Diffusion kinetics of explosively treated and plasma nitrided Ti-6Al-4V alloy

Ti-6Al-4V alloys, which were exposed to an explosive shock process, were nitrided in nitrogen plasma in the temperature range of 700-900°C for 3-12 h. During the plasma nitriding, the surface layer consisted of TiN (δ), Ti₂N (ε) and nitrogen solid solution layers (α-Ti). The growth rate of nitride and solid solution layers were found to be controlled by the diffusion of nitrogen. An effective nitriding was achieved due to high dislocation density and vacancy concentration. Based on the present layer growth data, an analytical model for multiphase diffusion was used to estimate the effective nitrogen atom diffusion coefficient in the nitride layers. The interface velocity equations were derived from Fick's law and a numerical method has been used to compute the diffusion coefficients of nitrogen in a binary multiphase Ti-TiN system. Depending on temperature and layer thickness, the activation energies of nitrogen in TiN and Ti₂N phases were found to be 18,950 (±2116) and 27,925 (±1105) cal/mole, respectively.