Cyclic hydrogenation stability of γ-hydrides for Ti₂₅V₃₅Cr₄₀ alloys doped with carbon

An automatic Sievert's apparatus equipped with a temperature-programmed desorption spectrometer was constructed to study the stability of annealed Ti₂₅V₃₅Cr₄₀C_x (x = 0 and 0.1) alloy under cyclic hydrogenation at 6 N H₂. The specimens were tested at 30 °C with a hydrogen loading of around 1.00 H/M, which enabled the phase transformation from β-to γ-hydrides. After 500 cycles, 83% and 90% of the initial hydrogen capacities were preserved for Ti₂₅V₃₅Cr₄₀ and Ti₂₅V₃₅Cr₄₀C_0.1, respectively. Therefore, a small amount of C doping was effective in reducing the hydrogenation degradation of Ti₂₅V₃₅Cr₄₀. The hydrogenation degradation of Ti₂₅V₃₅Cr₄₀ was examined by measuring the P–C isotherms, temperature-programmed desorption spectra, and X-ray diffraction patterns. The degradation was ascribed to intrinsic disproportionation, i.e., Ti_0.25V_0.35Cr_0.40 + 0.88H₂ → yTiH₂ + Ti_0.25−yV_0.35Cr_0.40H_1.76–2y, where the coefficient y indicates the amount of Ti-rich precipitate. The better cyclic hydrogenation stability of Ti₂₅V₃₅Cr₄₀C_0.1 was related to the suppression of intrinsic disproportionation by the presence of carbon atoms in the body-centered-cubic lattice. - Highlights: • The stability of γ-hydride for Ti₂₅V₃₅Cr₄₀ alloys was examined for 500 cycles. • The γ-hydride of Ti₂₅V₃₅Cr₄₀ alloy degraded by intrinsic disproportionation. • The disproportionation of γ-hydride can be suppressed through carbon inclusion