Design And Mechanical Property Of Al_xCo₁₅Cr₁₅Ni_70-x High-entropy Alloys At High Temperature

High-entropy alloys?HEAs?are newly-developed alloys with multiple principal elements and mixing entropy that is much higher than traditional alloys containing single principal element.It is believed that the mixing Gibbs free energy of HEAs can be reduced by the higher mixing entropy caused by the multiple principal elements.Consequently,the formation of the intermetallic compounds is suppressed and the formation of solid solutions is favored.In this work,based on the Ni-Al binary system,a series of Al_xCo₁₅Cr₁₅Ni_70-x HEAs with good high temperature properties was designed and arc-melted,with 15%?atomic ratio?Co and Cr are alloyed in,separately.The influence of Al content on the resultant phases and microstructures of the alloys was studied through XRD?X-ray diffraction?,SEM?scanning electron microscope?and TEM?transmission electron microscope?.The room and high temperature mechanical properties of the alloys were also characterized using an Instron universal test machine and a Gleeble 3500 system.It is revealed that,with the increase of Al content,the lattice structure of the Al_xCo₁₅Cr₁₅Ni_70-x0-x alloys evolves from FCC?face-centered cubic?to FCC plus BCC?body-centered cubic?,finally to BCC.In the Al content range of 12.5¹9.3 at%,the??phase which strengthened the alloys greatly at both room temperature and 700⁸50°C formed.The B2 phase with ordered BCC structure starts to appear at the Al content of17.5 at%.The volume fraction of B2 phase increased with the increase of Al content and at the Al content of 35 at%,only B2 phases were detected.It is noteworthy a eutectic alloy with typical lamellar structure was found at the Al content of 19.3%.This eutectic showed the best comprehensive mechanical property at both room temperature and high temperatures.A positive linear relationship between the yield strength and the volume fraction of the??phase was observed at room temperature,700 and 850°C.However,at 1000°C,the positive linear relationship was lowered and almost disappeared.The thermal stability and the hot deformation behavior of the eutectic Al_19.3Co₁₅Cr₁₅Ni_50.70.7 alloy were studied.The hot deformation behavior of the eutectic Al_19.3Co₁₅Cr₁₅Ni_50.70.7 alloy was studied with the Gleeble 3500 system.The constitutive equation of the Al_19.3Co₁₅Cr₁₅Ni_50.70.7 alloy was determined through the hot deformation tests.The thermal deformation constitutive equation can be used to guide the design of thermal deformation process.