Diffusion of nitrogen in solid titanium at elevated temperature and the influence on the microstructure

Nitrogen introduction to solid commercially pure titanium has been carried out at 1650 °C in an electric induction furnace using two different methods. An effective way to avoid the formation of the hard and brittle nitride layer (TiN and Ti2N) is reported. Microstructure and microhardness were examined on the cross-section of the nitrided samples. Multiple phase layers can be observed, and the phases in each layer were identified using X-ray Diffraction. The effects of the experimental conditions such as temperature and nitriding time on the kinetics of nitrogen diffusion were investigated. The nitrogen content within the samples was increased with increasing temperature and nitriding time. Correlations between microhardness and the nitrogen concentration have been developed for the phase(s) present in the core and the outer layers. The diffusion of nitrogen in solid titanium was simulated numerically, and the predicted nitrogen concentration profile in the rods and displacement of Ti–N phase interfaces show good agreement with the experimental observations. Energy-dispersive X-ray spectroscopy and the numerical simulation results suggest that β phase boundary composition is 2.8 wt. % N, the α phase exists within the compositional range of 5.5–6.5 wt. % and the Ti–N phase within the compositional range 7.0–15.0 wt. %, which differs from data extracted from a published phase diagram.