The Fe-N system: crystal structure prediction, phase stability, and mechanical properties

Abstract

Nitriding introduces nitrides into the surface of steels, significantly enhancing the surface mechanical properties. By combining the variable composition evolutionary algorithm and first-principles calculations based on density functional theory, 50 thermodynamically stable or metastable Fe-N compounds with various stoichiometric ratios were identified, exhibiting also dynamic and mechanical stability. The mechanical properties of these structures were systematically studied, including the bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, Pugh’s ratio, Cauchy pressure, Klemen parameters, universal elastic anisotropy, Debye temperature, and Vickers hardness. All identified stable and metastable Fe-N compounds were found in the ductile region, with most exhibiting homogeneous elastic properties and isotropic metallic bonding overall. As the nitrogen concentration increases, their bulk moduli generally increase as well. The Vickers hardness values of Fe-N compounds range from 3.5 to 10.5 GPa, which are significantly higher than that of pure Fe (2.0 GPa), due to the stronger Fe-N bonds strength. This study provides insights into optimizing and designing Fe-N alloys with tailored mechanical properties.