Mechanical properties and fracture behavior of an ultrafine-grained Al-20 wt pct Si alloy
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I. INTRODUCTION
AL-SI alloys are used in many application areas such as automotive, electronics, and aerospace industries due to their good wear resistance and low coefficient of thermal expansion. Among the most common applications are components, such as connecting rods, cylinder liners, pistons, engine blocks, and air conditioner compressors.[1,2] Al-Si alloys are currently produced by casting and powder metallurgy methods. A relatively slow cooling rate, associated with the conventional casting process, produces coarse and segregated primary Si or eutectic Si in the Al-Si alloys.[3] It is common practice to increase the Si content to improve the wear resistance and mechanical strength. However, with increasing Si content, above the eutectic composition (12.6 wt pct Si), primary Si crystals become coarse, resulting in poor mechanical properties of the Al-Si alloys. Thus, the distribution and size of the primary silicon particles, rather than the overall silicon content of the alloy, are important. Techniques such as modification,[4,5] ternary alloying,[6] spray deposition,[7] and rapid solidification processing[8] have been applied to refine the primary Si crystals and achieve their homogeneous distribution in hypereutectic Al-Si alloys. Consequently, their strength and wear resistance have increased. The wear resistance of Al alloy–SiC composites fabricated by a combination of infiltration and melt stirring processes with different sizes and volume fractions of SiC particles has been studied. The wear resistance of the composites has increased with the volume fraction of the SiC particles.[9,10] Of all the processes mentioned previously, rapid solidification of metallic melts has been known to produce altered SOON-JIK HONG, Postdoctoral Research Fellow, and C. SURYANARAYANA, Professor, are with the Department of Mechanical, Materials and Aerospace Engineering, University of Central Florida, Orlando, FL. 32816-2450. Manuscript submitted February 4, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
constitutional effects such as formation of supersaturated solid solutions, metastable intermetallic phases, and even amorphous alloys.[11–16] Additionally, the microstructural features (grain size and size of second-phase/intermetallic particles) are considerably refined and segregation effects are significantly reduced. The combined effects of supersaturation and refined microstructural features result in considerably higher strength and improved wear properties. Some results have been reported for Al-Si alloys fabricated by rapid solidification processing and hot extrusion. Increased strength and wear resistance were achieved using rapidly solidified hypereutectic Al-Si alloys containing additions of ternary alloying elements from the transition metals groups.[17,18,19] The presence of ternary compounds in these Al-Si ternary alloys, which were uniformly dispersed in the aluminum matrix, increased the strength and wear resistance. It has been shown in an earlier study that addition of transition metals such as Cr a
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