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KENJI MATSUDA, YASUHIRO UETANI, TATSUO SATO, and SUSUMU IKENOThe QЈ phase in an Al-1.0 mass pct Mg 2 Si-0.5 mass pct Cu alloy at a peak-aged condition of 523 K was observed by a high-resolution transmission electron microscope (HRTEM), in order to identify its structure and chemical composition, and was compared with the type-C precipitate in an Al-1.0 mass pct Mg 2 Si-0.4 mass pct Si alloy (the excess-Si alloy). The QЈ phase has similar features to the type-C precipitate, according to results of electron diffraction patterns and images taken by the HRTEM, that is, they have similar hexagonal crystal lattices (a ϭ 1.04 nm and c ϭ 0.405 nm) to each other. The type-C precipitate in the excess-Si alloy was the ternary Al-Mg-Si phase, and the QЈ phase was the quaternary Al-Mg-Si-Cu phase in the Al-Mg-Si-Cu alloy, as determined by energydispersive X-ray spectroscopic (EDS) analysis. KENJI MATSUDA, Associate Professor, and SUSUMU IKENO, Professor, are with the Faculty of Engineering, Toyama University, Toyama, chosen to obtain the peak-aged condition at 523 K for each 930-8555 Japan. YASUHIRO UETANI, Associate Professor, is with the alloy. The TEM samples were made by a twin-jet electrical Research Institute of Technology, Toyama Prefectural University, Toyama, polishing technique using a nitric acid-methanol solution. 939-0398 Japan. TATSUO SATO, Professor, is with the Faculty of Engi-The micro Vickers hardness of each alloy was also measured,

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The structure of the Guinier-Preston (GP) zone in an Al-1.6 mass pct Mg 2 Si alloy aged at 343 and 423 K was investigated by high-resolution transmission electron microscopy (HRTEM). The morphology of the GP zone is that of a fine plate having the dimensions of a monolayer in thickness, 2.5 nm in width, and less than 30 nm in length. Its elongated direction is parallel to the ͗100͘ m direction. The chemical composition of the GP zone in an Al-Mg 2 Si alloy indicated a ratio of Mg/Si ϭ 1.0. The zones aggregate with increasing aging time and exhibit a more complicated morphology because of the transformation to the metastable phase that takes place inside the GP zones.K. MATSUDA, Research Associate, and S. IKENO, Professor, are with

KENJI MATSUDA, SUSUMU IKENO, HIROAKI MATSUI, TATSUO SATO, KIYOSHI TERAYAMA, and YASUHIRO UETANI Differential scanning calorimetry (DSC) curves were obtained from Al-1.0 mass pct Mg 2 Si (balanced) and Al-1.0 mass pct Mg 2 Si Ϫ0.4 mass pct Si (excess Si) alloys, and precipitates corresponding to each peak at the DSC curve were interpreted by means of high-resolution transmission electron microscopy (HRTEM) observation in order to understand the precipitation sequence of metastable phases. Five peaks were obtained on the DSC curves, from which four were exothermic (A, C, D, and E) and one endothermic (B). Upon HRTEM observation, the peaks for the excess Si alloy were explained as follows: peak A-B: Guinier-Preston (GP) zones and random-type precipitates; peak B: dissolution of the GP zones and the random-type precipitates, precipitation of the ␤ЈЈ phase; peak C: ␤ЈЈ phase and precipitation of type B; peak D: dissolution of the ␤ЈЈ phase; precipitation of type A and ␤Ј phase; and peak E: dissolution of the type B, type A, and ␤Ј precipitation of the (␤ ϩ Si) phase. This result is quite different from that in the balanced alloy as follows: peak A-B: GP zones and random-type precipitates; peak B: dissolution of the GP zones and the random-type precipitates, precipitation of the parallelogram-type precipitate; peak C: parallelogram-type precipitate and precipitation of ␤Ј phase; peak D: ␤Ј phase, dissolution of parallelogram-type precipitate; and peak E: the ␤-(Mg 2 Si).