Thermodynamic evaluation of reaction products and layering in brazed alumina joints

Abstract

The joints formed by brazing Al2O3to itself or to a Ti-alloy (Ti-6Al-4V) with a Ag–Cu–Ti braze filler were investigated. In the brazing process, Ti in the braze alloy reduces Al2C3to form a series of reaction products which have a layered morphology. The formation of M6X-type compounds, Ti4Cu2O or Ti3Cu3O, at the interface is characteristic of these joints. The other reaction products also belong to the Ti–Cu–O system (with the reduced Al in solution), and hence this subsystem was chosen to assess the thermodynamic stability of the joints. The Ti–Cu–O 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, Ti4Cu2O and Ti3Cu3O, 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 Ti–Cu–O 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.