Elastic Properties, Thermal Expansion Coefficients, and Electronic Structures of Mg and Mg-Based Alloys
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MAGNESIUM (Mg) possesses the lowest density among the elements that are widely applied in structural materials. In addition to their high specific strength, Mg alloys also show an excellent castability and better recyclability than plastics. Because of these appreciable benefits, Mg-based alloys are prospective candidates for applications in the automobile industry. Recently it was reported that the addition of rare earth (RE) metals significantly improves the mechanical properties of Mg-based alloys, such as creep resistance at elevated as well as ambient temperatures through alloying solid solutions.[1–5] The strengthening mechanism in the creep resistance of Mg-RE alloys, however, has not been fully understood in the creep regime. Experimental observations suggest that the presence of precipitates due to the alloying addition cannot provide a prominent improvement in creep resistance, and alloy solid-solution hardening is shown to be the principal strengthening mechanism evident in Mg-Y alloys. The well-established Cottrell atmosphere-dragging model, however, cannot account for this solid solution strengthening issue in Mg-Y alloy.[6] Thus, a thorough investigation of the underlying mechanism responsible for alloy solid solution strengthening is essential to the research of KUIYING CHEN, Research Officer, is with the Structures and Materials Performance Laboratory, Institute for Aerospace Research, National Research Council Canada, Ottawa, ON, Canada K1A 0R6. Contact e-mail: [email protected] KEVIN P. BOYLE, Research Scientist, is with the Materials Technology Laboratory, Natural Resources Canada, Ottawa, ON, Canada K1A 0G1. Manuscript submitted January 15, 2009. Article published online September 1, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
Mg-based alloys. To our knowledge, there are no theoretical studies on solid solution strengthening mechanisms associated with alloying additions of sp-metals Al and Zn and RE Y that specifically address the electronic bonding characteristics. In this research, we present the first effort to investigate a possible mechanism of alloy solid solution strengthening, and to examine the elastic properties, thermal expansion coefficients, and electronic structures of Mg-based alloys using state-of-the-art ab-initio density functional theory (DFT) calculations. The information gained in this study is expected to provide essential guidance to the development of Mg-based alloys. This article is organized as follows. Section II briefly introduces and presents computational methodology on the evaluation of elastic coefficients and moduli for pure hcp Mg and Mg-based alloys. Section III examines the alloying effects on elastic properties; it also examines the bonding characteristics and electronic structures that are related to alloy solid solution strengthening. Thermal expansion coefficients of Mg-based alloys are given, with an analysis in terms of bonding characteristics. Section IV gives the conclusions.
II. COMPUTATIONAL METHODOLOGY OF ELASTIC COEFFICIENTS AND MODULI OF H
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