First-Principles Simulations For The Fe-N System

Nitrogen is a common added element in steel, and has remarkable influence on the property and performance, such as strength, hardness and corrosion resistance. In this paper, we try to understand some questions related to the point and suface defects of Fe-N system by first principle methods combined with transition state and thermodynamic theory. Four questions have been studied: The atomic mechanism of nitrogen diffusion into BCC iron; the interaction between nitrogen and point defects in FCC iron; the absorption and diffusion mechanism of N and C in BCC iron grain boundary(GB) and the BCC GB migration mechanism.(1) For BCC Fe, the computed data suggest that, depending on the coverage of N atoms, N prefers to stay on particular surface sites. Once pinned down well below the surface, N prefers to move into octahedral interstices rather than tetrahedral interstices. However, the tetrahedral interstices are crucial because they act as transition states and yield the saddle point energies of the corresponding minimum energy pathways. In comparison to carbon, we found that nitrogen prefers a different pathway form(100) surface to subsurface due to its strong repulsive interaction with Fe ions.(2) In FCC iron, it is found that spin-polarize will decrease the nitrogen solution enthalpy and vacancy formation energy. The interaction between nitrogens is repulsive, and weaken by the spin-polarize. There exists strong attractive interation between nitrogen and vacancy, casuing the formation of NVCs in the Fe-N system. Combined with a thermodynamic model, the DFT simulation predict that the main occupancy NVCs are vN2, vN3, vN4 and vN5. The concentration of vacancy is affected by the concentration of N, and will incease by about 8 orders of magnitude as the increase concentration of nitrogen, however, it does not always increase, and will reaches an saturated value. This phenomenon is in agreement with the prediction by the Kirchheim’s thermodynamic theory.(3) C and N have different influence on the GB structure and cohesive energy, e.g. the carbon atom will drag the two constituted grains of Σ5<100>(210) GB closely, while the nitrogen atom will push them away, however, the cohesive energy suggests that carbon will weaken, while nitrogen will strengthen the Σ5 GB. The two interstitial atoms will push away the constituted grains of Σ3<100>(112) GB and always weaken the GB. The chemical bond between interstitial atoms and Fe atoms of Σ5 GB is the key factor to determine the Σ5 GB cohesive energy, while interstitial atoms segregation at Σ3 will cause very large positive mechanical energy. The GB can be regarded as constructed by cellular, which related to the diffusion mechanism of interstitial atoms. The diffusion mechanisms of C and N in Σ5 GB are different. Compared with bulk diffusion barrier, it is found that the diffusion of carbon in fact has been decelerated by Σ5 GB, while the nitrogen atom has been accelerated. The diffusion mechanism of carbon and nitrogen in Σ3 is almost the same. Compared with bulk diffusion, the diffusion has been fiercely decelerated by Σ3 GB.(4) Symmetrical Σ5<100>(210) and Σ5<100>(310) tilt grain boundaries(GBs) in BCC Fe show similar GB energies, while the migration energy barriers are quite different, indicating that the Σ5(310) GB is intrinsically more stable. The influences of a vacancy and an interstitial atom(impurity) on GB migration have also been examined. A GB dislocation loop(GBDL) theory has been proposed to describe the GB migration mechanism, and the calculated energy barriers can then be used to predict the trend of GB migration with this model.