Defects Formation in LaNi 5 -based Alloys Investigated by In-situ X-ray Diffraction
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Defects Formation in LaNi5-based Alloys Investigated by In-situ X-ray Diffraction Yumiko Nakamura and Etsuo Akiba National Institute of Advanced Industrial Science and Technology (AIST) AIST Central-5, 1-1-1, Higashi, Tsukuba, Ibaraki, 305-8565, Japan ABSTRACT Defects formation in LaNi5-based alloys was investigated by X-ray diffraction (XRD). In-situ XRD data measured along the first hydriding-dehydriding P-C isotherms were analyzed by the Rietveld method. Lattice strain and crystallite size were evaluated from the peak profile. In LaNi5 hydride phase formed accompanying with 2 % of anisotropic lattice strain in direction caused by dense dislocations. Coexisting solid solution phase was not affected by the strain in the hydride phase. Lattice parameter a became smaller after cycles than before hydriding, which is related with vacancies formation. Only 5 % of Al substitution for Ni dramatically changed the defects formation behavior. Hydride phase did not show lattice strain in the first hydriding but showed small strain during dehydriding. Formation of both dislocations and vacancies are strongly affected by substitution of other elements for a part of Ni. INTRODUCTION LaNi5 is a representative intermetallic compound which has a reversible hydrogen absorbing property under ambient condition. The alloy phase transforms into hydride phase through two-phase coexisting region. The lattice parameters and the cell volume increase by 7 % and 24 % in the transformation from the alloy phase to the hydride phase, respectively. The hydriding reaction with such large lattice expansion in absorption and contraction in desorption could induce some kinds of lattice defects. Defects formation in hydriding has been investigated by several methods; X-ray or neutron diffraction, transmission electron microscopy (TEM) and positron annihilation lifetime spectroscopy (PALS). Diffraction and PALS measurements have been successfully performed under hydriding conditions [1-3] as ‘in-situ’ measurements as well as ‘ex-situ’ measurements. PALS technique can detect even a low concentration of defects and analytically distinguish kinds of defects contained [3], but it cannot distinguish a phase containing the defects in multi-phase system. Comparing with that, diffraction method is suitable to observe defects in multi-phase system though the sensitivity is less than PALS technique. In this method lattice strain or lattice contraction caused by defects formation are observed in each phase as well as phase transformation and structural change. In this paper, results of lattice strain formation in the first hydriding-dehydriding of LaNi5-based alloys investigated by in-situ XRD are reported. XRD data are measured at several hydrogen contents along P-C isotherms. They are analyzed by the Rietveld method where lattice
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strain and crystallite size are evaluated from the peak profile. The lattice strain formation is related with dislocations observed by TEM. Vacancies formation is also discussed by lattice parameter change after c
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