Formation mechanisms of hydrides in environmentally sensitive materials: Ti-based alloys

 Although the phenomenon was first recognized several decades ago [1], hydrogen-embrittlement remains an extremely important unsolved engineering challenge. High-performance Ti alloys are particularly prone to early fracture and reduced toughness in the presence of hydrogen or hydrides [2], which limit their possible manufacturing routes and industrial applications, leading to a risk of premature failure despite having potential for hydrogen storage [3]. The investigation of hydrogen within Ti alloys is hence essential in order to further understand the underlying mechanisms of hydrogen embrittlement and their consequences for hydrogen storage. In our work, we combined jet electro-polishing, cryogenic-focus ion beam (cryo-FIB), high angle annular dark-field scanning transmission electron microscopy (STEM), EELS and APT to show that hydride formation can originate from sample preparation, e.g., through electrochemical reactions during electro-polishing instead of the formation of a new “FCC Ti” phase (Figure 1) [4]. We further combined cryo-FIB, STEM and APT to prove that the standard site-specific FIB lift-out processes are not suitable to prepare Ti-based alloys due to the unexpected hydride formation caused by FIB milling at room temperature [5]. Lowering the temperature to -135 ^oC in the cryo-FIB is hence key to avoid hydride formation during sample preparation. This project has been realized in collaboration with Imperial College London in UK, RWTH Aachen and the APT group lead by Baptiste Gault in the department of Prof. Dierk Raabe (MPIE).