Precipitation in Two Al-Mg-Ge Alloys
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AGE-HARDENABLE Al-Mg-Si alloys attain strength by the precipitation of high numbers of nanometer-sized, partly coherent precipitates in the shape of needles. Initially, a high homogenization or extrusion temperature ensures that most solute atoms exist on Al lattice positions. Upon quenching to room temperature, the lattice becomes supersaturated with solute. These atoms will diffuse into small clusters on the Al lattice, aided by quenched-in vacancies. The alloys subsequently are aged. After a short time at the elevated temperature, favored clusters start reorganizing into periodic structures. The first ones are the fully coherent Guinier-Preston (GP) zones.[1] Further aging causes the less coherent b¢¢ phase to form. Together with the GP-zones, this is the phase that is found in high-number densities in the ternary Al-Mg-Si alloys at the maximum hardness condition.[2,3] Long aging times or high aging temperatures cause several additional precipitate types to form (Table I). The type of phase depends on alloy composition and thermomechanical treatment. The post-b¢¢ precipitates reported in the literature are b¢, U1, U2, and B¢.[2,4] The last three precipitates also are called type-A, type-B, and type-C precipitates, respectively.[5] The post-b¢¢ precipitates are needle- or lath-shaped and usually come with large cross-sections, are low in number, and, therefore, are associated with a decrease in alloy strength.[4] R. BJØRGE, Ph.D. Student, and R. HOLMESTAD, Professor, are with the Department of Physics, Norwegian University of Science and Technology, 7491 Trondheim, Norway. Contact e-mail: randi. [email protected] C.D. MARIOARA, Research Scientist, and S.J. ANDERSEN, Senior Scientist, are with the Department of Synthesis and Properties, SINTEF Materials and Chemistry, 7465 Trondheim, Norway. Manuscript submitted November 29, 2009. Article published online May 22, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
For sufficient temperature and aging time, eventually the incoherent equilibrium phase b-Mg2Si will form.[6] Depending on the Si content of the alloy, b often coexists with pure diamond Si particles. If Cu is added to the Al-Mg-Si system, then the fraction of b¢¢ in the alloys decreases in relation to other metastable needleor lath-shaped phases known as S, L, C, QP, and QC.[2,7,8] Prolonged aging leads to precipitation of the Q¢ phase,[7] which is similar to the equilibrium phase of the Al-Mg-Si-Cu system, Q.[9] The main growth direction of the precipitates is h001iAl, along which the precipitates are coherent with the matrix. Hence, one unit-cell dimension is 0.405 nm (the lattice parameter of Al) or a multiple of this. All metastable phases in the Al-Mg-Si(-Cu) alloy system are related structurally through a near-hexagonal network of Si atoms with sub-cell (SC) projected dimensions of a = b 0.4 nm.[7,10] The precipitates can be described as different arrangements of Al, Mg, and Cu on this silicon network. In b¢¢, this network is more distorted from the hexagonal symmetry than in the other precipitates, pro
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