The nitriding behavior of Ti-Al alloys at 1000 °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 nitrogen gas, for times ranging between 7 and 100 hours. The scales and subscales were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, energy dispersive and wavelength dispersive X-ray analysis, electron energy loss spectroscopy, and optical microscopy. Upon formation of a surface nitride scale, the subscale became enriched 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 scale and subscale and a shifting of the diffusion path toward the Al-rich corner of the ternary isotherm. The formation of Al-rich intermetallic phases 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 TiN was measured, as well as the composition of the ternary nitride "Ti2AlN."