Hydrogen-storage capacities and H diffusion in bcc TiVCr alloys

The H mass density of the bcc Ti35Cr65−xVx alloys (x = 18, 22) has been investigated as a function of pressure at various temperatures by tracing P–C isotherms from which the formation enthalpy of the γ hydride has been derived (ΔH = −39 ± 2 kJ/mol for the Ti35Cr43V22 alloy and ΔH = −35 ± 6 kJ/mol for the Ti35Cr47V18 alloy). Internal friction measurements have revealed the existence of a broad H Snoek-type of relaxation at around 150 K (f ≅ 1 kHz), which has been used to deduce information on H mobility. Combined Snoek and high-temperature absorption data have provided the following values for the H diffusion parameters W and D0: W = 0.32 ±0.01 eV; D0 = (4 ± 2) × 10−7 m2/s (Ti35Cr47V18 alloy) and W = 0.34 ± 0.01 eV; D0 = (2 ± 0.5) × 10−7 m2/s (Ti35Cr43V22 alloy). X-ray diffraction patterns have confirmed the fcc lattice structure of the hydride while differential scanning calorimetry and thermal desorption spectroscopy data exhibited peaks along the temperature scale which could be attributed to structural transitions of the hydrides. The occurrence of these transitions is accounted for in terms of a previously proposed atomistic model.