Materials Transactions Online

Materials Transactions, Vol.50 No.05 (2009) pp.943-947
© 2009 The Japan Institute of Metals

Stacking Faults and a Novel Structural Polytype in a Hydrogen-Storage (La0.8Mg0.2)Ni3.5 Alloy with Block-Stacking Superstructures

Ryo Ishikawa and Eiji Abe

Department of Materials Engineering, The University of Tokyo, Tokyo 113-8656, Japan

We investigated the microstructure of a hydrogen-storage (La0.8Mg0.2)Ni3.5 alloy with block-stacking superstructures by electron diffraction and Z-contrast scanning transmission electron microscopy (STEM), particularly focusing on the type of stacking fault structures and possible occurrence of novel structural variants. It was found that two major phases coexist in the alloy, which are of 5:19-2H type and 5:19-3R type superstructures that are constructed by the common structural blocks but with different stacking sequences. A high density of stacking faults were often observed, most of which were of inter-block-layer type that does not change the intra-block structure but simply alters local stacking sequence of the blocks. As a minor phase in the alloy, we identified a novel polytype structure represented as 5:19-12R, whose long-period block-stacking sequence is described as ABCA'CABC'BCAB'.

(Received 2008/11/10; Accepted 2009/1/19; Published 2009/3/4)

Keywords: hydrogen storage alloy, lanthanum-magnesium-nickel alloy, crystal structure, electron diffraction, high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), polytype, block-stacking superstructure

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