The temperature dependence of the Young's modulus and of the internal friction (IF) was investigated between 80 and 300K at acoustical frequencies in the hexagonal (C14) Laves-phase TiCr1.78 charged with hydrogen. In this compound H occupies tetrahedral interstitial sites which were grouped in interlinked hexagons. A mechanical relaxation was found at around 120K (f = 5.4 kHz), which appears to be due to tunnelling transitions of delocalized H from one hexagon to the other. The rate of H absorption was investigated at high temperature (660–1200K) and the H diffusion coefficient was derived from the pressure measurement as a function of time. A cumulative Arrhenius plot of IF and absorption diffusion data exhibits a non-exponential behaviour, which was due to a change in the diffusion mechanism from over-barrier hopping at high temperature to phonon-assisted tunnelling at low temperature.

Hydrogen Diffusion in the Laves-Phase Compound TiCr1.78. G.Mazzolai, B.Coluzzi, A.Biscarini, F.M.Mazzolai, A.Tuissi, F.Agresti, G.Principi, S.L.Russo: Materials Science and Engineering A, 2009, 521-522, 139-42