ANALYSIS OF THE NITROGEN ABSORPTION ISOTHERMS OF COLD-ROLLED FE-2 AT.PERCENT AL SPECIMENS WITH DIFFERENT ALN PRECIPITATE DIMENSIONS

Nitrogen absorption isotherms of cold-rolled Fe-2 at.% Al have been determined using thermogravimetric techniques to study the uptake of so-called excess nitrogen. The total mass increase by nitrogen absorption has been analysed assuming that nitrogen can be located at four different types of sites: contained in the AlN precipitates, at the precipitate-matrix interfaces, at dislocations and at the octahedral interstices of the ferrite matrix. The nitrogen solubility of the ferrite matrix is a few times larger than the equilibrium solubility as a result of lattice dilatation induced by the misfitting AlN particles. The distribution of nitrogen over these sites can be determined semi-quantitatively from the absorption isotherm data. This distribution depends on the size of the AlN precipitates. The dimensions of the AlN precipitates were varied by means of an annealing treatment at temperatures higher than those used in the absorption isotherm determination. interpretation of the enhanced nitrogen solubility of the iron matrix indicated that a partial transformation from cubic AlN into hexagonal AlN occurred on annealing at a relatively high temperature (1123 K). This partial transformation was confirmed by X-ray diffraction.