First-principles analysis on the nitrogen adsorption and diffusion in Ti alloy towards clarified diffusion mechanism in nitriding

Based on first-principles calculations, the adsorption and diffusion of the nitrogen atoms in Ti bulk and Ti surface layer were studied. Al doping is considered in the study of the surface layer. We first calculated and studied N in bulk Ti and obtained that the N atom preferentially occupies the octahedron(O) interstice of the α-Ti bulk, which behaves as a metallic N-containing solid solution at a lower N concentration. And the diffusion energy barrier is the lowest in the α-Ti bulk when it diffuses between two O sites along with the （1̅21̅0）direction. Then we studied N adsorption and diffusion of N on the α-Ti(0 0 0 1) surface. The N atoms are predicted to be adsorbed on the FCC and HCP sites of the Ti (0 0 0 1) surface, where the FCC site has lower E_ad (adsorption energy). In addition, after doping Al atoms on the Ti surface and sub-surface, the adsorption capacity of the N atom decreases. Especially when the Al atom exists on the Ti surface, this effect is very significant. And the Al atom also reduces the stability of the N atom in the subsurface O site at the same time. Finally, the diffusion process of N atoms from the surface to the subsurface was calculated and compared with and without Al doping. This paper provides fundamental insights into the diffusion mechanism in the nitriding treatment of titanium alloys. These calculation results can infer which titanium alloy is more suitable for nitriding.

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