Microstructural factors affecting superplastic properties in magnesium-based composites
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I. INTRODUCTION
MAGNESIUM alloys have an inherent advantage of being a lightweight material. Thus, there are many potential opportunities for the use of magnesium alloys as motor vehicle components. This is not only a consequence of magnesium’s relatively low density, but also a result of its good damping characteristics, dimensional stability, machinability, and low casting costs. Magnesium-matrix composites have yet an even greater potential to be used in high-performance aerospace and automobile applications because of their superior dimensional stability and strength compared to competitive materials. Despite these advantages, however, magnesium-based composites normally exhibit limited ductility near room temperature.[1,2] This has restricted their many potential applications. In order to exploit the benefits of magnesium-based composites, it is important to develop secondary processing procedures that can effectively produce complex engineering components directly from the wrought products. Superplastic forming is a viable technique to fabricate hard-to-form materials, such as a metal-matrix composite, into complex shapes. To date, it has been demonstrated that several magnesiumbased composites reinforced with ceramic particles or whiskers exhibit superplasticity. Kim et al.[3] reported the observation of extended ductility (100 pct) in a 13 vol pct SiC whisker-reinforced AZ91 in 1992. Subsequently, Nieh and Wadsworth[4] observed superplasticity in a 17 vol pct SiC particulate-reinforced ZK60 in 1995. A summary of the superplastic properties of magnesium-based composites is listed in Table I.[3–18] High-strain-rate superplasticity[3–9,12–16] or low-temperature superplasticity[17,18] has been observed in several fine-grained magnesium-based composites. For example, the variation in (a) flow stress and (b) elongation to failure as a function of strain rate is shown in Figure 1 for a doubly extruded (DE) ZK60/SiC/17p.[18] It is obvious HIROYUKI WATANABE and TOSHIJI MUKAI, Research Staff Members, are with the Osaka Municipal Technical Research Institute, Osaka 536-8553, Japan. KENJI HIGASHI, Professor, is with the Department of Metallurgy and Materials Science, College of Engineering, Osaka Prefecture University, Sakai 599-8531, Japan. Manuscript submitted May 30, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
that this composite exhibits a high-strain-rate superplasticity at 623 K and at a high strain rate of 10⫺1s⫺1 or a low temperature superplasticity at ⬃0.5 Tm (448 to 475 K), where Tm is the melting point of ZK60 (⫽908 K).[19] Although limited data are available for superplasticity in magnesiumbased composites, these excellent superplastic properties indicate the possibility of a technological breakthrough in superplastic forming. This article presents the guidelines for the enhancement of superplastic properties in magnesium-based composites. Microstructural factors, which may affect superplastic properties, were investigated. Effect of grain size of the matrix, presence of reinforcements, and their dis
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