Mechanical Property of MxN (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 MxN (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.