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

TL;DR

This study combines computational methods to predict and analyze the crystal structures, phase stability, and mechanical properties of various Fe-N compounds, revealing their potential for enhanced surface hardness and ductility.

Contribution

The paper introduces a comprehensive computational approach to identify stable Fe-N compounds and systematically evaluate their mechanical properties, aiding in the design of improved Fe-N alloys.

Findings

50 thermodynamically stable or metastable Fe-N compounds identified

Most compounds exhibit ductile behavior and isotropic metallic bonding

Vickers hardness ranges from 3.5 to 10.5 GPa, higher than pure Fe

Abstract

Nitriding introduces nitrides into the surface of steels, significantly enhancing the surface me-chanical 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 systemati-cally studied, including the bulk modulus, shear modulus, Young's modulus, Poisson's ratio, Pugh's ratio, Cauchy pressure, Klemen parameters, universal elastic anisotropy, Debye tempera-ture, 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. 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 signifi-cantly 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.