Study On Microstructure And Mechanical Properties Of （FeCoNiCr）_100-X（AlCu）_x High Entropy Alloys

Due to the limitation of the main elements,the degree of freedom of system design of traditional alloys is low,while the high entropy alloy can guarantee multiple components without producing complex harmful intermediate phase,which has a high degree of freedom and has great development potential.The mechanical properties of high entropy alloys can be improved by precipitation strengthening because of their hysteretic diffusion effect.However,most of the current studies on precipitation strengthening of high entropy alloys take single element addition as the variable,and there is a lack of discussion on the effect of multi-element addition.In this paper,（FeCoNiCr）_100-x（AlCu）_x（x=0,1,3,5,10,15,20,30）high entropy alloy was prepared by vacuum arc melting technology with equal atomic ratio FeCoNiCr alloy as the research object.Scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,small Angle neutron scattering（SANS）and other means to systematically analyze the phase and structure of the alloy and summarize the rules of mechanical properties.The effects of Aland Cu on the microstructure and mechanical behavior of FeCoNiCr alloy with different isoatomic ratios were investigated.The（FeCoNiCr）₈₅（AlCu）₁₅ high entropy alloy with the best comprehensive properties was selected by composition optimization.Finally,the influence mechanism of different heat treatment processes on the microstructure and mechanical properties of the alloy was studied.The results are as follows:The microstructure of（FeCoNiCr）_100-x（AlCu）_x high entropy alloy has a significant change due to the content of AlCu.The alloy gradually changes from a single FCC structure phase to a FCC+BCC biphase structure,and finally the BCC structure is the main phase.With the increase of（AlCu）content,the microstructure of the alloy changes from cell crystal to equiaxed crystal,dendrite and the mixture of cell crystal and dendrite.With the increase of（AlCu）content,the strength and plasticity of（FeCoNiCr）_100-x（AlCu）_x high entropy alloy increase first and then decrease.When x is less than 15,the strength and plasticity of the alloy are improved due to the effect of solution strengthening and fine-crystal strengthening.When x is greater than 15 and less than 30,the alloy becomes hard and brittle due to the increase of BCC phase,and the strength and hardness of the alloy continue to increase,while the plasticity decreases.When x is equal to 30,the strength and hardness of the alloy reach the maximum,but the strength and plasticity decrease sharply.When x equals 15,the alloy has the best strength matching.Before the aging temperature reaches 700℃,amplitude-modulated microstructure and ordered L1₂ structure（Al,Ni,Cu）nanophase begin to appear in the alloy,and will continue to grow,the tensile strength of the alloy continues to increase,plasticity continues to decrease.When the aging temperature reaches 700℃,the strong plasticity of the alloy decreases because the amplitude-modulated structure is consumed and a new precipitate phase is formed.The alloy treated at 700℃has the best strength and plasticity matching.