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,
