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.