MOLYBDENUM-NITROGEN AND MOLYBDENUM-CARBON COMPLEX FORMATION IN STEELS

The austenitic phase of four nitrogen containing duplex austenitic-ferritic stainless steels was studied by microhardness measurements and atom probe analysis.The hardness increased with nitrogen content, and atom probe analysis showed that a large fraction of the nitrogen atoms appeared as MoN molecular ions.It is argued that the observation of such molecular ions is due to the presence of Mo-N complexes in the austenite.Few CrN ions were observed, but the existence of Cr-N complexes cannot be ruled out.Comparison with earlier data from the martensitic and femtic matrices of high speed steels suggest that also in these a complex formation may occur: ions of the type MoC were sometimes observed, although to a much lesser extent than MoN in austenite.In the latter case, between 70 and 100% of the least abundant of Mo and N was observed as MoN.Although one of the outstanding merits of field ion microscopy is its ability to image a specimen with atomic resolution, it is often difficult to determine the exact atomic arrangement in materials from field ion micrographs.This is so since the contrast in the field ion image is determined not only by the chemical identity of the surface atoms but also by the position of the atoms relative to the specimen surface.The exact shape of the surface is in turn determined by the field evaporation process, which is sensitive both to the geometry and the chemical identity of the atoms.In practice, this restricts the use of field ion microscopy for exact atomic mapping to certain special cases; the detailed mapping of vacancy distributions in irradiated specimens is perhaps the most beautiful example [1,2].The spatial resolution of atom probe analysis is generally limited to about 1 nrn, so the sequence of recorded atoms during fied evaporation does not directly correspond to the arrangement of atoms in the specimen.Layer by layer field evaporation can give accurate data on short range ordering, but it does not directly give information on the atomic arrangement within each plane.However, it is suggested in this paper that when certain molecular ions are detected by the atom probe, these may be assumed to have occupied adjacent positions in the lattice.In these cases it should therefore be possible to detect complex or clustering formation using the atom probe.Many different sorts of molecular ions have been observed by atom probe.Of interest in this context are in particular ions containing C, N and 0, Oxygen very often forms metal-oxygen ions when present in the material, whereas carbon seldom appears as carbide ions (except molecular C2, C3 and CN).Nitrogen lies in between and forms nitride ions readily with some metals, reluctantly with others.Most of this paper concerns an atom probe investigation of the austenite phase in a series of nitrogen containing duplex stainless steels.The results are interpreted in terms of nitrogen complex formation, and the effect of such complexes of the strength of the steels are discussed.In addition, some data from an earlier study of high speed steels are also discussed and interpreted in terms of carbon complex formation.