Study On The Mechanism Of Brazing SiC/Zr With High-entropy Alloy And Network Composite Interlayer

SiC ceramics have excellent chemical stability,outstanding mechanical properties,and lower neutron absorption interface.It is expected to replace Zr alloy in nuclear fuel components and continuously expand its application in the field of nuclear energy.However,the high cost,brittleness,and difficulty in processing have limited its practical application.In order to fully utilize the advantages of SiC ceramics and metal Zr,they are connected by direct brazing.The key to direct brazing of ceramics and metals lies in the selection of brazing materials.In traditional active brazing materials,active elements（Ti,Zr,Hf,Nb,etc.）are prone to react with the interface to generate a large amount of brittle intermetallic compounds,which is the main reason that affects the joint performance.Active brazing materials containing Ag and Cu usually have lower melting points and poor high-temperature performance.In this paper,a CoCrFeNiCuSn high-entropy alloy with excellent high-temperature performance,good wettability,and the ability to enhance the formation of solid solution and reduce the generation of harmful phases is prepared as a brazing material alloy.In addition,in order to reduce residual stress caused by differences in the CET and improve joint strength,graphene and LiAlSiO₄ are loaded on Cu-foam with a unique 3D structure and good plasticity to form a composite intermediate layer vacuum brazing SiC ceramics and metal Zr with CoCrFeNiCuSn high-entropy alloy.The influence of brazing process parameters and Cu-foam loading on the interface structure,interface reaction mechanism,and mechanical properties of the brazed joint are studied.The CoCrFeNiCuSn high-entropy alloy and Cu-foam composite were vacuum brazed to SiC ceramics and metal Zr using an intermediate layer to achieve a reliable connection.The joint microstructure was mainly composed of a mixture of five phases:Zr（Fe,Cr）₂ Laves,α-Zr（s,s）,Zr₂Cu,（Zr,Sn）and HEAP.No changes were observed in the types of phases in the brazed joint under fixed 20-min holding time,only changes in phase distribution,volume fraction,and joint width occurred with the change in brazing temperature.Increasing temperature improved the flowability of the filler,reduced the defects（such as gaps and cracks）on the SiC ceramic side of the joint,and resulted in a gradual decrease in joint width.The brazing filler metal reacted vigorously with Zr to produce sufficient fusion.The eutectic reaction between Cr and Zr in the filler generated Zr Cr₂+Zr（s,s）,in which part of the Cr was occupied by Fe to form Zr（Fe,Cr）₂ Laves.The hard Zr（Fe,Cr）₂ was wrapped in the softα-Zr（s,s）,and their interaction significantly improved the joint’s strength and ductility.Under the high-entropy effect,HEAP and（Zr,Sn）were generated,which led to excellent thermal stability and high-temperature mechanical performance of the joint.The brazed joint reached a maximum shear strength of 221MPa at room temperature and an average shear strength of 207 MPa at600℃after being held at 1040℃for 20 min.The room temperature and high-temperature strength of the obtained joint were much higher than those of conventional brazed joints.Cu-foam was used as a substrate to obtain SiC/Zr joints by brazing with a composite interlayer composed of graphene,LiAlSiO₄,and graphene/LiAlSiO₄ loaded onto a CoCrFeNiCuSn high-entropy alloy.The effect of the holding time at 1040℃on the interfacial microstructure and mechanical properties of the joints was investigated.The addition of loaded particles and changes in the holding time did not significantly affect the types of phases in the joint,which mainly consisted of a mixture of Zr（Fe,Cr）₂ Laves,α-Zr（s,s）,Zr₂Cu,（Zr,Sn）,and HEAP,while the distribution of LiAlSiO₄ was found in the joint with the addition of LiAlSiO₄.When the holding time was 20 min,the joints loaded with graphene,LiAlSiO₄,and graphene/LiAlSiO₄ achieved maximum shear strengths of 122 MPa,147 MPa,and 235 MPa,respectively.Shorter or longer holding times resulted in decreased shear strength.The shear strength of joints brazed with only graphene or LiAlSiO₄ decreased,whereas their simultaneous loading resulted in a significant improvement in strength.The loading of graphene caused the Zr（Fe,Cr）₂ Laves to exhibit a fine mesh structure,limiting its ability as a reinforcing phase.In addition,LiAlSiO₄ was uniformly dispersed in the joint and adjusted the coefficient of thermal expansion,reducing residual stresses and improving the overall strength of the joint.