Effect of Y, Sr, and Nd additions on the microstructure and microfracture mechanism of squeeze-cast AZ91-X magnesium all
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I.
INTRODUCTION
THE magnesium alloy is the lightest structural alloy commercialized to date, and has excellent specific strength and elastic modulus together with a density of 1.74 g/cm3 (about two-thirds that of an aluminum alloy). Also, because of its superb capability to absorb vibration, impact, and electric waves, it is drawing special attention as an advanced material for coping with energy conservation and environmental pollution regulations. However, the magnesium alloy has a lower strength and elongation and is more susceptible to corrosion than other lightweight alloys; thus, it has had limited application as a structural material. Recently, many studies have been made to complement these shortcomings; for example, a high-purity AZ91 alloy (AZ91-HP) was fabricated by minimizing the content of Fe, Cr, and Ni, which diminish the resistance to corrosion.[1] It also has been reported that high-temperature properties and resistance to corrosion are greatly improved over existing magnesium alloys by adding Zr, Mn, Ca, and rare earth metals.[2–6] Furthermore, methods to improve the microstructural refinement and the solid solubility of alloying elements by applying the rapid solidification process are continuously under development.[7,8,9] Magnesium alloys are generally fabricated either by a casting process or by powder metallurgy. Some of the processes commonly used in casting, which is more economic than the complicated powder metallurgy process, are stir SUNGHAK LEE, Professor, is with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang, 790784 Korea. SEUNG HYUK LEE, formerly Research Assistant, Center for Advanced Aerospace Materials, Pohang University of Science and Technology, is Research Engineer, Advanced Device Research Laboratory, Hyundai Electronics Industries Co., Ltd., Ichon-kun, 467-860 Korea. DO HYANG KIM, Associate Professor, is with the Department of Metallurgical Engineering, Yonsei University, Seoul, 120-749 Korea. Manuscript submitted March 26, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
casting, compocasting, and squeeze casting.[10] Squeeze casting leads to microstructural refinement and the alleviation of casting defects due to the abrupt heat transfer and the squeezing-pressure effect which are when liquid phase metal is poured into a die and solidified under high pressure. This process is increasingly being used as a practically effective method because it produces more densified products and can be easily adapted to conventional die-casting devices.[11] To accelerate a wide-range application of magnesium alloys as structural materials allowing the maximum weight reduction, properties of final products should be further enhanced by improving the alloy composition and optimizing the fabricating factors.[12,13,14] Thus, the present study aims to present the effects of alloying elements by investigating the correlation between the microstructure and mechanical properties of squeeze-cast magnesium alloys and their microfracture mechanism.
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