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Journal of Electron Microscopy 48(4): 333-338 (1999)
© 1999 Oxford University Press

Changes in the long range ordered structure of Ag-Zn-AI alloys

Hiroyuki Hoshi, Kazuyoshi Takezawa* and Kenzaburo Marukawa

Department of Applied Physics, Faculty of Engineering Hokkaido University, Sapporo 060-8628, Japan

*To whom correspondence should be addressed. E-mail: ktake{at}eng.hokudai.ac.jp

The atomic ordering process in the p phase (bcc structure) of Ag-Zn-Al alloys having a composition near Ag6Zn2Al was examined by means of electron microscopic observations and electrical resistivity measurements. The high temperature disordered structure of this alloy can be frozen-in by rapid cooling to the room temperature. The ordering reaction by ageing at a low temperature near room temperature follows two steps. The first step is a transition to the C11b structure (MoSi2 type), whose unit cell is thrice as large as the one of body-centred cubic (bcc) structure. The ordering kinetics from bcc to the C11b phase can be expressed by the Austin-Rickett equation. The second step of ordering takes place by prolonged ageing, which brings the C11b structure to an orthorhombic one, which is tentatively called as C11b* structure. This structure has the unit cell 3×3 times as large as the bcc unit cell. A transformation strain accompanies the second transition. Tweed patterns are observed by an electron microscope at an early stage of the second ordering. Twins are then induced on (103) plane of the orthorhombic lattice. The twinning seems to effectively relax the transformation strain.

Keywords     Silver-zinc-aluminium alloy, long range order, C11b structure, transformation twin, transformation strain

Received     23 February 1999, accepted 5 April 1999


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