Improving Thermal Stability in Cu-Containing Al-Mg-Si Alloys by Precipitate Optimization
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INTRODUCTION
Al-Mg-Si-(Cu) alloys are important alloys for extruded products in marine, aerospace, and automotive construction applications. The popularity relates to low cost, ease of fabrication and machinability, and to a combination of attractive materials properties. The alloys are heat treatable. After extrusion, a few hours aging between 423 K and 473 K (150 C and 200 C) will nearly triple the strength. The aging creates a high density of nanometer-sized needle-like precipitates with surrounding strain fields which obstruct dislocations. Depending on alloy composition and thermo-mechanical processing, numbers, sizes, and types of metastable precipitates vary. They have different compositions, interfaces, coherence, and surrounding strain and will influence properties differently. This also opens the possibility for optimization of the precipitate microstructure for particular properties.
A. The Precipitates in Al-Mg-Si and Al-Mg-Si-Cu Alloys In Al-Mg-Si alloys, the precipitation sequence can be written[1–5]: SSSS ! solute clusters ! GP-zones ðpre-b00 Þ ! b00 ! b0 ; U1; U2; B0 ! b; Si (stable phases) ½1 Normally, b¢¢ is the most efficient strengthening phase in the ternary system. U1, U2, and B¢ (Type A, B, and C, respectively[2]) are less coherent. They form upon overaging, leading to coarse microstructures and significant loss of strength.[5] The initial supersaturated solid solution (SSSS) of Mg and Si is formed by quick cooling from a temperature typically above 773 K (500 C). If copper is added in sufficient amount, other phases form instead of b¢¢, b¢, and b[3,6–9]: SSSS ! solute clusters ! GP-zones ! ðb00 Þ; Q0 ; L; S; C; QC ! Q0 ! Q (stable)
CALIN D. MARIOARA and SIGMUND J. ANDERSEN, Senior Researchers, are with the SINTEF Materials and Chemistry, 7465 Trondheim, Norway. Contact e-mail: [email protected] JOSTEIN RØYSET, Senior Research Metallurgist, and ODDVIN REISO, Chief Engineer, are with the Hydro Aluminium R&D Sunndal, Romsdalsvegen 1, 6600 Sunndalsøra, Norway. SVERRE GULBRANDSEN-DAHL, Chief Scientist, is with the Norwegian University of Science and Technology (NTNU), 7491 Trondheim, Norway, and also with the SINTEF Raufoss Manufacturing AS, 2831 Raufoss, Norway. TOR-ERIK NICOLAISEN, Manufacturing Engineering Manager, and INGE-ERLAND OPHEIM, Process Development Manager, are with the Steertec Raufoss AS, 2831 Raufoss, Norway. JAN FREDRIK HELGAKER, formerly with the Norwegian University of Science and Technology (NTNU), is now with the DNV GL, P.O. Box 300, 1322 Høvik, Norway. RANDI HOLMESTAD, is with the Norwegian University of Science and Technology (NTNU). Manuscript submitted February 12, 2013. Article published online March 18, 2014 2938—VOLUME 45A, JUNE 2014
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B. The Similar Hexagonal Network of Si-Columns in the Precipitates All metastable precipitates in the Al-Mg-Si(-Cu) system have uniaxial morphology. They are needles or laths extending along h001iAl, adopting the lattice periodicity (0.405 nm). The Si-columns in this direction generally order in a hexagonal network, having 0.4
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