Microstructural details of hydrogen diffusion and storage in Ti–V–Cr alloys activated through surface and bulk severe plastic deformation

Structural observations were carried out on particles obtained after hydrogenation cycling of the Ti25V50Cr25 and Ti10V75Cr15 alloys processed by surface or bulk severe plastic deformation using the surface mechanical attrition treatment (SMAT) and high-pressure torsion (HPT) techniques, respectively. The produced particles differ in morphologies and fracture mode due to the differences in hydrogen diffusion paths. The fracture mode for the SMAT-processed samples with the gradient microstructure was mainly intragranular, whereas it was intergranular for the nanograined HPT processed samples. Hydrogen diffusion, which initiated at the grain boundaries on the surface, created Ti-rich and V-lean areas. The powders contained mainly β-VH monohydride and partly γ-VH2 dihydride, and an orientation relationship of (100)β//(110)γ and [001]β//[001]γ with an angular deviation of ∼2.5° was observed between the two phases using the electron backscattered diffraction (EBSD) analysis.