The Effects of Low Cu Additions and Predeformation on the Precipitation in a 6060 Al-Mg-Si Alloy
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THE 6xxx series (i.e., Al-Mg-Si alloys) of wrought Al alloys are heat-treatable materials used in many industrial applications on the basis of their characteristic properties: high strength-to-weight ratio, good formability, and corrosion resistance. Their main property is a significant increase in hardness during a final isothermal heat treatment (artificial aging) at an adequate temperature and time. This is due to the formation of large numbers of nanosized semicoherent metastable precipitates which yield interfacial strain into the Al matrix, hindering dislocation movement. The interfacial strain originates from the different atomic matchings at the interface, which ultimately depends on the precipitate structure (type). Therefore, alloy properties depend highly on the precipitate types and the microstructures they produce (sizes, numbers, orientations, etc.). The precipitates are formed from solid solution and are controlled by the alloy composition and thermomeTAKESHI SAITO, Ph.D. Candidate, and RANDI HOLMESTAD, Professor, are with the Department of Physics, Norwegian University of Science and Technology (NTNU), 7491 Trondheim, Norway. Contact e-mail: [email protected] SHINJI MURAISHI, Assistant Professor, is with the Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan. CALIN D. MARIOARA, Research Scientist, and SIGMUND J. ANDERSEN, Senior Scientist, are with the SINTEF Materials and Chemistry, 7465 Trondheim, Norway. JOSTEIN RØYSET, Principal Research Scientist, is with the Hydro Aluminum Research and Technology Development, 6601 Sunndalsøra, Norway. Manuscript submitted October 11, 2012. Article published online April 30, 2013 4124—VOLUME 44A, SEPTEMBER 2013
chanical history. The 6xxx alloys contain relatively low amounts of solute elements (up to 2 wt pct of Mg and Si), which become supersaturated in the fcc Al lattice after quenching from a solution heat treatment (SHT) to room temperature (RT). The solutes diffuse already at RT and nucleate atomic clusters. At elevated temperatures, during the artificial aging, the atomic clusters grow rapidly into metastable precipitates. Initially, fully coherent Guinier–Preston (GP) zones are formed with atoms on the Al matrix. The subsequent precipitates are seen to keep the alignment of some planes (fully coherent) only in one direction, corresponding to h001iAl, except for the final, completely incoherent equilibrium phase b (Mg2Si), which forms at higher temperature. As a consequence, the metastable precipitates in this alloy system have needle/lath/rod morphologies extending along h001iAl directions. The precipitation sequence of Al-Mg-Si alloys is as follows[1–7]: SSSS ! solute clusters ! GP zones (pre-b00 Þ ! b00 ! b0 ; U1; U2; B0 ! b; Si where SSSS stands for supersaturated solid solution. The main hardening precipitates are the highly coherent, needle-shaped GP zones and b¢¢ which form the finest microstructure. The U1, U2, and B¢ are larger, thicker needles/rods/laths and are also known
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