Effect of Icosahedral Quasicrystalline Fraction and Extrusion Ratio on Microstructure, Mechanical Properties, and Anisot
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AS the lightest structural metal, Mg alloy has been widely used in automobile, electronics, and aerospace industries due to its desirable combination of properties, such as the low density and high specific strength.[1] Most of the wrought Mg alloys, however, turn out to have a poor ductility because of their hexagonal closepacked structure, which allows an activation of only two independent basal slips at room temperature.[2] To refine grains in Mg alloys so as to improve their mechanical properties, a series of thermomechanical processing techniques has been conducted on wrought Mg alloys, including rolling,[3,4] forging,[5,6] and extruding.[7–9] Although such processing methods have beem demonstrated to enhance greatly the mechanical properties of wrought Mg,[3] most of the Mg alloys unfortunately exhibit strong basal textures, which lead especially to the yield asymmetry between compression and tension,[10] HUA HUANG, Ph.D. Student, is with the National Engineering Research Center of Light Alloy Net Forming, Shanghai Jiao Tong University, Shanghai 200240, P.R. China. GUANGYIN YUAN and WENJIANG DING, Professors, are with the National Engineering Research Center of Light Alloy Net Forming, Shanghai Jiao Tong University, and also with the State Key Laboratory of Metal Matrix Composite, Shanghai Jiao Tong University, Shanghai 200240, P.R. China. Contact e-mail: [email protected] ZHONGCHANG WANG, Assistant Professor, and CHUNLIN CHEN, Research Associate, are with the WPI Research Center, Advanced Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 9808577, Japan. Manuscript submitted June 14, 2012. Article published online January 24, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
which would impair its secondary deforming ability. In this regard, it would be desirable to obtain wrought Mg alloys with high ductility by weakening or randomizing texture.[11] In addition to textures, interfaces between the crystallite-strengthening phases and Mg matrix also impact adversely the properties of Mg alloys because microcracks prefer to initiate at the interfaces if alloys are imposed with tensile load.[12] For these reasons, the widespread use of wrought Mg alloys as structural materials is currently limited. Since the first discovery of quasicrystals in Al-Mn binary alloy by Shechtman et al.,[13] great effort has been devoted to the introduction of them to other materials so as to take full advantage of their excellent intrinsic properties such as high strength and hardness at elevated temperature, low friction coefficients, and low surface energy.[14] Such an attempt has been done as well in the field of Mg alloys. Indeed, the icosahedral quasicrystals (I phase) has been demonstrated to be able to act as a strengthening phase to realize stronger alloys with larger microhardness,[15,16] random textures,[17] improved ductility, and high-temperature mechanical properties.[18,19] In our previous studies, we also demonstrated a successful introduction of the icosahedral I-phase into the Mg-Zn-Gd-
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