THERMODYNAMIC EVALUATION OF REACTION-PRODUCTS AND LAYERING IN BRAZED ALUMINA JOINTS

The joints formed by brazing Al2O3 to itself or to a Ti-alloy (Ti-6Al- 4V) with a Ag-Cu-Ti braze filler were investigated. In the brazing pro cess, Ti in the braze alloy reduces Al2O3 to form a series of reaction products which have a layered morphology. The formation of M6X-type c ompounds, Ti4Cu2O or Ti3Cu3O, at the interface is characteristic of th ese joints. The other reaction products also belong to the Ti-Cu-O sys tem (with the reduced Al in solution), and hence this subsystem was ch osen to assess the thermodynamic stability of the joints. The Ti-Cu-O section was established experimentally at 945 degrees-C, and activitie s of elements in three of the three-phase regions were estimated based on the phase boundaries of the ternary section and available binary t hermodynamic 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 energ ies of formation suggest that these compounds are thermodynamically st able. The activity data were also used to generate activity diagrams f or Ti-Cu-O system. The layer sequences at the joints satisfied the sta bility requirements based on the ternary section and the activity diag rams, indicating that even though the interfaces formed in a matter of minutes, they were at local thermodynamic equilibrium.