Design And Properties Of Body-centered Cubic(BCC) High-Entropy Alloys For Nuclear Energy

Different from the design idea of the traditional alloy that uses one or two elements as the main element,the high-entropy alloy(HEA)usually consists of four or more elements,and the atomic ratio of each element is not less than 5.This new alloy design concept has broken and gone beyond the hedge and obstacle of the previous alloy design and broadens the scope of alloy research.Due to the high entropy effect,slow diffusion effect,lattice distortion effect and "cocktail" effect unique to HEAs,it is usually characterized by high strength,high hardness,wear resistance,corrosion resistance and excellent stability,so that the HEA has great theoretical research value and wide application potential.The classification of HEAs and medium-entropy alloys(MEAs)is reclassified by entropy value,which is a supplement to the previous definition and classification of alloys.The design concept of HEA was used to develop a new type of nuclear energy body-centered cubic(BCC)HEA.Then,the low-activation HEA VCrFeTaxWx(x=0.1,0.2,0.3,0.4 and 1)and the depleted uranium HEA U-Nb-Zr-X(Ti/Mo)were prepared by arc melting,aiming at investigating the influence of composition changes on alloy structure and properties.(1)First of all,the alloy design by using the law of HEA phase formation,to study the phase composition,microstructure,mechanical properties,corrosion resistance,thermal stability and thermal deformation of the low-activation VCrFeTaxWx system behavior,as well as the influence of Ta and W content changes on alloy structure and properties.The study found that the prediction of the alloy phase composition through the formation law of HEA phases was consistent with the experimental structure.When the Ta and W contents were at a molar ratio of 0.1 and 0.2,the alloy was mainly composed of the BCC phase.As the Ta and W contents increased,the BCC+Laves structure changed to 2BCC+Laves structure.The results of heat treatment and thermal analysis(DSC)showed that VCrFeTa0.1W0.1 and VCrFeTa0.2W0.2 alloys had good structural stability,and no phase transition reaction occured within 1400?.As the content of Ta and W increased,the microhardness increased and the plasticity decreased.The strength increased first and then decreased with the increase of Ta and W content.Among them,the compressive yield strength of VCrFeTa0.1W0.1 and VCrFeTa0.2W0.2 alloys was 1341 MPa and 1742 MPa,respectively,and their plasticity is 42.2%and 35.7%,showing excellent mechanical properties.The corrosion performance of VCrFeTa0.1W0.1 and VCrFeTa0.2W0.2 alloys in 3.5 wt.%NaCl solution found that VCrFeTa0.1W0.1 showed excellent corrosion resistance,and its corrosion current density and pitting potential were 0.066 ?A/cm2 and 1.058 VSCE.Research on the dynamic high-temperature compression deformation properties of VCrFeTa0.1W0.1 and VCrFeTa0.2W0.2 alloys at 600?1000? found that both alloys exhibit high resistance to high temperature softening at 600 and 800?,and VCrFeTa0.1W0.1 and VCrFeTa0.2W0.2 alloys had yield strengths of 1019 MPa and 1033 MPa at 800?,respectively,and had good plasticity.The linear expansion coefficient of the two alloys within 1000? was lower than 12×10-6/?.The sawtooth rheology of the VCrFeTa0.1W0.1 alloy with plastic instability in the as-cast compression process was also studied and discussed.The hardness value of this alloy reached 596 HV0.2 at 800? for 300 min,the yield strength reached 1612 MPa and the excellent elongation was 31%.This high-temperature hardening ability was very advantageous for its service under a high-temperature environment.The sawtooth rheological behavior of VCrFeTa0.1W0.1 alloy was discussed through the critical strain and the average stress drop amplitude.The stress drop amplitude of the alloy was related to the content and stability of the solute atomic air mass.High-temperature oxidation experiments were carried out on the VCrFeTa0.2W0.2 alloy with high oxidation resistance during the hot deformation process.The alloy belonged to the oxidation resistance class at 650?,the secondary oxidation resistance at 700 and 750?,the weak oxidation resistance at 800?,and the non-oxidation resistance at 900?.The oxidation activation energy of this alloy was 197.5 KJ/mol,which was close to that of Ni-based superalloys.The oxidation mechanism was changed from direct contact reaction control to diffusion-controlled oxidation mechanism from the oxide film to the diffusion layer.The effect of electron beam modification on the structure,phase composition,hardness and corrosion resistance of VCrFeTa0.2W0.2 high-entropy alloy was found.After electron beam bombardment,the lattice constant of the alloy decreased and the degree of grain refinement significant,the microhardness was increased,and the corrosion resistance was increased by about 3 times.From the perspective of entropy and surface energy,the improvement of alloy properties after electron beam modification was discussed.It was discovered that the increased solid solubility,the refinement of grains and the reduction of segregation were the main reasons for the increased hardness and enhanced corrosion resistance.The Refined Composite Multiscale Entropy method was used to reveal the relationship between the corrosion current fluctuation and corrosion resistance of the alloy after electron beam modification.(2)The depleted uranium HEA was designed by using the law of HEA phase formation.Based on UNb0.5Zr0.5,the design and experiment of the depleted uranium high-entropy alloy were carried out by adding different amounts of Ti and Mo to the base alloy.Combining theory and experiment,we finally succeeded in designing a UNb0.5Zr0.5Ti0.2Mo0.2 depleted uranium HEA with excellent comprehensive properties of yield strength and hardness of 1157 MPa and 297 HB,respectively.