Clustering and Vacancy Behavior in High- and Low-Solute Al-Mg-Si Alloys
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INTRODUCTION
OF
the commercial wrought aluminum alloys, the 6xxx alloys, with Mg and Si as main alloying elements, have some of the most desirable properties. Their high strength, good ductility and corrosion resistance, and low processing cost make them attractive structural materials for automotive and architectural applications. This has ensured Al-Mg-Si alloys a place in condensed matter science as materials of frequent study over many decades. In particular, the strengthening phases have been well investigated. These appear as needle-shaped precipitates along h001iAl, with lengths in the order of 10 to 100 nm, formed through diffusional phase transformations during heat treatment. Their metastable nature stresses the importance of aging the material at the right temperature for the right period of time to optimize the mechanical strength of an alloy. The sequence of precipitates appearing during artificial aging is commonly written[1,2]:
SIGURD WENNER, Postdoctoral Researcher, and RANDI HOLMESTAD, Professor, are with the Department of Physics, NTNU, Høgskoleringen 5, 7491 Trondheim, Norway. Contact e-mail: [email protected] KATSUHIKO NISHIMURA and KENJI MATSUDA, Professors, are with the Department of Materials Science and Engineering, University of Toyama, Gofuku, 3190 Toyama-Shi, Toyama 930-8555, Japan. TEIICHIRO MATSUZAKI and DAI TOMONO, Researchers, are with the Advanced Meson Science Laboratory, RIKEN Nishina Center for Accelerator Based Science, RIKEN, Wako, Saitama 351-0198, Japan. FRANCIS L. PRATT, Instrument Scientist, is with the ISIS Facility, Rutherford Appleton Laboratory, Chilton OX11 0QX, UK. CALIN D. MARIOARA, Senior Scientist, is with the Materials and Chemistry, SINTEF, Høgskoleringen 5, 7491 Trondheim, Norway. Manuscript submitted April 4, 2014. Article published online August 26, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
SSSS ! Atomic clusters ! GP-zones ! b0 ! b0 =TYPE-A/B/C ðU1/U2/B0 Þ ! b=Si:
½1
The fully coherent monoclinic phase b¢¢[3] dominates at peak hardness, and develops into four less coherent (but still needle-shaped) phases during over-aging: The hexagonal b¢[4], the trigonal TYPE-A,[5] the orthorhombic TYPE-B,[5] and the hexagonal TYPE-C,[5] which mostly nucleates on dislocations. The main equilibrium phase is the cubic b,[5] but diamond Si may also form. Whether a particular phase forms also depends on the ratio between Mg and Si in the chemical composition of the alloy. For instance, Si-rich alloys tend to form TYPE-A, TYPE-C and Si particles during prolonged over-aging, while Mg-rich alloys gain larger fractions of b¢ and TYPE-B.[6] Most of the crystal structures of the phases in (1) have been solved by transmission electron microscopy (TEM) (see e.g.,[6]). The powerful techniques high-resolution imaging and electron diffraction makes TEM the most applicable tool to study nano-sized precipitates in Al alloys. In addition to precipitate phases, it is important to figure out how the presence of smaller defects such as vacancies and atomic clusters vary with the ag
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