Study On Microstructures And Thermal Processing Behavior Of AlCrCuFeNi High Entropy Alloys

High entropy alloys are defined as a new kind of solid solution alloy,which consists of five elements and the atomic percentage of each component is between 5%and 35%.This novel design framework,which is different from the traditional alloy,broadens the application and research scope of metallic materials.The unique structures and multicomponent strengthening effects gives HEAs outstanding physical,chemical and mechanical properties that traditional alloys can not compare,such as high hardness,high strength,super wear resistance,high low temperature toughness,excellent thermal stability and so on.High entropy alloy is considered as a new type of structural and functional materials for a wide range of potential applications.Therefore,the research on HEAs has high industrial application and promising academic value.In the present work,an equalmolar AlCrCuFeNi HEA was prepared and used as the research object.The influence of Mo with large atoms radius on crystal structure,microstructure and mechanical properties were discussed systematically and the variation of microstructure and properties with composition was clarified.Due to improving the integrated performance,a 3.5kg alloy was fabricated by vacuum levitation melting and the effect of heat treatment on the microstructure and properties of the alloy was studied in detail.Bases on the isothermal compression tests,the hot deformation and dynamic recrystallization behavior of the alloy were investigated.In addition,a new kind of non-equal atomic Al0.4CoFeNiTi0.6 HEA was designed and prepared,and the thermal stability of the alloy was discussed.The main conclusions of the research are as follows:(1)The AlCrCuFeNi HEA exhibits simple BCC and FCC crystal structures where the content of BCC phases is higher than that of FCC phases.In AlCrCuFeNiMox alloys,the addition of Mo mainly dissolved into the CrFe rich BCC solid solution phase and increased the lattice constant of BCC phase.When Mo content is more than 0.3,the content of Mo reaches its limit and the Mo rich σ phase is precipitated.To a certain extent,the addition of Mo slows down the segregation of Cu at grain boundary.(2)The 3.5kg AlCrCuFeNi(at.%)HEA prepared via magnetic suspension melting was composed of matrix,petal-shaped structure and Cu enrichment region.This three different phases are confirmed to be the ordered B2,disordered A2 and FCC phase,respectively.Following homogenized at 1000 ℃ for 4h,the diffraction peaks of Al0.4Fe0.6 was detected,indicating that new phases have been precipitated from the BCC matrix.The atomic percentage of Cu content increases from 50.69 to 57.17 at.%at grain boundaries illustrate that homogenization treatment intensifies the element segregation in the copper segregation area.The as-cast alloy exhibits super Vickers hardness of 762 HV which is higher than that reported by others.(3)The rheological stress of the investigated AlCrCuFeNi HEA increases rapidly with increasing strain rate,and softens to a relatively stable state after reaching the peak value,showing typical dynamic recrystallization characteristics.Based on the high temperature compression test,the Arrhenius constitutive equation model established in the temperature range of 900～1050 ℃ and in the strain rate range of 0.001～1s-1 by the true stress can predict the flow behavior of the alloy well during high temperature deformation.(4)The processing map base on Prasad criterion indicate that the flow instability zones for AlCrCuFeNi alloy are in the region of 900～920 ℃/10-0.75～1s-1 and 1000～1050 ℃/10-0.5～1s-1,which should be avoided during TMP.During hot deformation,the dynamic recrystallization activation energy is 199.129kJ/mol.The alloy have stronger texture along<111>direction when(?)=0.001s-1 at 1050℃ with different strain rate,and the grains increases along<001>and<101>directions with the increase of strain rate.(5)The results of ΔSmix,ΔHmix,δ,Q and VEC for Al0.4CoFeNiTi0.6 HEA are calculated to be 12.92 J·K-1mol-1,-19.7 kJ/mol,6.27%,1.1 and 7.65,respectively.According to the requirements of forming solid solution phase in high entropy alloy,it can be predicted that the alloy can form simple BCC+FCC structure.(6)The Al0.4CoFeNiTi0.6 HEA project simple BCC and FCC phases which were further confirmed to be B2 and L12 structures.SEM-EDS results indicate that the FCC structure is in rich of Fe,Co and Ni while the BCC structure is in rich of Al and Ti.Following annealing treatment,a large number of shortly rod-like phases are precipitated from the ordered BCC phase.The precipitates are rich in Fe,Ni and depleted in Al and finally exhibit FCC structure,which makes the main phase of the alloy changed from BCC to FCC phase.(7)The yield strength,fracture strength and plastic strain of the as-cast Al0.4CoFeNiTi0.6 alloy are 2085 MPa,2136 MPa and 10.3%,respectively.Following annealing treatment,the annealed alloy projects a decrease of～33%in yield strength and a decrease of～8%in fracture strength.However,the plastic strain is increased from 10.3 to 21%.The hardness increased from 467 HV to 548 HV,indicating that the alloy exhibits excellent temper-resistant softening properties.