Preparation,Structure And Properties Of CoCrFeNi Based High-entropy Alloy Coatings

The high-entropy alloys(HEAs)have unique atomic structure,and show excellent mechanical properties such as high strength,high hardness,wear resistance and corrosion resistance.In particular,the recent application in coating has become a new research hotspot.According to environmental protection,high cost performance and excellent performances,HEA coatings(HEACs)as novel modified material show great usable space and potential application prospect.A series of novel Co Cr Fe Ni HEACs with high performance were prepared in the Q235steel and Ni201 substrates by vacuum hot pressing sintering(VHPS)and mechanical alloying(MA).The influence of the addition of refractory elements and ball milling time on the microstructural evolution of HEA powders have been studied.It provides theoretical basis and practical support for the application of HEA coatings in the complex environment.Moreover,the microstructures,microhardness,wear resistance and corrosion resistance of HEACs have been also explored in detail.It is expected to acquire high-performance HEACs.The main results are shown as follows:1.High-quality Co Cr Fe Ni Ta_(1-x)(x=0,0.5)and Co Cr Fe Ni Ta+Gr(Ta1+Gr)HEACs were prepared on Q235 substrate by MA and VHPS.After 120 h of MA,Co Cr Fe Ni Ta0.5(Ta0.5)is a Bore-Centered Cubic(BCC)solid solution,showing the Ta-rich BCC1 and Co Fe-rich BCC2 phases.The final ball milling products of Co Cr Fe Ni Ta(Ta1)consist of the high-entropy amorphous phase.The particle size of Ta0.5 and Ta1 HEA powders are in the range of 20?40?m and 10?25?m,respectively.It needs to be noted that the particle size of Ta1+Gr HEA powder is further refined between 10?20?m.Three kinds of HEA powders have uniform component distribution,which are close to the nominal composition.The HEACs prepared by VHPS-ed are uniform and compact,and possess the high bonding strength with the substrate.The phase structures of three kinds of HEACs are dual solid solutions with FCC structure,and intermetallic compounds.The microhardness of the HEACs are higher than 498 HV,which are the better than that of the substrate(169.36 HV).The Ta1+Gr HEAC presnts the highest microhardness,reaching 590.82 HV.In addition,the friction coefficients of three kinds of HEACs are all lower than that of the substrate,showing better wear resistance.The wear rate of Ta1+Gr HEAC is only 3.01×10^-5 mm³/N·m,which is about24.08%of the substrate showing the best wear resistance under the compound friction and wear condition.It accords with the law of positive correlation between wear resistance and hardness.According to the results of electrochemical corrosion in 3.5%Na Cl solution,the HEACs have the better comprehensive corrosion resistance than the substrate,which are characterized by lower corrosion current density(i_corr),more positive corrosion potential(E_corr)and a wider passivation region(?E_p).2.High-quality(Co Cr Fe Ni)_90-xHf₁₀W_x(x=0,10)HEACs were prepared on Ni201substrate by MA and VHPS.According to the XRD and DSC test results,the(Co Cr Fe Ni)₉₀HF₁₀(Hf10)HEA powder after MA of 140 h has a mixture of FCC solid solution and a small amount of amorphous phase.The phase component of(Co Cr Fe Ni)₈₀Hf₁₀W₁₀(Hf10W10)powder is composed of dual solid solutions with FCC and BCC structures and a small amount of amorphous phase.The particle sizes of two kinds of HEA powders are similar,ranging from 20 to 40?m.The content of each element is close to the nominal composition.The addition of W makes the refined powder particles.The HEACs are mainly composed of FCC solid solution and complex intermetallic compounds.It was found by microhardness test that the microhardness of two HEACs is the higher than that of the substrate(251.45 HV).The HF10W10 HEAC possesses the high mircohardness of 1131.60 HV,which is mainly attributed to the more precipitation of intermetallic compounds.The friction coefficient of two kinds of HEACs is lower than that of the substrate,and their wear resistance are significantly improved.The wear rate of Hf10W10HEAC is 3.10×10^-11 mm³/N·m,showing excellent wear resistance.In the corrosive environment of 3.5%Na Cl solution,the corrosion resistance of HEACs is better compared to substrate.W addition induces lower i_corr,more positvie E_corr and a wider?E_p.3.A preliminary exploration on the microstructures and properties of Ta0.5(Q235)and Hf W-containing system(Ni201)HEACs prepared by MA has been carried out.The mechanical alloyed HEACs are uniform and compact,showing a good continuity.The120-milled Ta0.5 HEAC bears a single BCC solid solution phase.The Hf10 and Hf10W10HEACs exhibit dual solid solutions with FCC and BCC structures.The microhardness of the three kinds of HEACs is obviously higher than that of their corresponding substrates,and the microhardness of Hf10W10 HEAC is up to 468.19 HV,which is 1.9 times that of Ni201substrate.In the corrosive environment of 3.5%Na Cl solution,the comprehensive corrosion resistance of the HEACs is better than that of the substrate.The Hf10 HEAC has a lower i_corr,which further improves the general corrosion resistance.4.According to above analysis and summary,it shows that the phase component of the MA coatings is mainly composed of solid solutions,while the VHPS-ed coatings possess more complex microstructures,revealing the increased number and type of intermetallic compounds.This is related to the precipitation of hard compounds from supersaturated solid solution under sintering condition.Compared with MA coatings,VHPS-ed ones have some advantages in wear resistance and microhardness,but the MA technique is more controllable and easy to handle.In conclusion,MA and VHPS studied in this study further expand the diversity of preparation methods of HEACs and provide necessary theoretical guidance and technical support for the development of potential applications.