Preparation and Properties of B₄C/W/AZ91 Composites with High Boron Content AITranslate

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Basic Information:

DOI：10.13373/j.cnki.cjrm.XY19050032

Chinese Library Classification Number:TG146.2

Citation Information:

Magnesium matrix composites have good application prospects in astronautics, weapons and other fields because of their light weight, high specific strength and high specific stiffness. The commercial B₄C/Al sheets with B₄C content of about 30% (volume fraction) has been widely used in nuclear power plants both domestically and internationally currently due to their excellent neutron absorption performance. In order to achieve desirable shielding performance of materials for fast neutrons and take into account certain gamma ray shielding performance, B₄C/W/AZ91 composites with high boron content were prepared by powder metallurgy method. Monte Carlo N Particle Transport Code (MCNP) program was used to calculate the shielding performances of B₄C/AZ91 composite material with B₄C content of 30% and 60% for Cf-252 source neutrons. The simulating neutrons was a point source, and the filling medium around the sample was air. The results of MCNP simulations showed that the fluence attenuation ratio of B₄C/AZ91 composite with 60% B₄C content for Cf-252 source neutrons was 8.3 at a thickness of 10 cm. The shielding performance of it was significantly higher than that of B₄C/AZ91 composite with 30% B₄C content, which had a fluence attenuation ratio of 5.3. X-COM program was used to calculate the shielding performance of B₄C/AZ91 composite with B₄C content of 59% and B₄C/W/AZ91 composite with B₄C content of 59% and W content of 1% for γ-ray whose energy was ranging from 0.01 to 10 MeV. 10 cm-thick AZB59W1 sample can completely shield X-rays in the energy range from 0.01 to 0.6 MeV and it had lower transmittance of γ-rays in the energy range from 0.05 to 10 MeV, which showed better shielding performance compared to the AZB59 sample without W elements. In this study, AZ91 magnesium alloy powder was chosen as the matrix of the composites, B₄C powder and W powder as the shielding components. The volume fraction of B₄C in the composites was range from 59% to 63%, and the volume fraction of W were zero and 1%. Hot pressing and hot isostatic pressing processes were used to realize the sintering of mixed powders, respectively. The sintering temperature of hot pressing sintering was 430~520 ℃. The hot isostatic pressing sintering temperature was 430 ℃. The results showed that the density of B₄C/W/AZ91 composite increased and then decreased with the increase of hot-pressing temperature. The density of the material reached the maximum at the hot pressing temperature of 500 ℃, which was 98.8% of the theoretical density. From the microstructure morphology, it could be seen that when the hot pressing temperature was 430 ℃, AZ91 alloy matrix was granular, and the interface between the matrix and the shielding component particles was relatively loose. When the hot pressing temperature was between 480 and 500 ℃, the interface structure was dense. The density of the material tended to decrease gradually with the increase of the volume percentage of shielding components, and the density of the composite material could reach 99% of the theoretical density when the volume percentage of shielding components is 60%. The densities of B₄C/W/AZ91 composite prepared by hot isostatic pressing were close to 100%, and the flexural strengths reached 465~480 MPa, showing good densities and mechanical properties. From the microstructure, it could be seen that after hot isostatic pressing at 430 ℃, the interface between AZ91 alloy matrix and the shielding component particles was well bonded and dense, and the interface between different particles was clear. During the cooling process of the material after hot isostatic sintering, Mg-Al solid solution and Al₁₂Mg₁₇ intermetallic compounds were formed in the AZ91 alloy region. As the volume percentage of B₄C in the material was as high as 59%, the fracture morphology was mainly shown as a smooth section with a large number of exposed B₄C particles and craters in AZ91 magnesium alloy matrix after B₄C particles were dislodged. According to energy dispersive spectrometer (EDS) analysis results, there were a large amount of Mg element and a small amount of Al element distributed in the matrix showing the Mg-Al solid solution. However, there were also some of significant Al element peak, where the content of Mg and Al elements significantly deviated from the ratio of Mg-Al elements in AZ91 alloy composition. According to Al-Mg binary phase diagram, this peak might correspond to Al₁₂Mg₁₇ intermetallic compound phase precipitated in the matrix. According to X-ray diffraction (XRD) results, the main phases in AZB59 composite material were Mg, B₄C and a small amount of Al₁₂Mg₁₇, while the main phases in AZB59W1 composite material were Mg, B₄C and W. AZB59W1 composite with a thickness of 32.6 mm was prepared using B₄C rich in ¹⁰B (the abundance of ¹⁰B in boron element was 99.2%) according to the optimized process. The fast neutron shielding performance of ¹⁰B-riched AZB59W1 composite was tested. When the thickness of the ¹⁰B-riched AZB59W1 composite was 32.6 mm, the neutron flux attenuation ratio of the composite to Cf-252 neutron source was 1.95. The simulated neutron shielding performance of ¹⁰B-riched AZB59W1 composite was calculated under the same condition using MCNP program. The calculated result showed that the neutron flux attenuation ratio for Cf-252 source neutron was 2.22, which was basically consistent with the testing values.