Effect Of Al And C On Microstructure And Mechanical Properties Of The Face-Centered Cubic High Entropy Alloys

The design concept of high entropy alloy is an innovative breakthrough compared to the traditional multielement alloy.Usually,high entropy alloy contains no less than five main elements,and the percentage of each element is no less than 5%but no more than 35%.Therefore,the microstructure and properties of high entropy alloy is different from that of the traditional alloy.Due to the synergistic effect among elements,the variation of types and contents of each element may cause the variation of microstructure,crystal structures and properties of the high entropy alloy.Therefore,to obtain the high entropy alloy with high properties will be of great significance in both theory and practice.In this paper,AlxFeCrCoNi(Cy),CoCrFeNiMnCx and CoFeNiMnCx high entropy alloys were produced by combining vacuum arc melting furnace and copper casting method,and the heat treatment was adopted as well by vacuum tube type heat treatment furnace.Then the effects of Al and C on phase structure and micro structure of FCC high entropy alloy were analyzed in virtue of optical microscopy,X-ray diffraction(XRD),scanning electron microscopy(SEM/BSE/EBSD)and transmission electron microscopy(TEM).Afterward,the hardness and tensile properties of the alloy were tested by micro hardness tester and material testing machine.The specific conclusions are as follows:In AlxFeCrCoNi(Cy)high entropy alloy,the crystal structure of the alloy gradually changed from FCC to FCC + BCC with the increase of Al element.When x = 0.4,BCC phase began to appear,and it will increase accompanied by the increase of Al element.The metallographic structure was changed from tetragonal dendrites(x<0.2)to columnar dendrites(x = 0.3),then further changed into equiaxed dendrites(x>0.4).After a heat treatment at 1100℃ for 24h,a large amount of acicular BCC phase appeared in the crystal of AlxFeCrCoNi(x=0.4,0.5)high entropy alloy.The addition of a small amount of Al will at the same time increase the strength and plasticity of the alloy.When x>0.3,the strength of the alloy will improved but the plasticity is reduced.In addition,with the addition of C elements,the alloy appears a new phase M7C3,increasing the strength while reducing the plasticity.C element can improve the solid solubility of Al atoms of high entropy alloy in AlxFeCrCoNi(Cy).The alloy with the best mechanical properties is Al0.4CoCrFeNiC0.05,of which the yield strength,tensile strength and elongation are 330MPa,621MPa and 56%respectively.In the high entropy alloy of CoCrFeNiMnCx,the crystal structure of the alloy will still be FCC after adding a small amount of C element(x<0.05),while the strength and plasticity of alloy will improved;when x>0.1,new phase M7C3 will appear,the strength of alloy will increase and the plasticity will reduce.Moreover,as C element increase,the shape will changed from thin rod into sheet,distributed around the grain boundary or aside grain boundary.CoCrFeNiMnC0.05 alloy will form plenty of nanoscale deformation twins under high strain.The trace element C can make the plastic deformation dominated by dislocation slip into a mixed deformation pattern consisting of dislocation slip and twinning.After the heat treatment,a new phase of M23C6 will appear,the strength and plasticity of the alloy will reduced.In high entropy alloy of the CoFeNiMnCx,lattice constant increase with the addition of C element,but the crystal structure of the alloy still be FCC.The strength increase with the increase of C content,and the plasticity decrease slightly.The alloy with the best mechanical properties is CoFeNiMnC0.2,of which the yield strength,tensile strength and elongation are 521MPa,778MPa and 38%respectively.After the heat treatment at 1200℃ for 48h,the spinodal decomposition appear,and the strength and plasticity of the alloy further improved.