The nitriding behavior of Ti-Al alloys at 1000 degrees C

The nitriding behavior of a series of alloys in the binary Ti-Al system has been determined at 1000 degrees C, under a controlled atmosphere of pure n itrogen gas, for times ranging between 7 and 100 hours. The scales and subs cales were characterized by X-ray diffraction, scanning electron microscopy , transmission electron microscopy, energy dispersive and wavelength disper sive X-ray analysis, electron energy loss spectroscopy, and optical microsc opy. Upon formation of a surface nitride scale, the subscale became enriche d in Al and resulted in the formation of a series of Al-rich intermetallic phases. This enrichment has been linked to the transport processes in the s cale and subscale and a shifting of the diffusion path toward the Al-rich c orner of the ternary isotherm. The formation of Al-rich intermetallic phase s in the subscale was shown to result in rapid "breakaway" nitriding of the TiAl and TiAl2 alloys. The stoichiometry of the binary nitrides AlN and Ti N was measured, as well as the composition of the ternary nitride "Ti2AlN."