Microstructure Control And Hot Corrosion Behavior Of Plasma Cladding High Entropy Alloy Coatings

With the continuous improvement of single unit capacity and parameters of power plant boiler,coupled with the wide application of low nitrogen combustion technology and ultra-low emission technology,the risk of high temperature corrosion of boiler heating surface is further increased,especially when burning high sulfur coal,which has a great impact on the safe operation of the boiler.One of the effective methods to solve this problem is to prepare high-temperature and anti-corrosion coatings on the heating surface of boilers.High entropy alloy coatings,due to their novel alloy design concept and unique high entropy effect,have characteristics such as high strength,high hardness,high thermal stability,and high corrosion resistance,which have great application potential in the field of surface protection of hot end components.This thesis designs and uses plasma cladding technology to prepare Fe Co Cr Ni Al_0.5Ti_0.5high entropy alloy coatings,compares and studies the effects of different coating powder states（simple mechanical mixing of powders,Vacuum atomized powders and mechanical alloying powders）on coating cladding formation,microstructure,and component distribution.The hardness and friction and wear resistance of the coatings are analyzed,and the simulated high sulfur coal ash hot corrosion behavior of the coatings at 650℃and 850℃is investigated,explore the mechanism of thermal corrosion resistance of coatings.The conclusion is as follows:（1）In the process of preparing Fe Co Cr Ni Al_0.5Ti_0.5high entropy alloy powder by mechanical alloying,the mechanical alloying sequence of each element in the high entropy alloy is related to the melting point and plasticity of the elements.Elements with low melting points are more likely to be alloying.With the increase of ball milling time,the alloying degree of the powder increases,forming a nano high entropy alloy powder.After 36 hours of ball milling,a dual phase solid solution powder with grain size of 13～15 nm,BCC as the main phase,and FCC as the secondary phase is formed.（2）The formation quality of Fe Co Cr Ni Al_0.5Ti_0.5high entropy alloy coating prepared by plasma cladding is good,and a typical cladding microstructure of cellular,dendritic,and equiaxed crystals is formed from the substrate to the surface.The high entropy alloy coatings obtained by plasma cladding in three different powder states all form a dual phase structure of FCC+BCC,with the highest BCC phase volume fraction of 31.2%prepared by simple mechanical mixing of powders.A large amount of dislocations accumulate in the nanocrystalline boundaries formed by mechanical alloying powder,and during the melting process,the internal lattice reorganizes,transforming the unstable BCC phase in the coating into a relatively stable FCC phase.（3）The wear resistance of Fe Co Cr Ni Al_0.5Ti_0.5high entropy alloy coatings is positively correlated with the coating hardness,and the hardness of coatings is directly proportional to the volume fraction of BCC phase in the coating.The highest hardness of the coating prepared by simple mechanical mixing powder is 650.5HV,which can effectively resist the grooves left by abrasive particles on the surface during the wear process.The main wear mechanism is abrasive wear.The coating prepared by vacuum atomization powder has high dilution rate,low hardness,and a small amount of delamination on the worn surface,resulting in delamination wear.The surface of the coating prepared by mechanical alloying powder is mainly subjected to abrasive wear,with adhesive pits appearing on the surface,indicating a small amount of adhesive wear during the wear process.The friction and wear resistance of the three high entropy alloy coatings is significantly better than that of HR3C and 316H.（4）Simulating boiler coal ash composition with 37.5%Na₂SO₄+37.5%K₂SO₄+15Fe₂O₃+10%Si O₂corrosion salt for 150h hot corrosion test,the results show that the hot corrosion kinetics curve of Fe Co Cr Ni Al_0.5Ti_0.5high entropy alloy coating prepared by plasma cladding shows a trend of rapid weight increase at first and then slow weight gain at 650℃and 850℃,and the corrosion weight gain of coating prepared by mechanical alloying powder is the lowest.The hot corrosion mechanism of high entropy alloy coating at 650℃is mainly oxidation corrosion.The hot corrosion at 850℃is the result of the interaction of oxidation and vulcanization.After150 hours of hot corrosion,the coating prepared by mechanical alloying powder forms a dense oxide structure of outer Cr₂O₃and inner Al₂O₃,which effectively hinders the invasion of corrosion medium,and the phenomenon of internal vulcanization is the lightest.The coating prepared by simple mechanical mixing and true air atomization powder could not quickly form a single dense oxide film at the initial stage of hot corrosion,and a mixed oxide layer was formed in the subsequent corrosion,and the loose mixed oxide layer became the channel of O and S into the inner channel of the coating.to form a thicker internal sulfide layer.Two kinds of heat-resistant corrosion-resistant steels showed obvious corrosion product shedding after corrosion at 850℃for 150 h,and the corrosion was serious.