Mechanical Property of M_xN (M = Fe, Cr) Nitrides in Nitriding Process of 1Cr11NiW2MoV Stainless Steel: First-Principles Calculation

The phenomenon, when firstly formed FeN phase doesn’t appear in 1Cr11NiW2MoV stainless steel in experiment, remains mysterious till now. Based on first-principles calculations, the phase transformations and mechanical properties of M_xN (M = Fe, Cr) nitrides were estimated by several parameters, such as binding energy, elastic constants and electronic structure. The results show that there only exists a quarter probability for Fe₄N phases and one-third probability for Fe₄N₂ phases. The phases can be easily formed due to negative formation enthalpy. However all of the formation enthalpies in Cr + N → CrN phase transformations possess negative value, and the formation enthalpy of CrN possesses the smallest value H_formation = ‒0.671 eV/atom. Then a small amount of the FeN phase found in the experiment formed due to its low formation probability. The calculated mechanical properties show that the largest ratio (G/B = 0.568) is for CrN phase. It possesses the largest Vickers hardness HV = 13.81 GPa and elastic Debye temperature 690 K. The electronic structure shows that the optimized mechanical properties of CrN phase come from its intensive electron density difference in Cr–N bond, its strongest Fe–Fe metallic bond, and FeN covalence bond.