Effects Of Al And Ti On Microstructures And Mechanical Properties Of CoCrNi Multi-principal-element Alloy

In recent years,multi-principal-element alloys are the new class of materials defined as alloys consisting of 3 or more principal elements with equimolar or nearly equimolar fractions.Compared with the traditional alloys,the multi-principal-element alloys exhibit many excellent properties,such as,high hardness,high strength,corrosion resistance and good high-temperature oxidation.In the present study,Al_xCoCrNi(x=0,0.1,0.2,0.3,0.4,0.5,0.6,0.8,1,1.2,denoted as Al-x afterwards)and Ti_xCoCrNi(x=0,0.1,0.2,0.3,0.4,denoted as Ti-x afterwards)were prepared by arc-melting and drop casting.The microstructure of the samples were investigated using X-Ray diffraction(XRD)and scanning electron microscopy(SEM).The mechanical properties material properties were tested by a material performance rig and Vickers hardness tester at room temperature.SEM results showed that the crystalline structure of the as-cast Al_xCoCrNi alloys changes from the initial face-centered cubic(FCC)structure to a duplex FCC plus body-centered cubic(BCC)structure and then a single BCC structure as the Al concentration is increased.In the range of 0≤x≤0.6,with the increasing Al,the tensile strength of the alloy is improved but accompanied with reduced ductility.For example,the tensile yield strength,ultimate strength and tensile elongation of Al-0.6 are 544 MPa,1109 MPa and14%respectively.Specifically,the Al-0.8,Al-1 and Al-1.2 alloys are brittle material.The hardness increase with the increase of Al contents,and the Al-1.2 alloy exhibits the hightest hardness wih a value of 659 HV.The microstructures of the as-cast Ti_xCoCrNi alloys change from the initial single FCC structure to a duplex FCC plus(Ti+Ni)-rich phase structure and then FCC+(Ti+Ni)-rich phase+Cr-rich phase structure as the Ti concentration was increased.With the increase of Ti concentration,the compressive yield strength increases at first and then decreases,the Ti-0.3 alloy exhibits the hightest yield strength wih a value of1712.6 MPa.In the terms of plasticity,the Ti-0,Ti-0.1 and Ti-0.2 alloys are plastic materials.The compressive ratios of Ti-0.3 and Ti-0.4 alloys are 13.6%and 27.9%,respectively.The hardness also increase with the increase of the Ti contents,and the Ti-0.4 alloy exhibits the hightest hardness wih a value of 608 HV.