Microstructural changes during tempering of Fe-N martensite: Formation of α″-nitride

Abstract

Abstract The decomposition of Fe-N plate martensite into α″-nitride (Fe16N2) and ferrite is attended by morphology changes which can be attributed mainly to the accommodation of the considerable misfit between the ferrite matrix and the α″ precipitates. Initially plate-like α″ precipitates, oriented nearly parallel to the (001) matrix plane, form locally in the martensite. Upon further tempering, the α″ precipitates become imperfect, with vacancies on the nitrogen sublattice (i.e. Fe16N2-x ), so that the misfit between the precipitate and ferrite matrix is reduced. After the decomposition is completed, the nitrogen concentration variations within the α″ precipitates disappear, that is the number of vacancies on the nitrogen sublattice reduces, and antiphase boundaries are observed. Finally a lamellar morphology of precipitate and matrix develops. Even after coarsening, the precipitate-matrix interface contains very few dislocations. High-resolution images reveal that the misfit between matrix and precipitate (about 10% in the c direction) is mainly reduced by, firstly, small orientation differences (1–3°) between the lattices of matrix and precipitate and, secondly local changes in the lattice spacing.