Vanadium (V) with the bcc structure is a promising material for hydrogen separation and purification because it exhibits higher hydrogen permeability than practical Pd-based alloy membranes. However, V is a metal that exhibits significant hydrogen embrittlement. In order to overcome this negative physical property, grain refinement by high-pressure torsion (HPT) and high-pressure sliding (HPS) processing methods is attempted in this research. These large strain processes are also expected to result in high hydrogen permeability at the same time. In this study, attention is focused on enhancing the hydrogen permeation performance by effectively utilizing the characteristics of grain boundaries and strains (or dislocations) that act as fast diffusion paths for hydrogen.

Based on the structural information obtained by FE-SEM/EBSD analysis and X-ray diffraction profile of V after HPT or HPS processing, the relationship between deformation microstructure, grain size and processing conditions can be investigated in detail. The giant-strained V with submicron grains used in the experiment is disc-shaped with a diameter of φ12 and thicknesses of 0.5 and 0.7 mm. The hydrogen permeation test is performed after heating to 300°C.

The microstructure of the V metal membrane is composed of high-density dislocations, random small-angle grain boundaries, large-angle grain boundaries, and coincidence boundaries. Therefore, if some grain boundary character distributions are also analyzed for the purpose of clarifying the effects on hydrogen diffusion behaviour in V, the characteristics of high-performance hydrogen separation membranes can be found. As a result, we try to understand and quantify the qualitative distribution of the amount of strain introduced near the surface of V subjected to huge straining, and discuss the relationship with hydrogen permeability. These studies will clarify the structure of grain boundaries and the characteristics of dislocations that act as fast diffusion paths for hydrogen.