Grain boundary study of technically pure molybdenum by combining APT and TKD

Molybdenum is an eligible material for high performance applications. However, its applicability is limited because of a brittle-to-ductile transition around room temperature, depending on the grain size and the content of interstitial impurities present at grain boundaries. The total amount of impurities in the current quality of molybdenum has become very small in the last decades. Therefore, the atom probe with its atomic resolution is the only suitable site-specific analysis technique. Nevertheless, a site-specific specimen preparation by focused ion beam (FIB) is required to study the grain boundary chemistry effectively. With a novel method, which combines re-sharpening of pre-electro-polished tips by FIB with transmission Kikuchi diffraction (TKD), a grain boundary can easily be positioned in the first 200 nm of an atom probe sample. Furthermore, the high resolution technique of TKD gives the opportunity to get crystallographic information of the mapped area and, therefore, an analysis of the grain boundary character to support the interpretation of the atom probe data files.
In the present study, APT specimens of technically pure molybdenum which contain grain boundaries were prepared by FIB in support of TKD and subsequently were measured in the atom probe. The difference of segregation content at unequal types of grain boundaries in the as-deformed and recrystallized state is discussed.