Impurity and texture driven HCP-to-FCC transformations in Ti-X thin films during in situ TEM annealing and FIB milling

A hexagonal close-packed (HCP) to face-centered cubic (FCC) phase transition has been observed in freestanding alpha-titanium (α-Ti) thin foils under two separate conditions: (1) after focused ion beam (FIB) irradiation, and (2) during in situ heating in a transmission electron microscope (TEM). The FCC phase is not found on the Ti single-component equilibrium phase diagram, however, both FCC structures were found to be stable after formation under these conditions. Here, we combine analytical TEM and Atom Probe Tomography (APT) investigations into the chemical nature of these anomalous FCC Ti-X phases. Both occurrences of the FCC phase were observed in thin films containing (initial) prismatic HCP surface plane texturing and appear to be facilitated by hydrogen and oxygen impurities. Our results suggest that the FIB-induced FCC Ti-X is a form of titanium hydride (δ-TiH2 and/or γ-TiH), while the thermally-induced FCC Ti-X appears to be tied to the incorporation of oxygen.