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A comprehensive description of the entire process of N 2 molecules on Ti surface and in Ti bulk as well as the underlying nitridation mechanism were explored via first-principles calculations. The reaction process was divided into four steps and the mechanism for the determined reaction path was analyzed. Utilizing the methodology of the climbing image nudged elastic band (CI-NEB), we investigated the diffusion energy barrier at each step of the reaction path, and determined the minimum energy path (MEP) for the complete reaction path. The calculated adsorption energies of N atoms and N 2 molecules show that they are most stable at the HCP site on the α-Ti (0001) surface. Charge transfer provided theoretical support for N 2 dissociation. We calculated the diffusion energy barrier of the N atom by migrating from the HCP site to the octahedral (O) site, which the N atom prefers to occupy. The diffusion coefficient is highest when the diffusion channel travels from one O site to the next layer of the O site via the next H site, it takes the most energy to complete, compared to those other steps. Lastly, we determined the rate-determining steps of the reaction pathway for the entire nitridation process. This study provides valuable insight into the fundamental mechanisms governing the nitridation process of N 2 molecules in titanium alloys. This study sheds light on the nitridation mechanism for N 2 molecules in titanium alloys. • A fundamental insight into the nitridation mechanism for titanium alloys. • N 2 adsorption is most stable at HCP site on the α-Ti (0001) surface. • Charge transfer reduces the dissociation energy of N 2. • The N atom prefers to occupy at the octahedral site in α-Ti. • The diffusion path of N atoms is O 1 -H-O 3 in α-Ti based on CI-NEB. [ABSTRACT FROM AUTHOR]