Hydrogen-storage capacities and H diffusion in bcc TiVCr alloys

Abstract The H mass density of the bcc Ti 35 Cr 65− x V x 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 (Δ H  = −39 ± 2 kJ/mol for the Ti 35 Cr 43 V 22 alloy and Δ H  = −35 ± 6 kJ/mol for the Ti 35 Cr 47 V 18 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 D 0 : W  = 0.32 ±0.01 eV; D 0  = (4 ± 2) × 10 −7  m 2 /s (Ti 35 Cr 47 V 18 alloy) and W  = 0.34 ± 0.01 eV; D 0  = (2 ± 0.5) × 10 −7  m 2 /s (Ti 35 Cr 43 V 22 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.