Microstructure Evolution And Properties Of Al-Cr-Fe-Ni-M Eutectic High-entropy Alloys

High entropy alloys（HEAs）have attracted a lot of attention because of their excellent mechanical properties,good corrosion resistance and excellent resistance to high temperature oxidation.It has been found that FCC single-phase alloys have excellent plasticity but low strength,and BCC single-phase alloys have excellent strength but poor plastic toughness.The proposed eutectic high-entropy alloys provide new ideas and methods for designing alloys with both strength and toughness,which are of great practical significance.Therefore,in this paper,we designed Al-Cr-Fe-Ni-M eutectic high-entropy alloy,added Mo,Nb and Cu elements to the Al-Cr-Fe-Ni alloy,and systematically studied the effects of the added elements on the phase composition,microstructure evolution and mechanical properties of the Al-Cr-Fe-Ni high-entropy alloy,and explained the variation of the alloy microstructure and properties with the composition.Firstly,Al Cr Fe₂Ni Cu Mo_x high-entropy alloy and Al Cr Fe₂Ni₂（Mo Nb）_x high-entropy alloy were designed and prepared in this paper.The composition of the Al Cr Fe₂Ni Cu Mo_xeutectic high-entropy alloy is BCC+B2+FCC+σphase,and the alloy shows a typical dendritic morphology,in which the dendritic region consists of BCC phase andσphase,and the interdendritic region is irregularly eutectic,consisting of ordered B2 phase and FCC phase.The Vickers hardness is 488 HV,the compressive fracture strength is 2030MPa,the yield strength is 1462 MPa,and the plastic strain limit is 18.18%.However,the addition of more Mo elements leads to the separation of the BCC solid solution phase and the ordered B2 phase in the alloy,and the excess Mo elements lead to the precipitation of theσphase on the BCC solid solution phase in the alloy,which deteriorates the room temperature mechanical properties of the alloy.Theσ-phase precipitation caused by the addition of Mo elements alone deteriorates the plasticity of the alloy,so the effect of the synergistic effect of Mo and Nb elements on the Al Cr Fe₂Ni₂ high-entropy alloy was further investigated.The Al Cr Fe₂Ni₂（Mo Nb）_x alloy mainly consists of FCC+BCC+B2 phase.When the content of Mo and Nb elements exceeds x=0.1,Fe₂Nb-type Laves phase is formed in the alloy,and the microstructure of the alloy is converted from multiphase grains to dendrite morphology,in which the dendrite region is composed of A2 and B2 phases produced by spinodal decomposition,and the interdendrite region is composed of the lamellar eutectic structure of FCC and Laves phases,and the eutectic reaction can be expressed as L→Laves+FCC.With the increase of Mo and Nb element content from 0.1 to 0.7,the room temperature compressive yield strength of the alloy increases from 878 MPa to 1549 MPa,while the plastic strain limit decreases from 43.7%to 8.6%.the Al Cr Fe₂Ni₂（Mo Nb）_0.1 alloy has a good balance of strength and plasticity,and the hardness and Young’s modulus of each phase in the alloy increase significantly,and the wear resistance of the alloy has been greatly improved.Secondly,considering the room-temperature and high-temperature mechanical properties of the alloy,the alloy preparation scheme was improved,and the bulk Al Cr Fe₂Ni₂（Mo Nb）_0.2 high-entropy alloy was prepared by induction melting technique,and the effects of the annealing heat treatment process on the phase composition,microstructure and room-temperature mechanical properties of the alloy were investigated in detail.The phase composition of the bulk Al Cr Fe₂Ni₂（Mo Nb）_0.2 alloy is B2+BCC+FCC+Laves phase,and the histomorphology of its different regions varies significantly,and the cooling rate has a great influence on the solidification process of the alloy.High-temperature annealing does not change the composition phase type of the alloy,but can change the distribution and volume fraction of the Laves phase by increasing the element diffusion rate.The alloy has excellent resistance to annealing softening and the fracture strength of the alloy increases from 2287.43 MPa to 2392.79 MPa and the plasticity increases by 4.7%after annealing treatment at 1200°C.This indicates that the annealing heat treatment process contributes to the improvement of the overall mechanical properties of the alloy and the alloy can also show excellent mechanical properties at high temperatures.The stress-strain curves of Al Cr Fe₂Ni₂（Mo Nb）_0.2 high-entropy alloy showed typical dynamic recrystallization characteristics under different thermal processing parameters.The Arrhenius-type intrinsic equation of the Al Cr Fe₂Ni₂（Mo Nb）_0.2 high-entropy alloy under the thermal deformation test parameters was constructed by using a linear regression method to analyze the hot compression test data.This equation provides a theoretical way to predict the flow stress under different processing conditions,while the combination with the DMM model allows the hot working diagram of the alloy to be drawn.When the true strain is 0.6,the optimal thermal processing range is T=1050°C to 1100°C andε?=0.0037to 0.031 s-1,in which the power dissipation rate can reach 57%to 60%.During the thermal deformation process,the thermal activation energy of dynamic recrystallization of the alloy can reach 333.16 k J/mol.At 1050°C,with the increase of deformation temperature and the decrease of strain rate,the weave strength of the alloy gradually weakens,and dynamic recrystallization grains with different deformation weave orientation precipitate at the grain boundaries,and the dynamic recrystallization phenomenon is gradually obvious.The decrease of strain rate and the increase of deformation temperature can enhance both the continuous and uncontinuous dynamic recrystallization processes of the alloy,further improving the high temperature deformation performance of the alloy.