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Precipitation and Hardening in Magnesium Alloys
JIAN-FENG NIE
Magnesium alloys have received an increasing interest in the past 12 years for potential applications in the automotive, aircraft, aerospace, and electronic industries. Many of these alloys are strong because of solid-state precipitates that are produced by an age-hardening process. Although some strength improvements of existing magnesium alloys have been made and some novel alloys with improved strength have been developed, the strength level that has been achieved so far is still substantially lower than that obtained in counterpart aluminum alloys. Further improvements in the alloy strength require a better understanding of the structure, morphology, orientation of precipitates, eects of precipitate morphology, and orientation on the strengthening and microstructural factors that are important in controlling the nucleation and growth of these precipitates. In this review, precipitation in most precipitation-hardenable magnesium alloys is reviewed, and its relationship with strengthening is examined. It is demonstrated that the precipitation phenomena in these alloys, especially in the very early stage of the precipitation process, are still far from being well understood, and many fundamental issues remain unsolved even after some extensive and concerted eorts made in the past 12 years. The challenges associated with precipitation hardening and age hardening are identied and discussed, and guidelines are outlined for the rational design and development of higher strength, and ultimately ultrahigh strength, magnesium alloys via precipitation hardening.
DOI: 10.1007/s11661-012-1217-2 The Minerals, Metals & Materials Society and ASM International 2012
I. INTRODUCTION
MAGNESIUM is the lightest of all commonly used structural metals, with a density approximately two thirds that of aluminum and one quarter that of steels. Magnesium is an abundant element, comprising 2.7 pct of the Earths crust, and it is available commercially with purity exceeding 99.8 pct. Magnesium has a relatively low melting temperature and high specic heat. Hence, magnesium and its alloys may, thus, be readily cast to near-net shape by conventional casting methods. Because of such attractive features, magnesium alloys have received considerable research over the last decade for potentially wider and larger applications in the automotive, aircraft, aerospace, and 3C (computer, communication, and consumer electronic product) industries. The annual production rate of magnesium metal was approximately 450,000 tons in 2001 and reached ~720,000 tons in 2008. Despite the considerable...