Characterization of H/D in Ti and its alloys with atom probe tomography

Ti and Ti‐based alloys have high affinity for hydrogen and can easily adsorb large amounts of hydrogen during manufacturing as well as in service. Severe problems may arise based on the extensively reported susceptibility of titanium alloys to both internal and environmental hydrogen embrittlement (HE). Atom probe tomography (APT), as a mass spectroscopy technique with near‐atomic spatial resolution, enables to characterize and visualize the 3D distribution of H within engineering materials. H was visualized as solute, hydride, and segregation at interfaces in a set of Ti‐alloys by APT, combined with transmission electron microscopy (TEM) and atom probe crystallography. However, some major challenges must be overcome when aiming at characterizing the ‘real’ H/D distribution at the near‐atomic scale within materials by APT and further identifying the underlying HE mechanisms. Firstly, a critical issue lies in the significant H ingress and hydride formation during the fabrication of specimens for APT measurement by conventional techniques. It is almost impossible to distinguish between H introduced during sample preparation and H originally within the material. Cryogenic focused‐ion beam (cryo‐FIB) is proved to efficiently inhibit the introduction of H from the environment during specimen preparation, and meanwhile, prevent out‐diffusion of preexisting H/D previously charged into the material. By using cryo‐FIB for sample preparation and semi‐correlative APT/TEM for chemical/structural characterization, the controversial face centered‐cubic (FCC) phase in Ti was unambiguously identified as FCC hydride instead of a new allotrope of Ti. Secondly, erroneous compositional measurements of titanium hydride and the influence of residual gas inside the analysis chamber on the quantification of H by APT have been recognized. The quantitativeness of H/D measurements by APT was then evaluated by systematically analyzing the composition of stable titanium deuteride (TiD~2, stoichiometry of 65.0‐66.6 at. % D) across a range of experimental conditions. It was found that the measurement of H/D by APT varies significantly with the intensity of the surface electrostatic field both globally and locally across the specimen surface. Experimentally, APT analyses with high‐voltage pulsing was recommended for measuring H/D as it is relatively independent of the applied experimental parameters and the local D composition is much more homogeneous, although approx. 15‐20at. % off from the nominal composition. At last, using cryo‐FIB for specimen preparation and APT with high‐voltage pulsing for atomic scale characterizing the 3D distribution of D and other matrix elements, the influence of D prechargingat ambient temperature and at high temperature on the chemistry and phase composition of the commercial alloy Ti6246 was investigated. With D pre‐charging at ambient temperature, γ(TiD)‐type deuteride formed, and Al was retained within the deuteride phase inTi6246 alloy. When exposed to gaseous D environment at high temperature, two types of deuteride, TiD~2 and C15‐type Laves phase Ti2ZrD~4, formed, and Al was repelled from the deuteride phases into the surrounding matrix causing transformation into α2‐Ti3Al or other intermetallic compounds TixAl