Thermodynamic evaluation of reaction products and layering in brazed alumina joints
- PDF / 1,309,774 Bytes
- 7 Pages / 576 x 792 pts Page_size
- 23 Downloads / 201 Views
The joints formed by brazing A12O3 to itself or to a Ti-alloy (Ti-6A1-4V) with a A g - C u - T i braze filler were investigated. In the brazing process, Ti in the braze alloy reduces A12O3 to form a series of reaction products which have a layered morphology. The formation of M6X-type compounds, Ti 4 Cu 2 0 or Ti 3 Cu 3 0, at the interface is characteristic of these joints. The other reaction products also belong to the T i - C u - O system (with the reduced Al in solution), and hence this subsystem was chosen to assess the thermodynamic stability of the joints. The T i - C u - 0 section was established experimentally at 945 °C, and activities of elements in three of the three-phase regions were estimated based on the phase boundaries of the ternary section and available binary thermodynamic data. The estimated free energies of formation of the two M6X-type compounds, Ti 4 Cu 2 0 and Ti 3 Cu 3 0, are —120 kcal/mol and —122 kcal/mol, respectively. The highly negative values for the free energies of formation suggest that these compounds are thermodynamically stable. The activity data were also used to generate activity diagrams for T i - C u - 0 system. The layer sequences at the joints satisfied the stability requirements based on the ternary section and the activity diagrams, indicating that even though the interfaces formed in a matter of minutes, they were at local thermodynamic equilibrium.
I. INTRODUCTION Reaction of dissimilar materials to form chemical bonds is an important step in the manufacture of joints and composites that make optimum use of both materials. Joining is also used to overcome the processing limitation of ceramics to form complex geometries. The ceramic-ceramic or ceramic-metal joints are made by techniques such as metallization, diffusion bonding, and brazing with variations for particular applications. Brazing of two components using a filler metal is a relatively simple technique that can produce a strong hermetic joint. To form a brazed joint, a metal alloy of suitable composition is sandwiched between the components to be joined; the assembly is then heated, usually in a vacuum and under a nominal load, to a temperature slightly above the melting point of the alloy, and finally cooled to room temperature. The heating and cooling cycle is designed to maximize the wetting and adhesion resulting from interfacial reactions and to minimize the thermal expansion mismatch stresses in the joint. An active element is usually added to the braze alloy and is defined as one that can reduce the ceramic and form a strong bond. Elements belonging to the IVB, VB, and VIB groups along with Ni and Pd are possible candidates.1"3 The most extensively 2244
J. Mater. Res., Vol. 9, No. 9, Sep 1994
http://journals.cambridge.org
Downloaded: 22 Mar 2015
studied active element is Ti,4'5 and Ti-containing braze alloys are commercially available (Wesgo, Inc., Belmont, CA). Braze alloys used to form joints at ~1000 °C are primarily composed of a low-melting material such as a Ag—Cu eutectic, with a minor fract
Data Loading...
