Journal
ACTA MATERIALIA
Volume 50, Issue 15, Pages 3845-3857Publisher
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/S1359-6454(02)00191-X
Keywords
rapid solidification; high angle annular dark field; scanning/transmission electron microscopy; magnesium alloys; crystal structure
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The microstructure of a nanocrystalline Mg97Zn1Y2 (at%) bulk alloy prepared by warm extrusion of rapidly solidified powders has been investigated by a combination of techniques, such as conventional high-resolution transmission electron microscopy (HRTEM), atomic-resolution high-angle annular dark field scanning-TEM (HAADF-STEM) with Z-contrast and energy-dispersive X-ray spectroscopy (EDS) with a sub-nanometer electron probe. We show that a novel long-period ordered structure is formed in the alloy, whose unit cell is composed of six close-packed planes of the magnesium crystal with a stacking sequence of ABCBCB' where A and B' layers are significantly enriched by Zn and Y. The lattice is distorted from an ideal hexagonal lattice of 6H-type (ABCBCB), which is probably due to an asymmetry of the chemical order with respect to the 6H-type stacking order. The present results demonstrate that the additional elements of a few atomic percent to Mg lead to formation of a long-period chemical-ordered as well as stacking-ordered structure, as directly revealed by a unique Z-contrast method. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.
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