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

• 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.

An automatic Sievert's apparatus equipped with a temperature-programmed desorption spectrometer was constructed to study the stability of annealed Ti25V35Cr40Cx (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 Ti25V35Cr40C0.1, 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., Ti0.25V0.35Cr0.40 + 0.88H2 → yTiH2 + Ti0.25−yV0.35Cr0.40H1.76–2y, where the coefficient y indicates the amount of Ti-rich precipitate. The better cyclic hydrogenation stability of Ti25V35Cr40C0.1 was related to the suppression of intrinsic disproportionation by the presence of carbon atoms in the body-centered-cubic lattice.

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