Structure of nitride layer formed on titanium alloy surface by N₂-gas exposure at high temperatures

Nitride treatment is used for hardening of titanium alloy surfaces. In this study, we prepare samples under two different conditions, (1) 810°C, for 600 min, and (2) 850°C, for 720 min. A depth-profile analysis of the surfaces of the samples is conducted through Doppler broadening measurements of positron annihilation gamma rays using a slow positron beam. Lager S-parameters are obtained near the surface for all samples. These parameters can be explained by the positron annihilation at the grain boundaries between the nano-grains confirmed by transmission electron microscope images. Furthermore, the variation in the W-parameters near the surface is independent of the variation in S-parameters, thus indicating a depth dependence of the W-parameters on the chemical composition of the precipitate at the grain boundaries, also confirmed by energy dispersive X-ray spectroscopy measurements. Variations in the W-parameters was observed to occur in the deeper regions, indicating that the nitrated layer is thicker in the sample prepared under condition (2) and the increased hardness of the sample surface measured using Vickers Hardness method is likely caused by this thicker nitrated layer.