High-resolution electron microscopy on the structure of Guinier-Preston zones in an Al-1.6 mass Pct Mg 2 Si alloy
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II.
INTRODUCTION
IT has been well known that the nuclei of metastable phases and composition fluctuation are found in an AlMg2Si alloy during the early stage of aging.[1,2,3] In Table I, previous studies on the aging sequence of this alloy system before the equilibrium phase appears are summarized. The specific heat and the electrical resistivity measurements have been performed over the entire range of aging temperatures, and the existence of clusters and Guinier–Preston (GP) zones have been proposed.[1–5] However, the structures of the zones formed during aging were identified only in a limited range of aging temperatures higher than 423 K using X-ray and electron diffraction techniques.[2,6–11] In our recent systematic work, new types of metastable phases were found in this alloy system. The morphologies, crystal structures, and crystallographic orientation relationship with the matrix of the metastable phases in this alloy system have been clarified using high-resolution transmission electron microscopy (HRTEM), a microbeam electron diffraction technique, and energy-dispersive X-ray spectroscopy (EDXS).[12,13,14] The changes in the number of each metastable phase have also been reported.[15] However, we have not yet clarified the structure and chemistry of clusters or GP zones during the early stage of aging. The aim of this study is to clarify the structure and chemical composition of the GP zones in an Al-1.6 mass pct Mg2Si alloy aged at temperatures lower than 423 K using HRTEM.
K. MATSUDA, Research Associate, and S. IKENO, Professor, are with the Department of Materials Science and Engineering, Toyama University, Toyama 930, Japan. H. GAMADA, formerly Graduate Student, Department of Materials Science and Engineering, Toyama University, is with the Department of Research and Development, Hokuriku Fabrication Center, Shin-Nikkei Co. Ltd., Toyama 933, Japan. K. FUJII, formerly Graduate Student, Department of Materials Science and Engineering, Toyama University, is Researcher, Industrial Research Institute of Ishikawa, Ishikawa 920-02, Japan. Y. UETANI, Associate Professor, is with the Research Institute for Technology, Toyama Prefectural University, Toyama 939-03, Japan. T. SATO, Associate Professor, and A. KAMIO, Professor, are with the Department of Metallurgical Engineering, Tokyo Institute of Technology, Tokyo 152, Japan. Manuscript submitted August 18, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
EXPERIMENTAL
An Al-1.6 mass pct Mg2Si alloy was prepared using 99.99 pct pure aluminum, 99.9 pct pure magnesium, and 99.9 pct pure silicon. The ingot obtained was formed into 0.2-mm thick sheets by hot and cold rolling. A solution heat treatment was performed at 848 K for 3.6 ks and was followed by quenching in chilled water (273 K). The aging treatment was performed at 343 and 423 K. Thin specimens for HRTEM were prepared by ordinary electrolytic polishing and were observed using the EM-002B (Topcon Co., Ltd., Tokyo, Japan) operated at 100 kV. Electrical resistivities of the specimens were measured usin
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