Gallium-enhanced phase contrast in atom probe tomography of nanocrystalline and amorphous Al-Mn alloys

Over a narrow range of composition, electrodeposited Al-Mn alloys transition from a nanocrystalline structure to an amorphous one, passing through an intermediate dual-phase nanocrystal/amorphous structure. Although the structural change is significant, the chemical difference between the phases is subtle. In this study, the solute distribution in these alloys is revealed by developing a method to enhance phase contrast in atom probe tomography (APT). Standard APT data analysis techniques show that Mn distributes uniformly in single phase (nanocrystalline or amorphous) specimens, and despite some slight deviations from randomness, standard methods reveal no convincing evidence of Mn segregation in dual-phase samples either. However, implanted Ga ions deposited during sample preparation by focused ion-beam milling are found to act as chemical markers that preferentially occupy the amorphous phase. This additional information permits more robust identification of the phases and measurement of their compositions. As a result, a weak partitioning tendency of Mn into the amorphous phase (about 2 at%) is discerned in these alloys. -- Highlights: → Atom probe tomography was used to examine solute distribution in Al-Mn alloys. → These alloys comprise different amounts of nanocrystalline and amorphous phases. → Standard analysis methods show uniform Mn distribution, even in the two-phase alloys. → However, implanted Ga ions were found to segregate to the amorphous phase. → Using Ga as phase identifiers, the subtle partition tendency of Mn was discerned.