Effect Of In-situ Autogenous VC On Microstructure And Properties Of CoCrFeNiAl_x High-entropy Alloys

CoCrFeNi and CoCrFeNi Al_0.3 single-phase high-entropy alloys have the advantages of simple phase structure,uniform structure,and excellent plasticity.However,its hardness and strength are low,which limits their practical application space to a certain extent,so how to effectively improve its hardness and strength is a problem that needs to be solved.VC has a high melting point and high hardness and can be used as a strengthening phase to improve the performance of the alloy.However,there are few reports on in-situ hard particle-reinforced high-entropy alloys,so it is very important to study the influence of VC particles on the microstructure and properties of CoCrFeNi Alx（x=0,0.3）high-entropy alloys.In this thesis,CoCrFeNi（VC）x（x is volume fraction,x=0%,15%,30%,45%）alloys and CoCrFeNi Al_0.3（VC）x（x is volume fraction,x=0%,15%,30%,45%）alloys were prepared by vacuum arc melting technology.The microstructure and properties of the alloy were analyzed by X-ray diffractometer（XRD）,scanning electron microscope（SEM）,energy spectrum analyzer（EDS）,Vickers hardness tester,universal electronic testing machine,friction and wear testing machine,and electrochemical workstation.The effects of different VC volume fractions on the microstructure,mechanical properties,and corrosion resistance of the alloy were systematically studied.The main research results are as follows:（1）After the addition of V and C,the phase structure and microstructure of the CoCrFeNi（VC）x alloy changed significantly,and the alloy changed from a single FCC phase to a multi-phase structure with the coexistence of FCC phase and carbide phase.With the increasing x value,the number and size of VC particles formed in situ in the alloy increase continuously.（2）The mechanical properties of CoCrFeNi（VC）x alloys are closely related to the content of V and C.After adding V and C,CoCrFeNi（VC）x alloys exhibit excellent wear resistance and high strength.With the increase of the x value,the hardness of CoCrFeNi（VC）x alloy increases gradually,the yield strength increases first and then decreases,and the wear resistance increases gradually.When x=45%,the hardness reaches the maximum(830.5HV_0.3),which is 6 times that of the CoCrFeNi high-entropy alloy(131.5HV_0.3).The wear resistance performance is the best,the friction coefficient is 0.41,and the wear rate is 2.4×10^-4mm³/（N·m）.But its brittleness is relatively large,and its compressive strength is 1280MPa.When x=30%,its hardness(465.5HV_0.3)is 3.5 times that of CoCrFeNi high-entropy alloy(131.5HV_0.3),and the yield strength reaches the maximum value（1475MPa）,and its 9 times the yield strength（160MPa）of CoCrFeNi high entropy alloy.The wear resistance is second only to CoCrFeNi（VC）_30%alloy,its friction coefficient is 0.46,wear rate is 2.8×10^-4mm³/（N·m）,and its comprehensive mechanical properties are the best.（3）The potentiodynamic polarization corrosion curve of CoCrFeNi（VC）x alloy in Na Cl solution shows that the corrosion potential of the alloy is between-0.128V～-0.228V,and the corrosion current density is 1.92E^-8～3.97E^-7A/cm².As the value of x increases,the corrosion current density decreases gradually,and the corrosion resistance of the CoCrFeNi（VC）x alloy decreases gradually.Among them,when x=0%and x=15%,the alloy has obvious passivation.（4）After adding Al,the phase structure and composition of CoCrFeNi Al_0.3（VC）x alloy did not change significantly.Compared with CoCrFeNi（VC）x alloys,the hardness,and strength of CoCrFeNi Al_0.3（VC）x alloys are further improved within a certain range.Among them,when x=0%,x=15%,and x=30%,the hardness and yield strength of CoCrFeNi Al_0.3（VC）x alloy are slightly higher than those of CoCrFeNi（VC）x alloy with the same x value.This is mainly attributed to the precipitation strengthening of VC particles and the solid solution strengthening of Al.When x=45%,the hardness of CoCrFeNi Al_0.3（VC）x alloy is lower than that of CoCrFeNi（VC）_45%alloy.The strengthening effect is significantly weakened.Like CoCrFeNi（VC）x alloys,with the decrease of x value,the corrosion resistance of the alloy shows a trend of decreasing gradually.