Al2O3/Ti interlayer/AISI 304 diffusion bonded joint: Microstructural characterization of the two interfaces

Sintered alumina and AISI 304 stainless-steel discs were joined at 900°C by solid state diffusion bonding, making use of a Ti foil acting as thermal stress relief interlayer. The microstructure of the two interfaces thus formed, that is, Al2O3/Ti and Ti/AISI 304 was investigated by a variety of characterization techniques such as scanning and transmission electron microscopy (TEM), energy-dispersive X-ray (EDX) microanalysis, X-ray diffraction and Vickers microhardness, with the following results: (i) precipitation of Ti3Al particles plus Al and oxygen diffusion were detected within the Ti and close to the ceramic; (ii) within the Ti foil, coexistence of α-Ti and β-Ti was observed and ascribed to Fe and Cr diffusion and to β-Ti decomposition during slow cooling from the bonding temperature; and (iii) adjacent to the Ti/steel interface a 5-μm-thick layer was found to contain TiFe and Fe2Ti, as well as sigma phase, M23C6 precipitates and TiC. An interpretation of these results in terms of element partition during the bonding process was attempted.