Effect Of Al Addition On Microstructure,Mechanics And Corrosion Behavior Of VCoNi High-entropy Alloy

High-entropy Alloys（HEAs）represent a class of solid solution alloys composed of multiple principal elements.It is a new type of metal materials developed in recent years.A variety of systems have been developed,showing unique mechanical,electrical and magnetic properties.Therefore,high-entropy alloys enrich the design idea of the alloy and provide a reference for the design of new high-performance alloys.The VCoNi alloy with face-centered cubic structure（FCC）is a kind of high-entropy alloy system developed in recent years,which has good plastic deformation ability and excellent low temperature mechanical properties.However,the relatively low room temperature strength hinders its application as a practical structural material.Alloying is generally considered to be an effective means of strengthening metallic materials.However,the effect of alloying on the microstructure evolution of VCoNi alloy and its relationship with mechanical and corrosion behavior has not been reported in the literature,and the relevant basic research is urgently needed.In this thesis,Al alloying was used to regulate the microstructure of VCoNi alloy,and its corresponding mechanical and corrosion behaviors were systematically studied by using quasi-static tensile,electrochemical and immersion experiments.The main research results were as follows:（1）Al_xVCoNi（x=0,0.13,0.23）alloys with high entropy were prepared by vacuum arc melting and copper mold drop casting.The effects of Al alloying on the microstructure of Al_xVCoNi alloys were investigated.It was found that with the increase of Al element content,the microstructure of the alloy changed from single-phase FCC to two-phases FCC+BCC,in which the BCC phase is the enrichment phase of Al element.（2）The test results of mechanical properties at temperatures from 298K to 873K showed that the strength of the alloy increase with the increase of Al content at the same temperature,while the ductility and toughness decrease.The BCC phase is the main reason for the increase of strength and ductility of the alloy.When the test temperature is 573 K,the stress-strain curves of the alloy showed obvious serrated flow characteristics,and showed a good strong plastic combination,which is attributed to the dynamic strain aging and dynamic precipitation of σphase.（3）Electrochemical tests and immersion test in 1M H₂SO₄ solution showed that the addition of Al reduces the corrosion resistance of VCoNi alloy.Among them,the corrosion morphology of Alo.23VCoNi alloy not only showed grain boundary corrosion,localized corrosion occurred at the two-phase interface and on the BCC phase,indicating that the addition of Al increased the alloy’s pitting sensitivity and reduced its local corrosion resistance.（4）In the study of the passivation film in the electrochemical impedance test and X-ray photoelectron spectroscopy test,the results show that after the addition of Al,the passivation film formed on the surface of the alloy is less stable and has higher solubility.At the same time,after the addition of Al element the protection of the passivation film on the surface of the alloy will be reduced,which will eventually lead to a reduction in the overall corrosion resistance of the alloy.