The debonding and fracture of Si particles during the fatigue of a cast Al-Si alloy
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I. INTRODUCTION
THE outstanding mechanical, physical, and casting properties of Al-Si alloys make them attractive for use in cheaper and lighter engineering components. However, to successfully utilize cast Al-Si alloys in long-life components, it is necessary to thoroughly understand their resistance to fatigue. Numerous studies have shown that when large-scale casting porosity is present in cast Al-Si alloys, the largest pores dictate the fatigue life of the material.[1–13] Moreover, in cast materials with a maximum pore size À100 mm, the fatigue life of a specimen can be estimated by integrating a Paris law crack-growth-rate equation. Ting and Lawrence[4] concluded that the crack-nucleation life from large pores encompasses a negligible fraction of the fatigue life of cast Al-Si alloys. However, continual advances in casting technology are driving the maximum pore size well below 100 mm in typical Al-Si castings.[6,12,14] Furthermore, the highly evolved design of industrial castings permits the control of porosity in regions that will experience large stresses during service. In parallel with these materials-processing developments, we need to further understand the micromechanisms of fatigue-crack growth in cast Al-Si alloys. Understanding local fatigue-crack growth mechanisms will facilitate the development of micromechanical fatigue models capable of KEN GALL, Assistant Professor, is with the Department of Mechanical Engineering, University of Colorado, Boulder, CO 80309. NANCY YANG and MARK HORSTEMEYER, Technical Staff, are with the Materials and Engineering Sciences Center, Sandia National Laboratories, Livermore, CA 94550. DAVID L. McDOWELL, Professor, and JINGHONG FAN, Research Scientist, are with the GWW School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332. Manuscript submitted April 1, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
predicting the fatigue life of cast Al-Si alloys that contain either significant or negligible porosity levels. In the automotive industry, one technologically important Al-Si casting alloy is A356. The microstructure of A356 consists of primary Al-1 pct Si dendrites and a eutectic with silicon particles (12 pct volume fraction) embedded in an Al-1 pct Si matrix. Other trace elements such as strontium, iron, and magnesium are added to improve the silicon particle morphology, to prevent die soldering, and to promote precipitation hardening, respectively. In the absence of intermetallic particles and large-scale porosity, the fracture and debonding of irregular-shaped silicon particles controls the ductility of A356 alloys.[15] When intermetallics are present in the material, they are expected to further limit the macroscopic ductility of the material.[16] Under monotonic loading, the debonding and fracture of second-phase particles causes the nucleation of voids in the surrounding Al-1 pct Si matrix material. In modeling the failure of cast Al-Si alloys subjected to monotonic loading, it is not critical to distinguish between particle debond
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