The Effect of Preaging Deformation on the Precipitation Behavior of an Al-Mg-Si Alloy
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TRODUCTION
AL-MG-SI(-CU) alloys are an important class of materials for structural applications and are the most commonly extruded alloys. The reasons behind are the good combination of properties, such as high strength/ weight ratio for stiffer, lighter constructions; good ductility, formability, and weldability; and good corrosion resistance. The material properties are a result of microstructure characteristics. As Al-Mg-Si alloys are heat treatable, they gain their strength mainly through the formation of metastable nanometer-scaled precipitates during aging. Generally, fine and homogeneous precipitate microstructures give superior material properties. Optimization of the material properties is achieved through the control of precipitate formation with respect to type, size, distribution, number density, and volume fraction, by manipulating alloy composition and thermomechanical processing. Microstructure characterization and quantification is therefore important for industrial alloy design, and it aids the fundamental understanding of these alloys. The precipitation sequence in undeformed Al-Mg-Si alloys is generally accepted to be[1] KATHARINA TEICHMANN, PhD Student, and KNUT MARTHINSEN, Professor, are with the Norwegian University of Science and Technology, Trondheim 7491, Norway. Contact e-mail: [email protected] CALIN D. MARIOARA and SIGMUND J. ANDERSEN, Scientists, are with SINTEF Materials and Chemistry, Trondheim 7465, Norway. Manuscript submitted October 18, 2011. Article published online May 31, 2012 4006—VOLUME 43A, NOVEMBER 2012
Atomic clusters ! GP-zones ðpre-b00 Þ½2;3 ! b00½4;5 ! b0 ;½6 U1;½7 U2;½8 B0½9 ! b; Si ðstableÞ All metastable precipitates in this system have needle/ lath/rod morphologies with longest dimension oriented along h001i Al directions, in which they are fully coherent with the aluminum matrix. At peak hardness conditions, a microstructure exists with very fine GP-zones and b¢¢ precipitates.[2,10] The coarser post-b¢¢ phases, namely b¢, U1, U2, and B¢ (the last three also called type A, B, and C, respectively[11]) generally appear after overaging, associated with a decrease in material hardness.[1] All metastable phases in the Al-Mg-Si(-Cu) system, including the disordered precipitates, are structurally related through a common network consisting of Si atomic columns with a projected near-hexagonal subcell (SC) a = b 0.4 nm, c = n*0.405 nm, n being an integer, as shown by Andersen et al.[7,12–14] The hexagonal c-axis of the network is parallel with the longest dimension of the precipitate. The Si network’s SC deviates most from a hexagonal symmetry in the case of the pre-b¢¢ and b¢¢ phases, probably because of their high coherency with the Al matrix. After extrusion, many products made of age-hardening Al alloys are subjected to different forms of deformation before the aging heat treatment takes place. Even without subsequent forming, extruded profiles often have to be stretched to straighten out a certain degree of bending and buckling after the extrusion.
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