Studies Of Microstructure And Mechanical Properties Of B₄C/6061Al Composites And Welded Joints With High Volume Fractrion Of Reinforcement Particles

B₄C is widely used as a neutron absorber in nuclear power plants because it contains of boron isotope 10 B,which is characterized by high thermal neutron absorption cross section and low cost.However,B₄ C ceramic material is difficult to be deformed due to its poor plasticity and toughness.Therefore,B₄ C particles are usually added into metal matrix to fabricate the neutron absorption materials.For example,aluminum matrix composites with high B₄ C content are often used as neutron absorption materials for spent fuel storage racks in nuclear power plants.With the increase of B₄ C content in the aluminum matrix composites increasing,the thermal neutron absorption capability of the composite increase,but the composite's mechanical properties and processing performance get worse and threshold for material preparation also increases.Adding high-content B₄ C particles in the metal matrix can cause a series of problems including splitting effect of B₄ C particles on the matrix,poor wettability and coordination ability between metal matrix and B₄ C particles,and many defects at the ceramic particles/matrix interface.These will undoubtedly increase the difficulty of fabricating neutron absorption materials with high B₄ C content.The microstructure morphology and bonding strength of the ceramic particles/matrix interface have significant effects on the mechanical properties and deformation performance of the neutron absorber composite and its deformatiom performance as well.In this study,the neutron absorption performances of the B₄C/6061 Al composites with different B₄ C content were investigated.More specifically,Monte Carlo method was used to study the relationship among B₄ C particle content,sheet thickness and the neutron transmittance of B₄C/6061 Al composites,aiming at establishing a theoretical foundation for optimizing the parameters for the fabrication of B₄C/6061 Al composites which includes B₄ C particle concentration and the sheet thickness of B₄C/6061 Al composites.According to the requirements of high B₄ C particle concentration and its uniform distribution,powder metallurgy was employed to fabricate B₄C/6061 Al composites with high volume fraction B₄ C contents?20%-40%?.It includes high-energy ball milling,vacuum hot pressing and rolling.The distribution uniformity of the B₄ C particle;the particle/matrix interface bonding mechanism;and the strengthening mechanism of B₄ C particles to the metal matrix were studied.The dynamic impact properties of B₄C/6061 Al composites were analyzed using Hopkinson pressure bar,and the microstructures of the specimens after impact tests were observed and the impact damage mechanism was investigated.Friction stir welding?FSW?was adopted to weld B₄C/6061 Al composites.The microstructures and mechanical properties of the welding joints were investigated.The main results were summarized as follows:Neutron absorption ability of the B₄C/6061 Al composite plate was found to increases with the increase of the B₄ C particle content and the plate thickness.The content of the B₄ C particle reaches 30% and the plate thickness is 3 mm,neutron absorption ratio of the B₄C/6061 Al composite plate reaches almst 100%.The relationships between the neutron absorption ratio?y?and the plate thickness?x?for B₄C/6061 Al composites with different B₄ C contents can be given as: y?10%?=e-0.98763 x,y?20%?=e-1.96735 x,y?30%?=e-2.91003 x,y?40%?=e-3.81738 x.B₄C/6061 Al composites with different B₄ C contents were fabricated using high-energy ball milling,vacuum hot pressing and rolling.High-density composites were fabricated with a vacuum hot pressing temperature at 640 oC;sintering pressure at 120 MPa;and the holding time at 60 min.After multi-pass rolling,distribution uniformity of B₄ C particles was found to be improved.Under the action of rolling pressure,the micro-porosity and-cracks in the material decrease and the particle strengthening effect was enhanced.With the increase of B₄ C particle content in the B₄C/6061 Al composites,defects are likely to appear,thus causing the decrease of the density of the material.The tensile strength of the composite was noticed to demonstrate a trend of first decreasing,then increasing.The presence of B₄ C particles in the composite is helpful for strengthening.The B₄ C particle content is less than 30%,Its strengthening mechanism included grain refinement,dislocation strengthening,load transfer and Orowan strengthening.However,when the B₄ C particle content continues to increase above 30%,the interfacial areas between particle/matrix increases rapidly;the particles start to agglomerate;and more pores and cracks appear,which cause the decrease of the tensile strength.A diffusion layer with a thickness ranging between 0.6 ?m and 1.55 ?m was noticed to appear at the particle/matrix interface,and the major phases there are Al,B₄ C,Al3BC and AlB2,etc.The variation of the layer thickness can be brefily explained as follows.For composites with different B₄ C particles contents,the self-revolving of the B₄ C particle during the plastic defromation of the metal matrix produces non-uniform frictional heat,thus leading to diffusion layers with different thickness.Under static loading,cracking tends to initiate at and expand around those large B₄ C particles,because of the stress concentration at the sharp corners of those large B₄ C particles and the relative weak bonding at the particle/matrix interface.After a dynamic impact,the failure mechanism includes the cracking of large size B₄ C particles,particle/matrix interface debonding;and the tearing of the metal matrix.Matrix alloy hinders the propagation of the cracking in the B₄ C particle at the particle/matrix interface.For B₄C/6061 Al composites with 20%-40% B₄ C paticle contents,the relationships between the plastic flow stress and the strain of the material could be described by the Johnson-Cook model.FSW was found to be an appropriate way to weld B₄C/6061 Al composites with high B₄ C particle content.Our studies shoe that B₄ C particles evenly distributed in the welding joint and the tensile strength of the joint can reach 240 MPa.The fracture occurs in the heat affected zone?HAZ?.The failure of the FSW welding joint is mainly due to the particle/matrix interface debonding and the tearing of the matrix alloy.The further the region is away from the B₄ C particle,the smaller the hardness and elastic modulus of the matrix in that region.Regions in which hardness and elastic modulus of the joint ranked from high to low were: weld zone?WZ?;thermomechanically affected zone?TMAZ?;base metal?BM?;and HAZ.