Microstructure And Properties Of FeCoCrNiMnN_x High-entropy Alloys Prepared By Powder Metallurgy

FeCoCrNiMn high-entropy alloy(HEA)has a simple crystal structure and good plasticity and toughness,but existing relatively low strength,then added with N element in order to improve the strength.The alloy prepared by the powder metallurgy has many advantages,such as fine grains,uniform composition and high utilization of raw materials,therefore FeCoCrNiMnNx(x=0,0.1,0.2,0.3,mol)HEAs were prepared by this method in this paper.Effects of N addition on microstructure and mechanical properties of FeCoCrNiMn HEA were investigated by X-ray diffraction,scanning electron microscope,transmission electron microscope,universal material testing machine and electrochemical workstation etc.The results show:During the mechanical alloying process of FeCoCrNiMnNx HEAs,the alloying elements were gradually alloyed and formed as face-center(FCC)solid solutions within 45h of milling.After the vacuum hot pressing sintering(VHPS),the microstructure of alloys were basically the same,which contained FCC matrix phases with traces of M23C6carbides(M=Cr,Fe,Co),a phases and Cr2N phases.Meanwhile,it also found some nano twins.Sintering temperature has an important effect on the microstructure and mechanical properties of FeCoCrNiMn HEA.In the temperature range(900℃～1000℃)sintering,the alloys have been fully sintered,and its microstructure show that the bulk light areas with low Cr content are FCC phases as a matrix,while the irregular dark areas with high Cr content are precipitations.As the sintering temperature increased,the hardness of the alloys first increased and then decreased,accompanied by a gradual increase in plasticity.From the considerations of comprehensive mechanical performance,the optimum sintering temperature of this alloy is 900℃.N element has a significant strengthening effect on FeCoCrNiMn HEA.As N content of the alloys increased,the yield strength,compressive strength and hardness gradually increased,accompanied by a decrease in plasticity,and the wear resistance also gradually increased,where the wear mechanism was slight flaking wear and abrasive wear.Adding N element is helpful to improve the high temperature oxidation resistance of FeCoCrNiMn HEA.The oxidation rate of the alloys decreased with the increase of N content;At800’C,the surface oxide layer of the alloys mainly consisted of Mn3O4 and a small amount of MnCr2O4;At 900℃,the surface oxide layer of the alloys were dominated by Mn3O4;At 1000℃,the surface oxide layer of the alloys occurred flaking,and there are also the other oxides such as CrO and Cr2O3 in addition to Mn3O4,MnCr2O4.In 3.5w.t%NaCl solution,FeCoCrNiMnN0.2HEA has the smallest corrosion current density;In 3.5w.t%NaOH solution,FeCoCrNiMnN0.3 HEA has the smallest corrosion current density;In 1 mol/L H2SO4 solution,the corrosion potential of the alloys first increased and then decreased with the increase of N content,FeCoCrNiMnN0.1 HEA has the smallest corrosion current density,which exhibits the best corrosion resistance.