The decomposition of Fe-N plate martensite, into an α’’-nitride (Fe16N2) and ferrite, was associated with morphological changes which could be attributed mainly to accommodation of the considerable misfit between the ferrite matrix and the α’’ precipitates. Initially plate-like α’’ precipitates, which were oriented almost parallel to the (001) matrix plane, formed locally in the martensite. During further tempering, the α’’ precipitates became imperfect; with vacancies on the N sub-lattice (Fe16N2-x) such that the misfit between the precipitate and ferrite matrix was reduced. When decomposition was complete, N-concentration variations within the α’’ precipitates disappeared. That is, the number of vacancies on the N sub-lattice decreased and antiphase boundaries were observed. Finally, a lamellar morphology of the precipitates and matrix developed. Even after coarsening, the precipitate/matrix interface contained very few dislocations. High-resolution images revealed that the misfit between matrix and precipitate (about 10% in the c-direction) was mainly reduced by small orientation differences (1 to 3°) between the lattices of the matrix and precipitates, and partially by local changes in the lattice spacing.

Microstructural Changes during Tempering of Fe-N Martensite - Formation of α′′-Nitride. K.Han, A.J.Böttger, H.W.Zandbergen, E.J.Mittemeijer: Philosophical Magazine A, 2002, 82[4], 715-33