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 (alpha') 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 alpha''-nitride and ferrite (alpha). The formation of alpha''-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 alpha'' precipitates nd the developing of a considerable misfit between the alpha'' precipitates and the alpha'/alpha matrix. In the initial stage of decomposition the misfit is relatively small, and precipitation of perfect alpha'' (i.e. Fe16N2) occurs in the martensite matrix. Upon tempering the matrix becomes ferrite and the misfit increases considerably (up to about 10 pet). This misfit is reduced by the formation of vacancies on the nitrogen sublattice in alpha''-nitride, i.e. precipitation of imperfect alpha'' (Fe16N2-x) takes place. After complete decomposition, upon continued annealing the nitrogen-concentration variations within the alpha'' precipitates disappear and the precipitates become of composition Fe16N2. Then the misfit is accommodated mainly by local deformation of the ferrite lattice.