Metastable phases in an Al-Mg-Si alloy containing copper
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DUCTION
IT is well known that the addition of copper to Al-MgSi alloys improves their mechanical properties, especially their ductility.[1] The metastable phase, however, has not yet been clarified in Al-Mg-Si-Cu alloys. Recently, the metastable phase containing copper has become known as the B⬘ phase, which Edwards et al.[2] have reported as behaving like the  ⬘ phase in Al-Mg-Si alloys. Arnberg and Aurivillius[3] proposed a crystal structure for h-AlCuMgSi and determined its chemical composition to be Al4Cu2Mg8Si7. They also reported that the B⬘ phase formed in this alloy is the same as the  ⬘ phase in a Cu-free Al-Mg-Si alloy. Dumolt et al.[4] reported that the crystal structure of B⬘ phase in the 6061 aluminum alloy has an orthorhombic structure or hexagonal one. These two types of precipitates have also been identified as being the type-B and type-C precipitates in our recent report,[5] which used high-purity Al-Mg-Si alloys without copper. Especially, it should be noticed that both precipitates appear in the high-purity Al-Mg-Si alloy without copper and contain no copper themselves. Laughlin and Miao[6] concluded, in their report on automotive 6000-series alloys, that all the Al-Mg-Si-Cu family alloys normally contain the quaternary phase Q. The Q⬘ phase is a metastable version of the Q phase and has the same crystal structure as the equilibrium Q phase. The Q⬘ phase has a lath morphology and a hexagonal structure, with the orientation of the long axis being parallel to the 具100典Al directions and {150}Al habit planes of the matrix.[6] Suzuki et al.[7] also reported that the Q⬘ phase existed in Al-Mg-Si alloys containing copper but did not determine its crystal structure. Perovic et al.[8] concluded that the AA6111 aluminum alloy has the  ⬙ and Q phases in the peak-aged condition, although the AA6016 aluminum alloy contains only one precipitate phase, identified as  ⬙. The AA6111 aluminum alloy used by them contains copper at about 0.75 mass pct; therefore, copper is an important element for precipitation of the quaternary AlMg-Si-Cu phase. KENJI MATSUDA, Associate Professor, and SUSUMU IKENO, Professor, are with the Faculty of Engineering, Toyama University, Toyama, 930-8555 Japan. YASUHIRO UETANI, Associate Professor, is with the Research Institute of Technology, Toyama Prefectural University, Toyama, 939-0398 Japan. TATSUO SATO, Professor, is with the Faculty of Engineering, Tokyo Institute of Technology, Tokyo, 152-8552, Japan. Manuscript submitted December 27, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
Recently, Caylon et al. have reported an interesting model for the Q, QC, and QP phases in the Al-Cu-Mg-Ag alloy and its composite materials.[9] They also extended their model to the metastable phases in Al-Mg-Si alloys. According to their report, metastable phases in Al-Mg-Si alloys can contain ternary elements, namely, Al, Mg, and Si, in the same way as the formation of the quaternary Q phase in Al-Cu-MgSi alloys. We have already reported on the precipitation sequence of Al-Mg-Si alloys containi
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