallic compounds.The coating prepared by plasma cladding contains two FCC phases.Disordered FCC Cu-rich precipitates with a scale of-5 nm are dispersed in dendritic FCC matrix and the ordered Ll2 precipitates with a scale of?1 nm are dispersed in interdendritic FCC matrix.The nanohardness of interdendritic FCC phase is 7.01 GPa,which is higher than that of the dendritic FCC phase(6.07 GPa).This is attributed to precipitation strengthening of ordered nanophase in intergranular FCC phase and dislocation strengthening in solid solution.The coating prepared by SPS contains two FCC phases and a B2 phase.The FCC matrix grains contain hierarchical nano-precipitates.The microhardness value of the SPSed coating is 455 HV0.1,which is much higher than those of most reported FCC-based HEAs,even some BCC-based HEAs.This enhanced microhardness is primarily attributable to hierarchical intragranular nanoprecipitates,which leads to precipitation hardening.The corrosion resistance of coating prepared by plasma cladding is better than that of the SPSed coating,which is mainly attributed to the low-level element segregation in plasma cladding coating.(4)On the basis of CoCrAl0.5NiCu0.5 system,the components and proportions of CoCrAl0.5NiCu0.5 HEA coating were further optimized and the Cu0.5CrAlFeNiTix system HEA coatings have been obtained.When Ti ratio is 0,the microstructure of the coating was consisted of copious eutectic grains or colonies.After adding Ti element,the Cu0.5CrAlFeNiTix system HEA coatings were transformed from eutectic to divorced eutectic microstructures:When Ti content is low,L21 phase is the primary phase,L21 phase and BCC phase form the eutectic lamellae,and most BCC phase remains in the intergranular region.The microstructures of coatings with medium and high Ti content are similar,the primary phase is transformed to the BCC phase,and the L21 phase and the BCC phase form the eutectic lamellae.In addition,a high density of hierarchical nano-precipitates was contained in the primary and eutectic phases.The hardness of the coatings increases with the increase of Ti content.The wear resistance of the coatings is one order higher than that of CoCrFeMnNiTix and CuCrFeNi2Tix system coatings,which is attributed to the strong-toughness coupling eutectic microstructure that was formed by disordered BCC phase with excellent toughness and ordered B2/L21 phases with high strength,and the precipitation strengthening caused by hierarchical nanoprecipitates.The wear mechanism of the coatings changes from abrasive wear without Ti to slight fatigue wear with Ti.The corrosion resistance decreases with the increase of Ti content,which is attributed to the main phase was transformed to the Cr-depleted L21 phase.