Mechanical Behavior,Mechanism Of High Strength And High Toughness Of B2-type High-entropy Alloys

(Ti,Zr,Hf)（Fe,Co,Ni,Cu）series high-entropy alloys have attracted more and more attention from researchers in recent years because of their excellent properties such as shape memory effect,high temperature stability and high yield strength.Despite its excellent high-temperature mechanical properties,the room temperature plastic deformation ability of high-entropy alloys in this system is generally poor,which is not conducive to its industrial application.It is necessary to investigate how to prepare an alloy that combines room temperature strength and plasticity and can serve in high-temperature environments.In this thesis,amorphous alloys,B2-type amorphous composites and B2-type bulk high-entropy alloys were prepared by vacuum arc melting and vacuum rapid cooling technology based on Ti Zr Hf Co Ni Cu near-isoatomic ratio high entropy alloy.A series of experimental test methods such as SEM,TEM,XRD,DSC and other experimental test methods characterized the microstructure and mechanical properties of these three types of materials.The thermodynamic and mechanical properties of Ti Zr Hf Co Ni Cu amorphous alloy and amorphous composites were investigated,the formation conditions of B2 phase of high-entropy alloy were discussed,and the mechanical mechanism of amorphous phase and B2-type intermetallic compound phase was explored.The strengthening and toughening mechanism of martensite phase transition,deformation twin and dislocation slip on high-entropy alloys was revealed.The research work mainly includes the following contents:（1）The microstructure and mechanical properties of amorphous phase and B2phase in high-entropy amorphous alloy and high-entropy amorphous composites were studied,and the martensitic phase transition process of B2→B33 was confirmed.It is found that with the precipitation of a small amount of B2 phase,the hardness,tensile strength and plasticity of B2 amorphous composites are improved compared with those of amorphous alloys.The enhancement of strength and hardness of amorphous composites is mainly related to lattice distortion at the phase interface and the diffusion distribution of B2 phase,while the improvement of plasticity is related to the plasticity induced by circular B2 phase by inhibiting cracking and promoting multiple distribution of shear bands and martensitic phase transition.（2）The microstructure of B2 bulk high-entropy alloy at room temperature and different annealing temperatures were studied,and its mechanical properties under room temperature and high temperature conditions were tested by compression experiments.The fracture morphology and microstructure after fracture were analyzed,and the strengthening mechanism of the alloy at room temperature was discussed.Compared with most existing high-entropy amorphous alloys,B2 and BCC phase high-entropy alloys,the alloy has better comprehensive mechanical properties.It is found that the alloy has good thermodynamic stability and maintains excellent mechanical properties at room temperature after water-cooled annealing.The influence of temperature on the mechanical properties of Ti Zr Hf Co Ni Cu high-entropy alloy was explored,and the alloy could still maintain the compressive strength of more than 1000MPa and the compressive plasticity of more than 20%at a high temperature of 873 K.（3）The microstructure and mechanical properties of Ti Zr Hf Co Ni Cu and Ti₂₅Zr₂₅Co_16.5Ni_16.5Cu₁₇ high-entropy alloys were comparatively investigated.The phase formation mechanism of B2 and BCC high-entropy alloys was explored according to the parameters of mixed enthalpy,mixed entropy,atomic size difference and valence electron concentration.It is found that Hf element may promote the transition of the phase structure of the alloy from BCC phase to B2 phase by affecting mixed entropy,atomic size difference and other parameters.The differences in mechanical properties of B2 and BCC high-entropy alloys are explained from the perspectives of lattice changes caused by atomic size differences,the structure of B2phase and BCC phase,thermodynamic parameter changes caused by entropy increase,and elemental solid solution.