| The refractory high-entropy alloys(RHEAs)mainly composed of refractory elements have great application potential in aerospace,nuclear power,gas turbine and other fields due to their excellent high-temperature mechanical properties,corrosion resistance and radiation resistance.In this paper,combined with the cladding service environment in the new nuclear power fuel system,the composition design and preparation of refractory high-entropy alloys are carried out,and the mechanical properties and oxidation mechanism of refractory high-entropy alloys under high temperature conditions are studied.These studies provide experimental and theoretical basis for the application of refractory high-entropy alloys,and have certain application value and theoretical significance.In this paper,Mo,Nb,Ta,Ti and V are screened out as the main constituent elements of high-entropy alloys based on the thermal neutron absorption cross-section and the activation of elements.The effects of Mo and Ta element content changes on the phase composition were investigated by empirical parameters and phase diagram calculation(CALPHAD)method.Pseudo-binary phase diagram and equilibrium phase diagram show that increasing Mo content will promote the precipitation of BCC#2 phase due to the increase of composition segregation,and increasing Ta content is beneficial to the formation of topologically close-packed phase.The results calculated by the Scheil model show that the alloy forms a single-phase BCC solid solution with dendritic morphology through non-equilibrium solidification.The effect of HIP treatment on the microstructure and mechanical properties of the alloy was studied.It was found that after HIP treatment,the distribution of elements was more uniform,and the defects such as pores in the as-cast alloy were suppressed.In the alloy with high Mo content,the TiN0.42 phase will be precipitated near the grain boundary by the precipitation reaction,resulting in the reduction of alloy plasticity.The effect of Al on the microstructure and mechanical properties of MoNbTaTiV high-entropy alloys was investigated.During the process of increasing the Al atomic ratio from 0 to 1,the alloys always maintained a single-phase BCC structure,the lattice constant decreased from 3.211 ? to 3.199 ?,the yield strength at room temperature increased from 1228 MPa to 1391 MPa,and the hardness increased from 421 Hv to 575 Hv.The improvement in properties were attributed to the solid solution strengthening of Al element,which depends on chemical interaction and electrical interaction.The Al-containing refractory high-entropy alloy exhibits serration behavior during the deformation process at high temperature.and the types of serrations change with the increase of Al content and temperature.The Al content has a power function relationship with the critical strain in serration behavior.A series of refractory high-entropy alloy composites were successfully obtained by introducing Si element into MoNbTaTiV alloy.These composites are hypoeutectic composites composed of BCC phase and Ti5Si3 silicide phase.The yield strength at room temperature reaches a maximum value of 2659 MPa at x=0.6,and then decreases due to the increase of the fraction of eutectic area.The yield strength of the alloys all increased with the increase of Si content at 800℃,1000℃ and 1200℃.When the Si content reaches x=1,a layered eutectic structure with orientation is obtained,which greatly improves the high temperature strength of the alloy.The strengthening mechanism including solid solution strengthening,dispersion strengthening of silicide particles and interfacial strengthening.The alloys have low plasticity,and the fractures below 1000℃ are all brittle fractures.However,during the compression at 1200℃,the plasticity of the alloy is greatly improved because the silicide participates in the deformation through dynamic recovery and dynamic recrystallization.The dynamic recrystallization mechanism of BCC phase includes discontinuous and continuous dynamic recrystallization mechanism.The dynamic recrystallization mechanisms of Ti5Si3 phase includes three mechanisms:discontinuous dynamic recrystallization,continuous dynamic recrystallization and geometric dynamic recrystallization,and the latter two mechanisms are dominant.The oxidation behavior and microstructure of AlMoNbTaTiV,CrMoNbTaTiV,SiMoNbTaTiV high-entropy alloys in air atmosphere were investigated at 700℃,900℃ and 1100℃.It is found that the AlMoNbTaTiV alloy can form a stable oxide protective layer composed of complex oxides,such as AlNbO4 and Al2TiO5 at 700℃and 1100℃ oxidation.The oxidation type is a selective oxidation.At 900℃,the protective oxide layer on the surface is warped and cracked due to the expansion of the oxides,and the protective effect is lost.At 1100℃,a stable and loose oxide layer is formed,which effectively slows down the oxidation rate.The CrMoNbTaTiV alloy performs well at the oxidation under 700℃ and 900℃.The dense oxide layer composed of complex oxides such as Cr2TiO5,CrTiO3,CrNbO4 and CrTaO4 near the matrix/oxide interface can effectively block the migration of cations.However,at a high temperature of 1100℃,due to excessive internal stress,the oxide layer is cracked,and the oxidation rate is accelerated,resulting in catastrophic oxidation.Due to the hypoeutectic microstructure of SiMoNbTaTiV alloy,oxygen can rapidly diffuse along the grain boundary to accelerate the oxidation. |
PDF Full Text Request