Creep strength of magnesium-based alloys
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I. INTRODUCTION
MAGNESIUM has the lowest density among the elements widely used as structural materials. In addition to their high specific strength, magnesium alloys have excellent castability and better recyclability than plastics. Owing to these benefits, they are prospective candidates for applications in automotive components. Automotive engine components made of Mg alloys are used at elevated temperatures from 423 to 473 K and are required to have sufficient creep resistance. However, it is often claimed that Mg alloys are inferior to Al alloys in creep resistance.[1] The objective of this article is to evaluate the inherent creep resistance of Mg alloys in comparison with Al alloys and to discuss how to improve their creep resistance. In the first part of this article, the creep rates of Mg-Al solid-solution alloys having an hcp lattice structure are compared with those of Al-Mg solidsolution alloys with an fcc structure. The merit and demerit of Mg alloys will be discussed in terms of creep resistance. It has been reported recently that the addition of rare-earth (RE) elements significantly improves the mechanical properties of Mg alloys at elevated as well as at ambient temperatures.[2–6] The strengthening by the RE elements is more substantial than any other elements that have been added to Mg alloys. However, the strengthening mechanism of MgRE alloys has not been fully understood in the creep regime and will be discussed in the second part of this article, with special attention paid to Mg-Y alloys. The experimental results on Mg-Y and Mg-Al alloys have been reported in more detail elsewhere.[6,7]
KOUICHI MARUYAMA, Professor, and MAYUMI SUZUKI, Assistant Professor, are with the Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai, 980-8579, Japan. HIROYUKI SATO, Associate Professor, is with the Department of Intelligent Machines and System Engineering, Faculty of Science and Technology, Hirosaki University, Hirosaki 036-8561, Japan. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11-15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
II. COMPARISON OF CREEP RESISTANCE BETWEEN Mg AND Al ALLOYS In order to develop an Mg alloy with better creep resistance than Al alloys, we ought to recognize the origins of the so-called poor creep resistance of Mg alloys. In the case of solid-solution alloys, the melting temperature of the alloy, solute concentration (N ), and atomic-size misfit (␦ ) of the solute atom in the solvent are the most important parameters determining creep resistance. These parameters are required to be as similar as poss
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