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

Abstract

The nitriding behavior of a series of alloys in the binary Ti-Al system has been determined at 1000 °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.”