The Fabrication,Microstructure And Properties Of B₄C/Al Neutron Absorber Composite Plates

Due to the large thermal neutron absorption cross section of ?10?^B isotope and its low cost,boron carbide?B₄C?is widely used as a neutron absorber in nuclear power plants.However,a thin B₄ C plate is difficult to be fabricated due to its brittleness and poor toughness.Therefore,B₄ C particles are usually added into an aluminum alloy matrix to fabricate structural and functional materials.Usually the higher the content of the B₄ C in the fabricated plate,the stronger the thermal neutron absorption capacity of the plate.However,high content of B₄ C particles in the plate can cause torn-out of the aluminum alloy matrix;increase the particle/matrix interface;and deteriorate the plastic deformation between the particles and the matrix.In this paper,?10?^B areal density??10?^BAD?was proposed to evaluate the neutron shielding performance based on MCNP,and the direct relationship between ?10?^B areal density??10?^BAD?of the material and its neutron absorption ratio was obtained.The optimum thickness with good neutron shielding capability of a B₄C/6061 Al composite plate with 30 wt.% B₄ C particle was found.Furthermore,the hot deformation behaviors of 30 wt.% B₄C/6061 Al neutron absorber composites were investigated through isothermal compression tests.The corresponding constitutive equations and processing maps were proposed.Along with the Deform software,the extrusion die and rolling process of the composite material were optimized,and its influence on the forming performance was studied.Finally,microstructure of the fabricated composite plate was characterized;the anisotropy of tensile and fatigue properties,and the corresponding fracture mechanism were further analyzed.Conclusions of above studies are listed as follows:?1?A direct relationship between ?10?^BAD value?x?and neutron absorption ratio?y?of the composite plate can be mathematically expressed as: y =-1.15117exp[-x/0.00669] +0.99963.It is found that when the value of ?10?^BAD of the plate is over 0.034 g/cm2,its neutron shielding performance can reach 99%,which means that the fabricated B₄C/6061A1 composite plate can shield 99% of neutron radiation.The corresponding thickness of the 30 wt.% B₄C/6061A1 plate has to be over 3 mm in order to meet the design requirements.?2?Optimum hot working conditions for B₄C/6061 Al composite plates are found to be at 760–803 K/0.01–0.05 s^-1 with a maximum dissipation efficiency of 44.7%.Softening mechanisms?DRX and DRY?can neutralize the hardening mechanisms?generation and accumulation of dislocations?of the plastic deformation zones?PDZs?.During hot deformation,material instabilities are typically caused due to void formation,particle de-bonding,and matrix cracking.?3?Problems like small plastic deformation and cracking formation in B₄C/6061 Al composite plates with high B₄ C content during the extrusion process can be solved through optimizing the extrusion die design by using the Deform software.The die cavity can be optimally shaped to avoid the cracking formation at the edge of the plate during extrusion.The edge cracking problem in rolling due to the tensile stress can be prevented by using a multi-pass and small deformation rolling process.An aluminum-based neutron absorption plate with 30 wt.% B₄ C particle having preferred properties can be successfully fabricated,up to a size of 4800 mm × 190 mm × 3 mm.?4?The tensile strength of longitudinal specimens of B₄C/6061 Al composite is higher than that of transverse specimens.EBSD analysis showed that the anisotropy of mechanical properties can be attributed to the distribution of B₄ C particles along the rolling direction.The Al matrix grains of the composites were recombined under the influence of PDZ,led the grain with an average grain size of 700 nm tends to be randomly distributed.Strengthening mechanism analysis indicates that the grain refinement can provide more strengthening effect.The actual strength to be lower than the theoretical calculation strength,this can be attribute to the present of B₄ C particle fracture caused by secondary processing.?5?The tensile fatigue strength of B₄C/6061 Al composite sheet is better than that of matrix aluminum alloy.The fatigue strength of longitudinal specimen is 162 MPa and the fatigue strength of transverse specimen is 119 MPa.The SEM analysis of fatigue fracture shows that the area of B₄ C particles exposed on the fracture increases with the propagation of fatigue crack.?6?The relationship between the crack growth strength factor??K?and the corresponding fracture of the B₄C/6061 Al composite plate shows that the crack propagation rate in an E-D specimen is lower than that in a T-D specimen at low ?K levels(?K ? 9 MPa·m^1/2).At high ?K levels(?K ? 9 MPa·m^1/2),fatigue crack growth rates in E-D and T-D specimens are found to be basically at the same level.The propagation of the crack is found to be greatly dependent on the deflection angle of the crack,the size of the plastic zone at the crack tip,the crack driving force.