Evolution of Grain Boundary Phases during the Homogenization of AA7020 Aluminum Alloy
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THE mechanical properties of aluminum alloys are largely dependent on alloying elements present in solid solution. These elements increase the strength mainly through solid solution or precipitation hardening.[1] During casting of aluminum alloys, a large fraction of alloying elements segregate to the liquid and form constitutive particles in the grain boundary (GB) regions or inside the grains.[2] These particles decrease the hot workability and, therefore, limit the range of process parameters applicable during subsequent extrusion.[3] It is well known that coarse residual particles (>1 lm) deteriorate the extrudability of these alloys especially when they are located in the GB regions.[4–9] Therefore, it is necessary to dissolve these particles in the structure in order to attain high mechanical properties and extrudability. Although there have been a number of investigations on the homogenization treatments of aluminum alloys in recent years,[10–20] most of the efforts have been focused on the nature and evolution of the Al2Mg3Zn3 (T), Al2CuMg (S), (CuZnAl)2Mg, and MgZn2 (g) phases[10–13] and the formation and distribution of dispersoids[14–20] during homogenization. In addition, in comparison with other aluminum alloys, the information on the 7XXX A.R. EIVANI, PhD Researcher, and H. AHMED, Postdoctoral Researcher, are with the Materials Innovation Institute, 2628 CD Delft, The Netherlands. Contact e-mail: [email protected] J. ZHOU and J. DUSZCZYK, Associate Professors, are with the Department of Materials Science and Engineering, Delft University of Technology, 2628 CD Delft, The Netherlands. Manuscript submitted May 1, 2008. Article published online January 21, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
series aluminum alloys is rather scarce in the literature. Although some researchers have studied the microstructural changes and the evolution of the eutectic and low melting point phases during homogenization in the case of 7XXX series aluminum alloys, there is still a lack of information in the case of AA7020 aluminum alloy. Moreover, there is no comprehensive quantitative study on the dependence of the particles on homogenization treatment parameters. Most of the investigations have been focused on the qualitative studies of the microstructural evolution and phase transformations during homogenization. For example, Lim et al.[10] investigated the effects of constitutional changes and preheat conditions on the evolution of constitutive particles, the M, T, and S phases, and dispersoids in AA7175 and AA7050 alloys. Senkov et al.[21] studied the effect of homogenization treatment on the microstructural evolution of four newly developed 7XXX series aluminum alloys to obtain optimized conditions. Jackson and Sheppard[22] studied the effect of homogenization treatment on the microstructural changes of AA7075, 7150, and 7049. They focused on the evolution of the microstructure, low melting point phases, and the M, T, S, Al3Mg2 (b), b, Al18Cr2Mg (E), and CrAl7 phases. Fan et al.[23] studied the evolution of micro
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