Despite diligent work on titanium hydrides the crystal structures of some titanium hydride phases are still not well established. Transmission electron microscopes (TEMs) can support crystallographic data by direct imaging of unknown structures with superior resolution. Direct imaging of hydrogen atomic columns is extremely challenging due to their weak scattering. However, an introduction of TEMs with corrected spherical aberration has made such experiments feasible. In this work, direct imaging of hydrogen atomic columns in titanium hydrides was performed in order to provide additional support to incomplete crystallographic data for titanium hydrides.
The commercially pure titanium grade 2 annealed at 900 °C for 1 h was examined in this work. Samples for transmission electron microscopy (TEM) were prepared by mechanical grinding of 3 mm discs and subsequent thinning by twin-jet electropolishing in perchloric acid solution using Struers TenuPol-5. The titanium grade 2 contains 0.24 at.% of hydrogen, which can create hydride phases. Face-centered tetragonal γ-hydride phase was found in a lentil-like shape in the α-Ti matrix and in a shape of long thin lamellae at either the α-Ti matrix/β-Ti phase interface or originating at α-Ti matrix grain boundaries. This phase introduces hydrogen-induced diffracted reflections that do not coincide with the diffracted reflections coming from Ti atoms. For that reason, the presence of the hydrogen can be observed even using the selected area electron diffraction (SAED) technique. The hydrogen-induced reflections translate also to a phase-contrast pattern (PCP). Compared to the titanium columns, the hydrogen columns produce much broader peaks/dips that enable us to differentiate between the positions filled with hydrogen and the vacant columns in the structure.
Annular bright-field scanning transmission electron microscopy (ABF-STEM) and high resolution transmission electron microscopy (HRTEM) techniques were employed. As both techniques rely on phase-contrast imaging, they can produce imaging artifacts that can be easily mistaken for genuine structural features caused by the presence of the hydrogen. To increase robustness of our results we support them with simulations based on theoretical models of the hydrides. Multislice method was used to simulate the dynamic scattering effects. This helped us to find optimal imaging conditions with regard to resolution, maximal contrast for the hydrogen positions and the absence of imaging artifacts for a wide range of thickness and defocus values.
The project was supported by GACR GBP108/12/G043.
To cite this abstract:Kamil Daněk, Viera Gärtnerová, Martin Němec; Direct imaging of hydrogen atomic columns in hydride phases in titanium grade 2. The 16th European Microscopy Congress, Lyon, France. https://emc-proceedings.com/abstract/direct-imaging-of-hydrogen-atomic-columns-in-hydride-phases-in-titanium-grade-2/. Accessed: December 6, 2019
EMC Abstracts - https://emc-proceedings.com/abstract/direct-imaging-of-hydrogen-atomic-columns-in-hydride-phases-in-titanium-grade-2/