COMPOSITION AND MORPHOLOGY OF FE-N OFF-EQUILIBRIUM PHASES IN A NITRIDED FE-1.5WT.PERCENT-MO SINTERED ALLOY

Specimens of an Fe-1.5wt.%Mo sintered at 6.9 and 7.5 g cm(-3) were sub jected to both gas and ion nitriding treatments, under different opera ting conditions (gas composition, length of treatment and temperature) . The composition, morphology and distribution of the Fe-N phases were investigated by means of optical and scanning electron microscopy, el ectron probe microanalysis and X-ray diffraction. The Fe-N reactivity effects were shown and explained on the basis of both the surface chem ical potential and the diffusion potential, taking into account the pe netration of nitriding media through open porosity, grain boundaries a nd volume. No effect of Mo-N interactions or Mo segregation could be o bserved. Significant differences were found between gas and ion nitrid ing with regard to phase composition and thickness of surface compound layers as well as depth and features of diffusion zones. In particula r it was shown that the material density plays an important role in th e reaction mechanisms characterizing gas treatments.