The Role of Misfit-Strain Energies during Precipitation of α"-Nitride in Fe-N Martensite

The structural changes occurring during tempering of Fe-N martensite (α') have been investigated using X-ray diffraction and (high resolution) transmission electron microscopy. The first stage of decomposition of Fe-N martensite leads to the formation of α"-nitride and ferrite (α). The formation of α"-nitride involves both a local enrichment and ordering of N atoms ; the Fe sublattice remains of b.c.t. type. The structural changes which take place can be largely understood as the outcome of the competition between ordering of nitrogen atoms in α" precipitates and the developing of a considerable misfit between the α" precipitates and the α'/ α matrix. In the initial stage of decomposition the misfit is relatively small, and precipitation of perfect α" (i.e. Fe16N2) occurs in the martensite matrix. Upon tempering the matrix becomes ferrite and the misfit increases considerably (up to about 10 pct). This misfit is reduced by the formation of vacancies on the nitrogen sublattice in α"-nitride, i.e. precipitation of imperfect α" (Fe16N2-x) takes place. After complete decomposition, upon continued annealing the nitrogen-concentration variations within the α" precipitates disappear and the precipitates become of composition Fe16N2. Then the misfit is accommodated mainly by local deformation of the ferrite lattice