Investigation On Microstructures And Properties Of Semi-Solid Al₈₀Mg₅Li₅Zn₅Cu₅ Light-Weight High Entropy Alloys

Light-weight high entropy alloy has a high development potential in the industrial field as light-weight structural materials due to its excellent comprehensive properties.However,its structure is mostly composed of body-centered cubic（BCC）phase and intermetallic compound.Although its compressive strength is high,it still has the disadvantage of poor plasticity.The semi-solid forming process has the characteristics of low deformation resistance and good fluidity,which can realize the forming of low plastic alloy.In this paper,Al₈₀Mg₅Li₅Zn₅Cu₅light-weight high entropy alloy is taken as the research object,and the semi-solid forming technology is proposed to control the microstructure and mechanical properties of the alloy.The semi-solid billet of Al₈₀Mg₅Li₅Zn₅Cu₅light-weight high entropy alloy is prepared by semi-solid isothermal treatment（SSIT）method and strain-induced melting activation（SIMA）method.The effects of pre-deformation,isothermal temperature and isothermal time on the semi-solid microstructure evolution and properties are studied.Through the uniaxial isothermal compression experiment of Al₈₀Mg₅Li₅Zn₅Cu₅light-weight high entropy alloy,the effects of strain temperature and strain rate on the true stress-strain curves were analyzed,and the semi-solid thixotropic behavior of Al₈₀Mg₅Li₅Zn₅Cu₅light-weight high entropy alloy was investigated.The main research contents of this paper are as follows:（1）The semi-solid Al₈₀Mg₅Li₅Zn₅Cu₅light-weight high entropy alloy was prepared by SSIT method.It was found that with the increase of isothermal temperature,the average grain size of semi-solid Al₈₀Mg₅Li₅Zn₅Cu₅light-weight high entropy alloy increased,and the spheroidization time of grains decreased.With the extension of holding time,the average grain size increases,and the shape factor increases first and then decreases.Better semi-solid microstructures can be obtained at 520℃for 30 min,and the shape factor and average grain size are 0.90 and 56.4μm,respectively.Semi-solid Al₈₀Mg₅Li₅Zn₅Cu₅light-weight high entropy alloy has a lower coarsening rate than conventional aluminum and magnesium alloys under the hysteresis diffusion of high entropy alloys.In the mechanical properties tests,the hardness of semi-solid Al₈₀Mg₅Li₅Zn₅Cu₅light-weight high entropy alloy is 145 HV at 520℃for 30 min.The compressive strength of the as-cast alloy was 496.7 MPa,and the plastic strain was 9.5%.The semi-solid treatment significantly improved the strength and plasticity of the as-cast alloy,which increased by 36%and 108%,respectively.The polarization curves of as-cast and semi-solid alloys were measured and compared in 3.5 wt.%Na Cl solution.It was found that the corrosion rate of semi-solid alloy was smaller,and the passive current density i_pand polarization resistance R_pwere 2.27×10^-5A/cm²and 389.06Ω·cm²,respectively.The semi-solid Al₈₀Mg₅Li₅Zn₅Cu₅light-weight high entropy alloy formed a more stable passivation film,showing better corrosion resistance.（2）The semi-solid Al₈₀Mg₅Li₅Zn₅Cu₅light-weight high entropy alloy was prepared by SIMA method.It was found that when the alloy was held at 500℃for 15 min,with the increase of the pre-deformation amount,the microstructure of the alloy changed from dendrites to spheroidized grains.When the deformation is 20%,the minimum average grain size is 29.1μm and the maximum shape factor is 0.86.With the increase of isothermal time,the average grain size increases slowly from 24.9μm to 42μm.The shape factor increases first and then decreases.The coarsening rate K of semi-solid alloy prepared by SIMA method is 35.8μm³/s,which is lower than that of semi-solid alloy prepared by SSIT method,and a uniform and round semi-solid structure is formed in a relatively short time.The results show that the pre-deformation and heat treatment process have a great influence on the mechanical properties of the alloy.With the increase of pre-deformation,the hardness of the as-cast alloy with 30%deformation is increased by 33%,which is 195 HV,compared with the as-cast alloy without deformation treatment（146 HV）.When the deformation is 20%,the hardness is up to212 HV after holding at 500℃for 20 min.In the compression tests,the compressive strength of the SIMA semi-solid alloy was 558.4 MPa,which was 12%higher than that of the as-cast alloy,and the plastic strain remained basically unchanged at 10.1%.In the electrochemical corrosion tests,it was found that the self-corrosion potential of the SIMA semi-solid alloy was greater than that of the as-cast and semi-solid samples,and the polarization curve had obvious passivation characteristics.Compared with the other two alloys,the passivation region of the semi-solid alloy by SIMA method is narrow,and theΔE₃is only 0.31 V,but the corrosion current density j_corr（0.009 A/cm²）measured in Na Cl solution is small.（3）It was found that there were three different stages in the process of hot compression deformation,which were work hardening stage,deformation resistance reduction stage and steady flow stage.When the strain temperature is constant,the peak stress increases with the increase of strain rate.At 500°C,the peak stress increased from 9.4 MPa to 32.5 MPa when the strain rate increased from 0.01 s^-1to 1 s^-1.When the strain rate is constant,the peak stress decreases with the increase of strain temperature.When the strain rate is 1 s^-1,the peak stress decreases from 32.5 MPa to 0.96 MPa when the strain temperature increases from 500°C to530°C.The constitutive equation of Al₈₀Mg₅Li₅Zn₅Cu₅light-weight high entropy alloy semi-solid zone established by Arrhenius hyperbolic sine equation is:exp（566307/8.314T）（1-1.5f_L）^4.578（?）=7.367×10³⁷［sinh（0.682σ）］^1.255...