Research On Brazing Parameters And Bonding Mechanism Of The SiC/SiC Joints With Au-based High-temperature Filler Alloy

Developing new materials for the nuclear-fuel cladding is critical important,in order to improve the safety of nuclear reactors.SiC ceramic is considered as potential material for nuclear-fuel cladding due to its excellent mechanical and irradiation properties.Complex structures fabricated by the SiC ceramics are difficult to be obtained through common machining because of their high-hardness and high melting-point.Besides,the nuclear-fuel cladding is continually serviced in high temperature,strong oxidation or other special environment.Therefore,it is important to study the high-temperature bonding between SiC ceramics.In this work,(Au₇₉Ni₁₇Pd₄)₉₆Ti₄?wt.%?filler alloy was designed and employed to join the SiC ceramics at?1150-1300??for 10 min and 1250?for?10-90?min.The typical microstructure of the obtained joints and the melting process of the filler alloy were annalyzed.Based on that,the joining mechanism of joints was discussed.Effect of brazing temperature and soaking time on the microstructure and mechanical property was investigated.Besides,the fracture modes of the obtained joints using different brazing parameter are studied,and the relationship between the physical properties of the phases within the joints and the residual stress was analyzed.The typical microstructure of SiC/SiC joint brazed using(Au₇₉Ni₁₇Pd₄)₉₆Ti₄?wt.%?could be written as SiC/reaction layer/braze/reaction layer/SiC.The reaction layer was mainly composed of TiC and Au?Si,Ti?.The braze was mostly constituted by Ni₂Si,Pd₂Si and Au?Ni,Si?with fine TiC particles dispersed on it.The filler alloy that composed of multiple foils started to melt at about 950?and the foils transform to liquid phase at about 1150?.TiC,Pd₂Si and Ni₂Si particles were inferred to form during heating,while Au?Si,Ti?and Au?Ni,Si?were solidified at about 1064?in the cooling process.Considering the selected parameters in this work,the increasing brazing temperature would lead to the coarsening of reaction layers and TiC particle in brazing seam.The extension of holding time could lead to the similar results.The morphologies of Pd₂Si and Ni₂Si obtained at various brazing parameters.The filler alloys were well bonded with the substrates.It was residual stress that led to failure of joints in shear tests.To reduce the stress level,the microstructure of joint should be modified by optimizing brazing parameter or chemical composition of filler alloy.Considering this,a series of Au-Ni-Pd filler alloys were used to join the SiC ceramics.The results show that the volume fraction of Pd₂Si in the joint increases when increasing the Pd content in the filler alloy,which is beneficial to reduce the residual stress level and improve the high-temperature strength of the joint.