Inward diffusion of Al and Ti3Al compound formation in the Ti-6Al-4V alloy during high temperature gas nitriding

Microstructure and composition analysis was performed on the nitrided Ti-6Al-4V alloy by using coupled techniques of X-ray diffraction analysis and glow discharge spectroscopy. Titanium nitride (TiNx) appeared at the outmost surface, but the low solubility of Al in the TiNx phase made Al atom to move inward. Such Al-rich phase as Ti3Al compound was formed by Al accumulation at the boundary between TiNx and α(N)-Ti phase (N diffusion zone). Interestingly, the Ti3Al had very low concentration of N, and it may inhibit N atom from diffusing inward as a diffusion barrier. The flat region of V concentration profile with about 2 at% also appeared in the region of α(N)-Ti phase because there was a solubility limit of less than 2 at% V in the α-Ti phase (hcp).