First-principles Study on Nitrogen Diffusion in α/γ Phase in HAZ of Ferritic Stainless Steel

Using a first-principles method, the properties of the α-Fe/γ-Fe interface and the diffusion behavior of N atoms in interfaces with different nitrogen content were investigated. The results show that the interface separation energy of α-Fe/γ-Fe interface without N atoms is maximum at 7.34 J·m^-2. When the interface contains N atoms, the interface separation energy decreases, and with increasing nitrogen content, the bonding between N-Fe changes from ionic to covalent. The diffusion of N atoms in the α-Fe/γ-Fe interface follows the order of interface＞α-Fe/γ-Fe, with the fastest diffusion occurring at the γ-Fe（saturated N-α-Fe）/α-Fe interface. As the nitrogen content increases, the diffusion barrier increases, and when it reaches saturation, the activation energy for diffusion increases by 3-4 times, with the lowest energy state near the tetrahedral position. At a high temperature of 1273 K, the diffusion coefficient in the α-Fe/γ-Fe interface reaches a maximum at-20 m²·s^-1.Upon application of an electric field, the diffusion coefficient increases from -50 m²·s^-1 to -18 m²·s^-1.