In situ SEM-based microstructural characterization of hydrogen effects in titanium alloys

Abstract

The role of hydrogen (H) in titanium alloys is important crucial in two aspects: (i) H embrittlement in structural titanium alloys and (ii) H storage capability in the form of solid H. In both of these aspects, the diffusion, trapping of H, and formation of hydrides within the alloy microstructures are crucial for understanding the mechanisms. However, it is challenging to observe and analyze the H effects in titanium alloys, even if it accompanies obvious phase transformation, due to the complexity of H interactions with various defects and constituents in the alloy microstructures. To overcome these difficulties, we developed novel in situ H-charging techniques that are compatible with scanning electron microscopy (SEM). The first generation of the in situ H-charging setups employs an electrochemical cell to insert H into a disc-type bulk specimen, which allows investigating the effect of H-ingress in a relatively short time scale but only at room temperature. The second generation of techniques utilizes a microscale heater and H gas environment, which would enable to study H-induced microstructural evolution in a pressurized H environment at an elevated temperature up to 1073K. These SEM-compatible and versatile techniques enable real-time imaging, crystallographic analysis, and micromechanical analysis during H-ingress into a specimen. Through several case studies on titanium-based H-storage alloys and structural alloys, we demonstrate how in situ SEM techniques can be used to address the complex challenge of H-induced effects on material microstructures.