Reconstruction and Quantitative Characterization of Multiphase, Multiscale Three-Dimensional Microstructure of a Cast Al
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INTRODUCTION
EUTECTIC and hypereutectic Al-Si base alloys frequently are used for cast automotive components such as pistons because of their low thermal expansion coefficient, high wear resistance, and good castability.[1] It is well known that the mechanical properties of Al-Si base alloys are microstructure sensitive,[2] and the microstructure is governed by the alloy chemistry as well as casting process parameters. Therefore, visualization, characterization, and modeling of threedimensional (3D) microstructures of these alloys are of interest for the development of quantitative processing–microstructure–properties relationships. Cast microstructure of eutectic and near-eutectic Al-Si alloys typically are comprised a-Al cells that contain Si platelets (or fibrous rod-like Si particles in the modified alloys), Fe- and/or Cu-rich intermetallic inclusions of complex morphologies,[3–5] as well as gas and shrinkage pores.[6,7] Because of the divorce nature of the Al-Si eutectic, depending on the local cooling rate and composition, primary Si particles (i.e., the Si H. SINGH, Research Associate, A.M. GOKHALE, Professor, and Y. MAO, Graduate Student, are with the School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 303320245. A. TEWARI, Staff Researcher, is with the India Science Laboratory, General Motors, Bangalore, India. A.K. SACHDEV, Laboratory Group Manager, is with Research & Development, General Motors, 30500 Mound Road, Warren, MI 48090. Contact e-mail: [email protected]. This article is based on a presentation given in the ‘‘3rd Shape Casting Symposium’’ that occurred during the TMS Spring Meeting in San Francisco, CA, February 15–19, 2009, under the auspices of TMS, the TMS Light Metals Division, the TMS Solidification Committee, and the TMS Aluminum Processing Committee. Article published online August 28, 2009. METALLURGICAL AND MATERIALS TRANSACTIONS B
particles formed during solidification of the alloy before the eutectic reaction) also can form in the eutectic and near-eutectic Al-Si alloys.[8] The primary Si particles often are sparsely spaced, coarse, and have complex faceted polyhedral shapes.[9,10] Therefore, the 3D microstructure of cast Al-Si base alloys contains numerous constituents of complex shapes/morphologies; some of these constituents/phases are spaced sparsely; spatial correlations exist among the constituents/phases; and the length scales of different types of microstrucutral features can span over two orders of magnitude (e.g., eutectic Si platelets have thickness on the order of few microns, whereas gas pores can be of a few microns to a few hundred microns in size[6,7]). These geometric attributes of the microstructure affect the mechanical properties such as strength, toughness, and abrasion resistance and, therefore, must be included in the microstructure-based modeling and simulations of the material behavior. Most of the previous microstructural studies of as-cast, Al-Si base alloys involved observations on two-dimensional (2D) metallographi
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