Reactive phase formation at AlN–Ti and AlN–TiAl interfaces

Ti specimens with embedded AlN particles, as well as AlN–Ti and AlN–TiAl diffusion couples were annealed at 900 to 1100°C for up to 40 h. The microstructure and phase composition of the reacted interfaces were characterized using electron probe x-ray microanalysis (EPMA) and electron backscatter diffraction (EBSD) in scanning electron microscope and x-ray diffraction (XRD). In AlN–TiAl diffusion couples, a ternary Ti2AlN compound was formed at the interface. A more complex AlN–TiN+Ti3AlN–Ti3Al–(Ti)Al–Ti reaction zone was observed at the AlN–Ti interface. The morphology of the two-phase (TiN+Ti3AlN) layer changed with time so that the ternary nitride dominated after longer exposures. The kinetics of the reaction zone growth was two-stage, with a significant decrease of the growth rate at the second stage. Such behavior is believed to be caused by the changing morphology of the (TiN+Ti3AlN) layer accompanied by decreasing diffusivity of the components.