Stability of microstructure and its evolution during solid-state annealing of image–Inconel 600 brazed couples Original Research Article

The stability of the microstructure of alumina–Inconel 600 brazed joints was investigated under simulated in-service conditions by subjecting them to prolonged heat treatment at 400 and 560 °C. The evolution of the microstructure and microchemistry of the brazing zone was examined using extensive microanalysis of the constituent phases. The layered structure of the brazing zone transformed to homogeneous, near-equilibrium, two-phase microstructure after heat treatment at 560 °C. Solid-state interdiffusion was identified as a primary factor responsible for such copious modification of the microstructure. Intermediate temperature heat treatment at 400 °C revealed that the mechanism of transformation of the microstructure was globulization of the Ni overlayer and dissolution of Mo from the metallization layer into the Ni-rich phase. The migration behavior of each of the elements, in response to heat treatment, was analyzed. Cr was found to diffuse out of Inconel and form a layer of image at the alumina–brazing alloy interface. The bond strength of the interface was high enough to cause cohesive failure in the alumina side of the joints.