The sequence of precipitation in the Al-Mg-Si-Cu alloy AA6111
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UDY of the precipitation processes in Al-Mg-Si-(Cu) alloys has been of considerable interest in the past several years due to the desire to employ these alloys for automotive panel applications and to achieve significant weight reduction. These studies, which have significantly benefited from the atomistic scale characterization methods such as high-resolution transmission electron microscopy (HRTEM) and atom probe techniques, have led to the detailed characterization of previously known metastable precipitates and the identification of new phases in alloys with various compositions (e.g., References 1 through 6). The earlier investigations in this area had suggested a rather simple sequence of precipitation in ternary Al-Mg-Si alloys, commencing with solute/vacancy clusters and proceeding through primary Guinier–Preston (GP) zones and the metastable precursors of the equilibrium Mg2Si (i.e., b) phase, b0 and b9, until the equilibrium was achieved.[7] However, more recent studies have given evidence for the precipitation of other metastable phases, which can form in the Al-Mg-Si alloy system, depending on the alloy composition. A comprehensive survey of these findings and a first-principle study of the crystal structure and stability of these phases have been given by Ravi and Wolverton.[8] The addition of Cu in percentages comparable to those of Mg and Si, which create the AlMg-Si-Cu family of 6000 series alloys, introduces Cu containing phases to the sequence of precipitation leading to the precipitation of quaternary phase Q at the equilibrium condition.[5,9–16] A recent atom probe study provides evidence that Cu atoms are present in the early solute clusters in an Al-Mg-Si-Cu alloy.[6] The presence of Cu in precipitates in more advanced stages of aging, which includes b0 phase precipitation, has also been reported.[5] In addition to the precipitation of clusters/GP zones and b0 phase, the formation of precursors of Q phase during aging to the peakstrength condition in Al-Mg-Si-Cu alloys with various chemistries has been reported by several research teams.[9,11,13–16] The details of structure, compositional variations, and stabilXIANG WANG, Research Associate, is with the Department of Materials Science and Engineering, McMaster University, Hamilton, ON, Canada L8S 4L7. SHAHRZAD ESMAEILI, Assistant Professor, is with the Department of Mechanical Engineering, University of Waterloo, Waterloo, ON, Canada N2L 3G1. Contact e-mail: [email protected] DAVID J. LLOYD, formerly Principal Scientist, Novelis Global Technology Centre, Kingston, ON, Canada K7L 5L9. Manuscript submitted November 30, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
ity of equilibrium Q phase have also been published.[8,12,15,17] In AA6111, the present authors have found that a precursor phase of Q is present in the alloy microstructure from early stages of aging at 180 °C, and it gradually increases both in number density and size.[14,18] However, these precipitates contribute only a small portion of the total volume fraction of precipitates in t
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