https://www.repository.cam.ac.uk/bitstream/1810/248572/1/Ali%20et%20al%202015%20Acta%20Materialia.pdf

Microstructural evolution and characterisation of interfacial phases in Al2O3/Ag-Cu-Ti/Al2O3 braze joints

Interfacial reactions between sapphire and Ag-Cu-Ti-based active braze alloys

Infrared brazing of Al2O3 and alloy 42 using a silver-base active braze alloy was investigated at 900 °C for 0 to 300 seconds, with a heating rate of 3000 °C/min. Experimental results show that Ti3(Cu, Al)3O intermetallic with various amounts of Al is observed in the reaction layer and plays an important role in the early stage of reactive wetting. A two-layer structure is observed at the reaction interface brazed at 900 °C for 5 seconds. The reaction layer close to the alumina contains large amounts of Al, so the mass balance of the system is maintained. The growth of the reaction layer is not rate controlled by diffusion within the first 120 seconds. After 120 seconds, the rate controlling mechanism of the reaction layer becomes the diffusion control, satisfying the parabolic law. Dynamic wetting angle measurements using a traditional vacuum furnace at the heating rate of 10 °C/min demonstrate that the wetting angle rapidly decreases within the first 150 seconds, especially 0 to 80 seconds, and eventually stabilizes after 600 seconds.

Microstructural Evolution at the Bonding Interface during the Early-Stage Infrared Active Brazing of Alumina

Metallurgical reactions controlling the Ti activation in brazing of Al2O3 have been studied by means of microstructural and thermodynamic analysis. The reactions of titanium with oxygen and copper are shown to be decisive in active brazing. Themiscibility gap in the Ag–Cu–Ti system divides the liquid braze into Ag-rich (L1) and TiCu-rich (L2) liquids. The liquid L2 reacts with alumina forming the mixed oxide (Ti, Al)4Cu2O. Besides with alumina, Ti reacts with oxygen that the filler alloys usually contain and forms a brittle ribbon composed of TixO and Ti–Cu–O phases in the braze. The formation of Ti oxides next to the alumina is possible only in the filler alloys of highest Ag content.

Metallurgical reactions controlling the brazing of Al2O3 with Ag–Cu–Ti filler alloys

Titanium is an extremely reactive element which is known to promote the wetting of ceramics by liquid metals. The wetting process is believed to be controlled by the segregation of a very thin layer of Ti to the metal/ceramic interface where it undergoes a localised redox reaction with the ceramic. This hypothesis is consistent with cross-sectional microanalysis studies of metal–ceramic interfaces where thin layers of substoichiometric titanium oxides have been identified. The study of titanium segregation is ideally suited to neutron reflection because of the negative scattering length of the dominant isotope. In this study we have characterised the chemical composition of three interfaces between metals and a sapphire (Al2O3) single crystal using specular neutron reflection. The interfaces are: Sn/sapphire, Sn/sapphire containing a thin (≈20 nm) Ti interlayer introduced by physical vapour deposition (PVD), and an interface between sapphire and a commercial Ag-Cu-Ti alloy used to braze ceramics. We have evaluated the neutron reflection data collected from the CRISP reflectometer at the ISIS facility using a multilayer model of the interface. The results indicate that the 20 nm evaporated layer cannot be distinguished from interface roughness while the interface between sapphire and the commercial braze alloy contains a thin (≈70 nm) titanium suboxide layer.

Neutron reflection studies of titanium segregation to metal-ceramic interfaces

Ceramic-to-metal (C/M) joining is of particular importance because of physical and economic limitations that restrict the size and complexity of ceramic components. The active filler metal method is one of the C/M joining techniques and involves the use of elements with a strong affinity for O[sub 2], e.g., Ti or Zr. Active metal elements, typically Ti, have been shown to be effective in wetting various oxides. In the Ag-Cu-Ti/Al[sub 2]O[sub 3] system the primary concern has been focused on the formation of Ti-contained phases as a result of chemical reaction between Al[sub 2]O[sub 3] and the Ti-containing brazing alloy. Previous studies suggested that Ti in brazing alloy reacts with the Al[sub 2]O[sub 3] ceramic surface to form compounds such as TiO. These compounds act as a transition layer between the Al[sub 2]O[sub 3] and the brazing alloy. The chemical reaction products at the C/M interface are important because they influence the mechanical strength of the joint. These products are affected by the composition of the brazing alloy and the process conditions such as the heat-treatment temperature and time. Consequently, microstructural studies of interfacial reaction products must be carried out to understand the relationships between the structure and the properties ofmore » the joint. The purpose of the present study is to investigate the phase evolution and the microstructural characteristics of chemical reaction products formed in the interface between the commercial alumina and the Ti-containing brazing filler metal at various heat-treatment conditions.« less

Woobong Byun

Papers

Variations of Phases and Microstructure of Reaction Products in the Interface of Al2O3/Ag-Cu-Ti Joint System with Heat-Treatment