Design and Performance of High Strength and Toughness(B4C+Gd)/Al Neutron Shielding Composites

Author:Xu Zhong Guo

Supervisor:jiang long tao


Degree Year:2018





With the rapid development of nuclear power technology,more stringent requirements are imposed on spent fuel storage structural materials.In the 13th Five-Year Plan for the Development of Nuclear Industry,China predicts that the installed scale of nuclear power will reach 120 million to 150 million kilowatts by 2030,and the storage of spent nuclear fuel in nuclear power stations will face the upper limit by 2030.Therefore,how to further improve the high-density storage of spent fuel is a major challenge for the structural materials of spent fuel storage in China.In order to solve the problem that the neutron shielding performance and mechanical properties of spent fuel storage structural materials are mutually restricted,the content of B4C and Gd in the composite system was systematically designed and(15%B4C+1%Gd)/Al composites were successfully prepared by atmospheric hot pressing and hot extrusion.The types,the defect forms and the microstructure evolution of Gd-containing phases during thermal deformation and their effects on mechanical properties and neutron shielding properties were systematically studied,and the strengthening mechanism was studied.In this paper,the upper limit of Equivalent B areal density(EBAD)in neutron shielding materials has been found,and the relationship between material density,thickness and equivalent B content have been established.With the thermal neutron shielding coefficient of 99%as the design standard,the upper limit of EBAD was determined to be 0.1105g/cm2,and the content relationship of B4C and Gd satisfying the thermal neutron shielding performance requirement was determined.Based on the strengthening and toughness design of the composite,the volume fraction of B4C was determined to be 15%.Comparing possible Gd-containing phases and their properties in composite systems,it was determined that the Al-Gd-O phase with higher mechanical properties was formed in-situ in the composite by introducing Gd2O3 and Al2O3 into the composite system,and the volume fraction of Gd was determined to be 1%.The composites were prepared by oxygen-containing atmosphere hot pressing sintering process and hot extrusion process,and the preparation process of the composites was systematically studied.The research on the ball-milling process have shown that the ball milling process of 300r/min-1h in the oxygen-containing atmospheric environment can make the powder mix evenly and Gd powder was oxidized to Gd2O3,which meets the design requirements of the material system.The hot pressing sintering process of(15%B4C+1%Gd)/pureAl was 660℃-2.5h,and the hot pressing sintering process of(15%B4C+1%Gd)/6061Al was 640℃-2.5h.In combination with the hot extrusion process,composites having a density of>99.5%can be obtained.Hot extrusion can improve the interfacial bonding strength of Gd-containing phases and Al matrix,the uniformity of distribution of Gd-containing phases,refine Al crystallites and increase the density.The tensile strength of the T6 state(15%B4C+1%Gd)/6061Al composite after hot extrusion was 409 MPa,and the elongation increased from 3.8%to 5.6%.The microstructure and mechanical properties of(B4C+Gd)/Al composites during hot rolling were investigated.The study on B4C and Al matrix shows that the B4C size decreased gradually with the increase of hot rolling deformation,and the grain size of Al matrix decreased from 1.3μm to 0.7μm.The Gd-containing phase formed in-situ in the composites was mainly Al5Gd3O12,in addition to the formation of a very small amount of Al11GdO18,Al2Gd4O9 and Al2Gd3 phases,and the main internal defects were stacking faults.As the amount of hot rolling deformation increased,the size of the Gd-containing phases decreased,and when the amount of deformation was 79.3%,the size of the Gd-containing phase was about 0.5μm.The results of thermodynamic analysis showed that the Gibbs free energy of Al5Gd3O12 phase was lower than that of other kinds of Gd-containing phases.During the hot rolling process,the Al5Gd3O12 phase did not undergo phase transformation due to lower than the phase transition temperature.With the increase of hot rolling deformation,the yield and tensile strength of the composites did not change much.When the hot rolling deformation reached 79.3%,the agglomeration of the Gd-containing phases were basically disappeared.Therefore,the strength of the composites was improved.The mechanical properties of Gd-containing composites were always higher than those of composites without Gd.Comparing the yield strength of the four composites prepared in this paper,the yield strength of Gd-containing composites under different hot rolling deformations was always 30-40 MPa higher than that of composites without Gd.The strengthening effect was manifested by dislocation strengthening due to the mismatch of elastic modulus and the mismatch of thermal expansion coefficients between the Gd-containing phases and the Al matrix,Orowan strengthening,and load transfer enhancement.The neutron shielding properties of the composites were investigated.The thermal neutron shielding performance of the(15%B4C+1%Gd)/Al composite was much higher than that of the 15%B4C/Al composite,which was comparable to the 30%B4C/Al composite.The Am-Be neutron source shielding test results showed that the linear attenuation coefficient of(15%B4C+1%Gd)/Al composite was 0.110cm-1,which was higher than 15%B4C/Al composite.The thermal deformation improved the neutron shielding performance of the(15%B4C+1%Gd)/Al composite due to the improved distribution uniformity of the Gd-containing phases and the refinement of the size of the Gd-containing phase,and the thermal neutron absorption cross section was improved to 25.1 cm-1,the linear attenuation coefficient was increased to 0.121 cm-1.At the same time,the function A(x1,x2)indicating the overall uniformity of B4C and Gd-containing phases in the composites was introduced into the formula of EBAD,and finally introduced into the formula of the thermal neutron shielding coefficient a,and the modified thermal neutron shielding coefficient formula was:α=1-exp(-42.3×A(x1,x2)×EBAD).