An in-situ neutron diffraction study of the ageing of CaNi5Dx at 80 °C and 9 bar

Abstract

The intrinsic ageing of the CaNi5Dx system has been studied at 80 àand 9 bar over a period of 13 days. The system displays a distinct two-phase mixture of the intermediate a' and ߠphases, whose phase proportions approach 40 and 50 wt.%, respectively at the end of the ageing period. The change in proportion of each intermediate phase and free fcc Ni suggests two periods of Ni depletion. The amount of free Ni in the system is doubled through the ageing period. The orthorhombic symmetry of the a' and ߠphase is preserved, albeit with a strong depletion of three of the four Ni positions from the ߠphase during the second half of the ageing period. That one half of the hexagonally equivalent 2c Ni position does not release Ni indicates that low symmetry geometrical ordering in orthorhombic symmetry is responsible for the rapid loss of recoverable hydrogen capacity in this system.