APT Characterization of Nanometer Scale Features in RPV Steels and Nanostructured Ferritic Alloys: Insight, Challenges and Opportunities

APT tools and techniques, many pioneered by Professor Smith and his Oxford group, have been used for more than 20 years to study nm scale Cu-Mn-Ni-Si precipitates in irradiated RPV steels, and more recently to characterize Y-Ti-O nanofeatures in nanostructured ferritic alloys. While enormously valuable, the APT results have sometimes not been fully consistent with those found by other characterization techniques. We show specific examples of well-known phenomena that standard reconstruction treatments do not treat. Perhaps the most important is the local evolution of the tip morphology due to evaporation field differences leading to variations in the magnification factor and associated trajectory aberrations. For lower evaporation field the topology changes increase the number of atoms nominally in the feature volume. We focus on quantifying the tip morphology evolution, and suggest approaches to improved reconstructions. We also discuss low evaporation field features affects on increase in multiple hits.