Phase stability, magnetism, elastic properties and hardness of binary iron nitrides from first principles

First-principles calculations were performed to investigate the phase stability, magnetism, elastic properties and hardness of four main binary iron nitrides: FeN, ζ-Fe2N, ε-Fe3N and γ′-Fe4N. The linear relation ΔHf = -65.912x - 11.154 kJ mol-1 was found between the formation enthalpies and their nitrogen content x. Based on this linear approximation, the formation enthalpies of ε-Fe3N1.1, ε-Fe3N1.3 and γ″-FeN0.91 were estimated as -28.95, -31.59 and -42.56 kJ mol-1, respectively. The values were consistent with the experimental results. The calculated elastic constants indicated that the FeN and γ′-Fe4N phases were significantly anisotropic, the ζ-Fe2N exhibited some anisotropy to a certain degree. While the ε-Fe3N showed a relatively strong isotropic character. The ratios of bulk modulus (B) to shear modulus (G), 3.41 for FeN, 2.69 for ζ-Fe2N, 2.39 for ε-Fe3N and 3.09 for γ′-Fe4N, indicated their ductile nature. Magnetism calculations showed that the FeN was antiferromagnetic, the ζ-Fe2N, ε-Fe3N and γ′-Fe4N were paramagnetic. Besides, the Vickers hardness of these iron nitrides were calculated, and the strong Fe-N covalent bonds were found to play important roles in the hardness of iron nitrides. © 2014 Elsevier B.V.