Research On Microstructure And Properties Of AlCoFeNiCu High-entropy Alloys Prepared By Mechanical Alloying

As a novel alloy, high entropy alloys have broken-through the design mode ofconventional alloys that applies a single principal element, and become a new focus onmaterials research. Through the reasonable metallurgy composition design, high entropyalloys can achieve high strength and high hardness, excellent corrosion resistance, goodthermal stability, distinctive electrical and magnetic properties, as well as many otheroutstanding properties.In this paper, AlCoFeNiCu system high entropy alloys were prepared by mechanicalalloying (MA) and spark plasma sintering (SPS) or vacuum hot press (VHP). The alloyingbehavior of MA, phase formation rules, microstructure and mechanical properties ofAlCoFeNiCu system alloys with different contents of Al and Cu were studied. Also, theeffects of Cr, Ti, C on the microstructure and mechanical properties of AlCoFeNiCu systemhigh entropy alloys were investigated. VHP and different processing parameters of SPS wereused to analyze the influence of preparation technique on microstructure and mechanicalproperties of the alloys.The results reveal that in AlxCoFeNiCu1-x(x=0.25,0.5,0.75) high entropy alloys, withthe increase of Al element and the reduction of Cu element, the crystal structure first consistsof an FCC (face-centered cubic) structure and then become FCC+BCC (body-centered cubic)structures. Segregations of both Al and Cu elements were found and the microstructures of thealloys consist of white areas and grey areas. The grain size of Al0.25CoFeNiCu0.75alloy is lessthan1μm. Some nanocrystalline areas as well as a small amount of nanoscale twins withFCC structure were found in Al0.25CoFeNiCu0.75alloy. The compressive strength and hardnessof AlxCoFeNiCu1-xhigh entropy alloys increase while the compression ratio decreases withincreasing Al content. Bulk Al0.25CoFeNiCu0.75alloy displays superior mechanical properties,its yield strength, compressive strength, compression ratio and Vickers hardness are1598MPa,1889MPa,13.1%and482HV, respectively.The addition of Cr element facilitates the formation of BCC phase during MA, whilereduces the formation of BCC phase in bulk high entropy alloys after SPS. Compressivestrength of AlxCoFeNiCu1-xhigh entropy alloys decreases with the addition of Cr. When themolar rate of Al content is x=0.25, a higher hardness and a lower plasticity can be achievedby adding Cr. However, Cr can soften and plasticize the alloys when the molar rate of Alcontent is x=0.5or0.75. The addition of Ti element can also facilitate the formation of BCCphase during MA. It can enhance the strength and hardness, while reduce the plasticity of the bulk high entropy alloys. The yield strength, compressive strength and hardness ofAl0.5CoCrFeNiCu0.25Ti0.25alloy reach2046MPa,2279MPa and628HV, respectively.Al0.5Co0.3CrFeNiC0.2high entropy alloy that adding C element forms a main FCC phase, aBCC phase, an ordered BCC phase and Cr23C6carbide. Nanoscale twins present in partialFCC phase.Bulk high entropy alloys sintered by VHP show higher compressive strength and density,as well as lower plasticity compared with which sintered by SPS. Superior mechanicalproperties of the bulk high entropy alloys can be achieved by SPS than by VHP. The increaseof soaking time during SPS will reduce the yield strength and enhance the plasticity ofAl0.25CoFeNiCu0.75alloy. The effect of heating rate on the mechanical properties ofAl0.25CoFeNiCu0.75alloy is limited. And the increase of sintering temperature can enhance theplasticity of the alloy, while its yield strength first increases and then becomes lower.