First-principles study of the stability of NbC and NbN precipitates under coherency strains in α-iron

The first stages of precipitation in Fe–Nb–C–N alloys exhibit mono-atomic coherent platelets, analogous to the Guinier–Preston zones. With an ab initio approach, we evaluated for various NbX (X = C, N) bulk phases their formation enthalpies and their stability under strains similar to those induced by the coherency of the platelets with the α-iron matrix. It appears that the nitrogen- and to a lesser extent the carbon-rich precipitates are likely to adopt a structure connected to the bulk CW hexagonal phase, according to an inedited ‘prismatic on cube’ orientation with the body centred cubic matrix.