Effect Of Ni Content On Microstructure And Properties Of CoCrCuFeMnNi_x High-entropy Alloys

Five-element high-entropy alloys(HEAs)made of Co,Cr,Cu,Fe,Mn and Ni have been widely studied,but there are limited researches on the six-element HEAs of CoCrCuFeMnNi serie.In this study,based on the investigation of our research group on the microstructure and mechanical properties of Co_xCr_yCu Fe Mn Ni system six-element HEAs,CoCrCuFeMnNi_x(x=0,0.5,1.0,1.5,2.0 mol)HEA powders and bulks were prepared by mechanical alloying and hot pressing sintering processes.The effects of Ni element on mechanical alloying behavior,thermal stability,mechanical properties,oxidation resistance and corrosion resistance of the alloys were systematically studied.The main research results are as follows:(1)In the initial stage of mechanical alloying(5?15 h),all Ni₀,Ni_0.5and Ni_1.0alloys are composed of BCC+FCC two-phase solid solution structure.With the prolongation of milling time,BCC phase gradually disappeared and finally formed a single FCC structure.However,Ni_1.5and Ni_2.0alloys with high Ni content remain a single FCC structure during the whole mechanical alloying process.The annealing experiments of powder at 700?900?shows that the increase of Ni content can improve the thermal stability of CoCrCuFeMnNi_xHEA powders.(2)After hot pressing and sintering,all of Ni₀,Ni_0.5,Ni_1.0,and Ni_1.5alloys are composed of Cr-rich FCC1 main phase,secondary Cu-rich FCC2 phase and a small amount of?phase.With the increase of Ni content,the content of FCC2 secondary phase gradually decreases.Increasing Ni content gradually decreases the FCC2secondary phase content until it disappears completely when the Ni content increases to 2.0 mol,results in the alloy being only composed of FCC phase and a small amount of?phase.The hardness and yield strength of CoCrCuFeMnNi_xbulk HEAs first increase and then decrease with the increase of Ni content.Ni_1.0alloy has the highest hardness and yield strength,which are 400HV and 1121MPa,respectively.The strengthening mechanisms of all alloys are solid solution strengthening and second phase strengthening.(3)The oxidation resistance of CoCrCuFeMnNi_xbulk HEAs increases with the increase of Ni content.The Ni_2.0alloy has the smallest oxidation weight increment and the thinnest oxide layer thickness,showing the optimal oxidation resistance.The oxidation mechanism of the alloys is selective oxidation mechanism of Mn and Cr elements.Due to the increase of Ni content reduces the volatilization of Cr₂O₃and enhances the hysteretic diffusion effect of the HEA and the thermal stability of the FCC phase,the number of Mn ions diffusing to the surface of the alloy decreases and the oxidation resistance of the alloy improves correspondingly.(4)The i_corrof CoCrCuFeMnNi_xbulk HEAs in 3.5%Na Cl solution decrease with the increase of Ni content.Ni_2.0alloy has the smallest i_corr,higher polarization resistance R_CPEand relative thickness CPE₁^-1of passivation film,consequently.it shows good corrosion resistance.The main corrosion resistance mechanism of CoCrCuFeMnNi_xbulk HEAs is that Ni promotes the formation of Ni(OH)₂passivation film and inhibits further corrosion of the corrosive medium.Meanwhile,the decrease of Cu-rich FCC2 phase weakens the corrosion degree of the original battery in the alloy.