Microstructures And Deformation Of Al-Co-Cr-Fe-Ni High Entropy Alloys

High entropy alloys(HEAs)is a kind of new multi-component metal alloy materials with many excellent properties,which has many advantages such as high strength,high hardness,strong corrosion resistance and good high temperature and low temperature properties.It has broad application prospect.However,with the continuous improvement of the requirements of engineering applications,it is necessary to develop high-entropy Al-Co-Cr-Fe-Ni alloy with high strength,high plastic with more balanced mechanical properties.The alloy composition system is designed based on the valence electron concentration and atomic radius difference.It is an important method to promote the strengthening and toughening of Al-Co-Cr-Fe-Ni system HEAs through different rolling and heat treatment processes.The phase formation and its effect on the properties of Al-Co-Cr-Fe-Ni high entropy alloys remain to be further studied.In this study,Al_xCoCrFeNi(x=0.25,0.55,0.75,1.25)and Al Co Co Cr_0.5Fe_xNi_2.5(x=0.5,1.5,2.5,3.5)systems of HEAs were prepared by vacuum arc melting technique.The samples were cooled in furnace by annealing at different temperatures after deep cold rolling and cold rolling respectively.With using XRD,OM,TEM,SEM and different characterization methods to observe the microstructure,and carrying out a hardness and tensile property test of alloys,rolling and heat treatment on the two systems in order to get the influence of the microstructure and mechanical properties of high entropy alloys,and based on the average valence electron electronic concentration(VEC)and the difference of atomic radius,to explore the discipline of alloy phase formation,the main conclusions are as follows:1.With the increase of Al content(0.25-1.25),the phase structure of Al_xCoCrFeNi system high entropy alloy underwent FCC?FCC+BCC two-phase?BCC transition.When the content of Al increases to a certain extent,the BCC phase is ordered and B2 is formed.It is concluded that Al is the element that promotes the stability of BCC phase,and the hardness of the alloys keep rising with the increase of the volume fraction of BCC phase and the degree of ordering.2.In the HEAs with constant valence electron concentration of 8.00,the atomic radius difference decreases with the increase of Fe content,and the volume fraction of FCC phase gradually increases,indicating that Fe is a stable element of FCC phase in Al Co Co Cr_0.5Fe_xNi_2.5 HEAs.The increase of Fe content also leads to the gradual disappearance of the ordered FCC phase(L1₂),and the hardness of the alloy decreases with the increase of the volume fraction of FCC phase(or the decrease of the volume fraction of BCC phase).At the same time,TEM confirmed the K-S orientation relationship between the BCC phase and the FCC phase in the high entropy alloy of AlCoCr_0.5Fex Ni_2.5.3.After cryogenic rolling and annealing of FCC single-phase Al_0.25CoCrFeNi alloy,it is found that the precipitation of B2 phase occurs after recrystallization,and the B2 phase particles tend to gather in the grain of the recrystallized grain.It is found that recrystallization annealing has an obvious softening effect on the alloy after cryogenic rolling,a good match of strength and plasticity can be achieved by adjusting the recrystallization and second phase precipitation of the alloy through deformation+annealing.4.After rolling and annealing,it is found that the cold-rolled double-phase AlCoCr_0.5Fex Ni_2.5 high entropy alloy has a phase transition from FCC to BCC after annealing,which changes the phase composition and distribution in the alloy,and produces recrystallization and annealing twins.It is an important way to regulate the mechanical properties of alloy.