First-principles study of the stability of NbC and NbN precipitates under coherency strains in alpha-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 a-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. (C) 2010 Elsevier B.V. All rights reserved.