Role of initial stage nitridation on the mechanical properties of an α-Fe(100) nanofilm in NH3

Abstract

Nitrogen is one of the most significant non-native interstitial elements that is present in the structure of Fe. Initial stage nitridation dramatically influences the mechanical properties of steel, especially for micro to nanoscale applications, but is not yet fully understood. By means of reactive force field molecular dynamics (ReaxFF MD) simulations, the initial stage of the nitridation process of nanofilm Fe, as well as its role on the mechanical properties of the material, were investigated. To clarify the temperature effect, nitridation was simulated in the range of 500–900 K, demonstrating that the adsorption of both N and H atoms into Fe was enhanced by thermal actuation. Corresponding tension test simulations were performed, manifesting that the Fe nanofilm nitrided at 600 K presents the highest yield stress. Further analysis shows that there is a competitive mechanism between the inward diffusion of N atoms that enhances the strength and simultaneous adsorption of H atoms, which leads to brittleness of the material as the temperature increases. Hence, an intermediate temperature could lead to optimal mechanical properties due to the balance of improving the strength while controlling the brittleness of the material. To probe the deformation mechanism, evolutions of partial dislocation and twin boundary at plasticity beginning for pure Fe and the nitrided Fe nanofilm are discussed. The present results show the nitridation strengthening technology of Fe in NH₃ and its related microscale mechanism, which may theoretically support the technical design and improvement in the properties of steel.