Lattice defects in V–Ti BCC alloys before and after hydrogenation

•Strain contrast with spacing smaller than 20 nm is observed in as-cast and hydrogenated V–Ti alloys.•Twin boundaries and stacking faults are introduced into the V–Ti alloys after hydrogenation.•These planar defects are formed due to accommodation of shear stress during hydrogenation.•Densities of twin boundaries are higher in the alloys with smaller contents of V.•Effective absorbing capacities at ambient condition depend on the densities of twin boundaries.

Microstructures of V–Ti BCC alloys before and after hydrogenation/dehydrogenation are investigated using transmission electron microscopy. Strain contrast with spacing smaller than 20 nm is observed both in the as-cast alloys and hydrogenated alloys. Twin boundaries and stacking faults with spacing of 5–20 nm are introduced into the V–Ti BCC alloys after hydrogenation. These are parallel to {11¯1} planes of FCC hydrides. The twin boundaries and stacking faults are formed due to the accommodation of shear stress during hydrogenation accompanied by large anisotropic expansion along the c-axis direction in V–Ti BCC alloys. Density of twin boundaries is high in the alloys with small contents of V, which have a small effective hydrogen capacity at ambient condition. Dependence of microstructure evolution in the V–Ti alloys during hydrogenation on the V/Ti ratio is also discussed with the thermodynamic stabilities from measurements of Pressure–Composition isotherms and mechanical properties.