Design, Preparation And Performance Research Of High-temperature Reinforced Aluminum-based Boron Carbide Materials

With the rapid development of nuclear power industry in China,the country will produce nearly 1,000 tons of spent fuel per year by 2020.In order to alleviate the pressure of increasing spent fuel on storage tank capacity,dry storage is the main development direction of spent fuel storage in the world.As a result,the spent fuel dry storage and transportation vessel has put forward higher performance requirements for the aluminum-based boron carbide?B₄C/Al?neutron absorption material as the key material.The main purpose is to change the B₄C_p/Al composite originally positioned as the functional material to the structural-functional integration material,so that its mechanical properties under high temperature service conditions are improved essentially.In this paper,a new type of structure-function integrated neutron absorption material has been systematically studied from the aspects of composition design and selection,powder metallurgy preparation,performance test and evaluation of B₄C_p/Al composite.Monte Carlo?MC?method was used to design the matrix composition of B₄C_p/Al composite and the effect of the second phase particle on the neutron absorption function.The mechanical properties of B₄C_p/Al composites were tested on a high temperature tensile testing machineto simulate the high temperature environment of dry storage and transportation container.The homogenization mechanism of B₄C_p/Al powders,the densification mechanism of semi-solid hot isostatic pressing?HIP?and the strengthening mechanism of nano-second phase on B₄C_p/Al composites were analyzed by scanning electron microscopy?SEM?and high resolution transmission electron microscopy?HRTEM?.The main results of this paper are as follows:?1?Based on the analysis of the working condition of the high temperature strengtheningneutron absorption material,the service environment model of the neutron absorption material was established.The influence of B₄C_p content,thickness of the shielding material and the second phase particles on the shielding performance of the composite were simulated by MCNP neutron transport software.The results indicated that when B₄CP content was in the range of 6wt%?12wt%and thickness was in the range of 6mm?15mm,the B₄CP content had a great influence on the thermal neutron shielding performance of the composites.Under the premise of 6061A1 alloy was selected as the matrix of the composite material,and the B₄CP content of the material was about 12wt.%,and the thickness was about 12mm,the composite can shield 99.9999%of the thermal neutron.The shielding effect of B₄C_p/Al composites on thermal neutrons was not linear.The first 1/3 thickness of B₄C_p/Al composites shielded about 99%of the thermal neutrons,and the second 2/3 thickness shielded less than 1%of the remaining thermal neutrons.The physical realization of the shielding function of the composite material can be given the minimum value by ¹⁰B surface density,and the design value of thermal neutron shielding rate can be guaranteed by the thickness change of ±0.5mm.The nano-Al₂O₃ particles(Al₂O_3np)had no obvious effect on thermal neutron shielding of composite materialswithin 1?15wt.%range.The radiation damage behavior of He and Li ions produced by ¹⁰₅B?n,??₃⁷Li nuclear reaction on B₄C_p and Al substrates was simulated by MC software.The results indicated that the higher incident energy of He ion,the range of He ion in B₄C_pwill more longer,and the shorter the range of Li ion in B₄C_p,In-situ damage of Al matrix caused by He and Li ions is much greater than that of B₄C_p.B₄C particle can be defined as three layers:semi-probabilistic sedimentary layer,relatively probabilistic sedimentary layer and total sedimentary core,and the binding capacity of He and Li particles in different deposits is quite different too.When the radius of B₄C_p particles is larger than 4.45?m,increasing the size of B₄C_p particles will be beneficial to reduce the radiation damage of B₄C_p/Al composites caused by secondary ions.?2?The research on the preparation of B₄C_p/Al neutron absorbing materials by powder metallurgy method was carried out.The homogenization distribution mechanism of B₄C_p and the densification mechanism of semi-solid hot isostatic pressing?HIP?during the preparation of Al/B₄C_pcomposites were also studied.The results indicated that with the increase of milling time,the laser particle size distribution spectrum of 6061A1 alloy powder and B₄C_p powder will had a "double peak"effect,which was mainly due to the flat "cake" in the process of ball milling tough 6061Al powder.The ball milling process can realize the full dispersion of boron carbide particles in 6061Al powder matrix.The mechanism was that the B₄C_p form obvious mosaic behavior on the aluminum "cake" after a long time of ball milling.Semi-solid HIP can densify B₄C_p/Al composites,and which densities were close to the theoretical densities,much better than those prepared by vacuum sintering at the same sintering temperature and time conditio ns,the main mechanism was that the6061Al liquid overflow in the semi-solid temperature region and fill the voids in the powder body.?3?The high temperature service temperature of neutron absorbing materials is taken as an experimental parameter.The mechanical properties of 606Al alloy and B₄C_p/Al neutron absorption material before and after adding Al₂O_3np particles at room temperature and high temperature were tested by high temperature tensile material testing machine.The results indicated thatif 6061 Al/12wt.%B₄C_p composites were prepared by nanoparticle strengthening method,the threshold value of the addition of Al₂O_3np particles was 2.5wt.%,then the tensile strength of nanoparticle reinforced B₄C_p/Al composites at room temperature was more than 300Mpa,and the yield strength was more than 240Mpa.The tensile strength and yield strength of 6061 Al/12wt.%B₄C_p composites were improved by about 40%and 46%respectively by the second phase particles of Al₂O_3np at 350 C.And which were been improveed by about 67%and 65%respectivelyT at 450?.The results showed that the Al₂O_3np can obviously improve the high temperature properties of 6061 Al/12wt.%B₄C_p composites,and the plasticity and toughness of the composites decreased either.?4?Scanning electron microscopy?SEM?and high resolution transmission electron microscopy?HRTEM with XDS and EELS?were used to study the microstructure and strengthening mechanism of Al/B₄C_p composite strengthened by Al₂O₃nP particles.The results indicated that the grain size of 6061Al matrix in 6061 Al/12wt.%B₄C_p composite was about 12pm,6061 Al/12wt.%B₄C_p+2.5wt.%Al₂O_3np composite was about 4?m.Micron B₄C particles and nano-scale Al₂O₃ particles inhibited the grain growth of 6061Al matrix,while Al₂O_3np inhibited the grain growth more obviously.Al₂O_3np were mainly in?-Al₂O₃ phase and pinned in the grain boundary segregation zone of 6061Al/12wt.%B₄C+2.5wt.%Al₂O_3np composite in discontinuous state.This kind of distribution can restrain the diffusion of alloying elements and grain boundary softening at high temperature,and change the fracture mode of composites at high temperature,so that the fine grain strengthening effect of composites can be effectively maintained at high temperature.