In-situ observations of low-cycle fatigue damage in cast AM60B magnesium in an environmental scanning electron microscop
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INTRODUCTION
THE low density, excellent castability, and moderate strength of cast magnesium alloys make them strong candidates for intricate, lightweight structural components. The use of cast magnesium as a structural material requires a detailed understanding of the material response to cycling loading conditions. Previous work has characterized the fatigue properties of various wrought[1–6] and cast[5–15] magnesium alloys. However, a detailed understanding of fatigue mechanisms operating in cast magnesium alloys is still needed in order to develop microstructure-based fatigue-life prediction tools, and thus, facilitate optimal casting design. The purpose of this study is to examine the mechanisms of low-cycle fatigue damage in cast AM60B magnesium in situ in two different environments. A complimentary study[16] examines high-cycle fatigue mechanisms in cast AM60B magnesium in situ as a function of environment. KEN GALL, Associate Professor, is with the Department of Mechanical Engineering, University of Colorado, Boulder, CO 80309. Contact email: [email protected] GERHARD BIALLAS, Postdoctoral Associate, and HANS J. MAIER, Professor, are with Lehrstuhl f˙ur Werkstoffkunde (Materials Science), University of Paderborn, 33095 Paderborn, Germany. PHIL GULLETT, Member of the Technical Staff, is with Sandia National Laboratories, Livermore, CA 94550. MARK F. HORSTEMEYER, Professor, is with Mechanical Engineering, Mississippi State University, Mississippi State, MS 39762. DAVID L. McDOWELL, Professor, is with the George Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332. Manuscript submitted February 21, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS A
Previous work has shown that the fatigue behavior of wrought and cast magnesium alloys can sometimes be characterized by phenomenological strain-life methods.[7,9,12] However, many cast magnesium alloys can demonstrate considerable scatter in strain-life data due to inherent variations in the microstructure caused by different local solidification parameters.[6,11,17,18] Traditional strain-life fatigue-prediction tools lead to highly conservative allowable strain limits and overdesigned components in the presence of extreme data scatter. An alternative approach for as-cast materials consists of understanding and quantifying the link between microstructure and properties through the operant deformation mechanisms.[19] Models based on the microstructure and operant deformation mechanisms can be used to optimally design castings, to avoid microstructures susceptible to fatigue in highly stressed regions.[19] Fatigue cracks in magnesium have been observed to form at slip bands,[4,16] subsurface inclusions,[4] or casting pores[14,16] during high-cycle fatigue. Wrought magnesium alloys, or high-pressure die castings, usually show preferential crack formation at slip bands and inclusions, while as-cast magnesium alloys typically show crack formation at casting pores. The fatigue response of magnesium alloys is considerably s
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