High Temperature Wetting And Brazing Of Non-oxide Ceramics By Al-Ti Alloys:Interfacial Behavior And Joint Mechanical Properties

The carbide and nitride ceramics have broad application prospects in the aerospace and nuclear industry due to the high strength,high temperature resistance and excellent anti-irradiation,etc.In general,the carbide and nitride ceramics need to be joined with themselves or other metals to realize the fabrication of the ceramic/ceramic or metal/ceramic composite structures.Al-Ti alloys have been widely concerned for its high strength,high melting temperature,low density,and excellent high-temperature mechanical properties.In this paper,four Al-Ti alloys with different compositions were firstly fabricated by vacuum arc refining,and the wetting and brazing of non-oxide ceramics by the four Al-Ti alloys were further carried out.The wettability,interfacial interactions and joint mechanical properties involving shear strength and fracture were analyzed and discussed.The evolution rules on microstructure and phase composition of wetting interfaces,joint interlayers and interfaces were analyzed systematically by SEM,EDS,TEM and XRD.Moreover,the work of adhesion,interfacial energy,and electronic properties of the related interfaces were evaluated by first-principles calculations to establish the relationships among the microsturecutre,interfacial bonding and shear strength of joint and to clarfy the influence mechanism of phase composition on interfacial bonding and joint performance.Wetting and interfacial behavior of molten Al-（10,20,30,40）at.%Ti alloys on C-terminated4H-SiC at 1500°C and 1550°C were investigated experimentally,and theoretical bonding strength,structure stability and electronic structure of interfacial reaction products/C-terminated4H–SiC interfaces were evaluated by first-principles calculations.The wetting experiments show that the Al–Ti/SiC systems present excellent wettability with contact angle of less than 15°except the Al–40Ti/SiC system performed at 1500°C×30 min.The SEM-EDS and TEM analyses demonstrate that the reaction products are mainly composed of Al₄C₃,Ti C,Ti₃SiC₂,Ti₅Si₃CX andτphase,and their formation and evolution can be mainly affected by the Ti concentration in the Al–Ti alloys and wetting temperature.Moreover,the calculated results demonstarte that the SiC/C-terminated Ti C interface presents the highest work of separation,and its electronic property reveals that the localization of electrons and formation of covalent bond between interfacial C atoms lead to the excellent bonding strength of SiC/Ti C interface.The effects of Al-Ti alloy composition and interlayer thickness on the joint strength,microstructure and phase evolution,as well as the joint fracture behavior were investigated.The average joint shear strength fluctuates in the range of～100?260 MPa while employing the interlayer thickness of～50μm.Moreover,the average joint strength climbs markedly with the interlayer thickness decreasing from～100 to 25μm while employing the Al-20Ti alloy as filler metal,arriving at the maximum of～315 MPa.Meanwhile,the（Al）phase in the interlayers is reduced gradually till to disappear while employing the thinnest brazing interlayer,leaving the Al₄C₃,Ti C and（Al,Si）₃Ti phases in the interlayer.The joint fractures of SiC/Al-20Ti/SiC joint mainly occur in the SiC ceramic substrate near the interlayer/ceramic interface.The wettability,interfacial behavior and mechanical properties of of Al-Ti/AlN systems were investigated.The wettability of Al-Ti on AlN ceramic is sharply enhanced when the Ti concentration is over than 10 at.%,with the equilibrium contact angle from～80?90°to below20°.However,the decrease of contact angle is not obvious with the temperature increasing.The reaction products are mainly composed of Ti N,Al and Al₃Ti phases,and the content of Ti N increases with Ti concentration.Furthermore,the increased temperature can significantly enhance the interfacial reactions between Al-Ti and AlN.Moreover,the calculation results show that the work of adhesion of the AlN/Ti N interface is higher than that of the AlN/Al interface.The brazing experimental results show the interlayer of AlN/Al-Ti/AlN joint is mainly composed of AlN and Al₃Ti phases.The interlayer thickness is the key factor for affecting the joint strength,and the maximum average joint strength of～93.5 MPa is obtained while employing the interlayer thickness of 10μm.The joint fractures mainly occur in the interlayer and the ceramic substrate.The wettability,interfacial behavior and joint mechanical behaviour of Al-Ti alloy/Si₃N₄systems were investigated.The wettability is significantly improved with the Ti concentration or wetting temperature increasing,and meanwhile the interfacial reactions are intensified.The interfacial products consist mainly of Ti₅Si₃and AlN phases.The brazing experimental results show both the brazing temperature and holding time have an obvious effect on the shear strength of Si₃N₄/Al-Ti/Si₃N₄joint.The maximum average joint strength is～139.5 MPa while brazing at1550°C×30 min.The fracture of Si₃N₄/Al-Ti/Si₃N₄joints is closely related to the interfacial microstructures.From the typical SEM images,the Si₃N₄/Al-Ti/Si₃N₄joint fractures occur only in the interlayer,or in the ceramic substrate,with the main propagation of crack in the interlayer,or with the crack initiation in the interlayer and the crack propagation in the ceramic substrate for a long distance.