FORMATION OF ALPHA-IRON NITRIDE IN FEN MARTENSITE - NITROGEN VACANCIES, IRON-ATOM DISPLACEMENTS, AND MISFIT-STRAIN ENERGY

The precipitation of alpha '' iron nitride in FeN martensite (containi ng about 5.9 N/100 Fe) is studied as a function of tempering time succ essively at 333 and 373 K by means of X-ray diffractometry (XRD). On t he basis of peak (=lattice parameter) shifts and intensity changes of main and superstructure reflections, it is concluded that, during temp ering, the structure of the formed alpha '' precipitates changes. At 3 33 K, initially stoichiometric Fe16N2 is formed and, with further temp ering, structural vacancies are introduced, i.e., Fe16N2-x is formed. At 373 K, the vacancies are partially filled. The change of the alpha '' structure at 333 K is caused by the change of the matrix from marte nsite to ferrite, i.e., the misfit between the alpha '' and the matrix is initially relatively small and then increases. By allowing nitroge n-deficient alpha '', the increase of the misfit can be diminished at the expense of same increase of the volume of alpha ''. Gibbs free ene rgy calculations (at 333 K), in which differences in the misfit energy are decisive, indeed show that, in a martensite matrix, Fe16N2-x is f avored, whereas, in a ferrite matrix, Fe16N2-x is favored. Further, in this article, methods are presented to calculate structure factors fo r nitrogen-deficient alpha '' and to obtain, from measured (=strained) lattice parameters, strain-free alpha '' lattice parameters from whic h the composition can be deduced.