AN ATOM-PROBE INVESTIGATION OF SOME CORRELATED PHASE-TRANSFORMATIONS IN CR, NI, MO CONTAINING SUPERSATURATED FERRITES

At 300-400-degrees-C aged ferrite of duplex stainless steels may undergo complex demixing processes which depend on the alloying element nature and content. In addition to the spinodal decomposition of the delta ferrite solid-solution into alpha (Fe-rich) and alpha' (Cr-rich) domains, which characterizes the unmixing process, variations in the alloying element content lead to a more or less important precipitation of an intermetallic G-phase. For a given Cr-content, at least four elements (C, Si, Ni and Mo) induce a particular sensitivity to G-phase precipitation. Two duplex stainless steels, the ferrite of which exhibits almost the same C, Si and Cr but different Ni and Mo contents have been carefully studied by atom probe. Because of its high spatial resolution, the atom probe is an attractive tool for the investigation of the fine-scale spinodal decomposition and precipitation process. In order to get both reliable chemical and spatial information, technical improvements as well as refinements in the statistical analysis of the experimental data were implemented. Characterization of both the spinodal decomposition and G-phase precipitation was carried out. Pertinent microstructural aging parameters were defined in such a way as to follow both kinetics. The investigations we conducted, demonstrate that G-phase precipitation is induced by the spinodal decomposition of the ferrite phase. The influence of Ni and Mo contents on G-phase precipitation is discussed.