Thixoforming of A356/SiC and A356/TiB 2 Nanocomposites Fabricated by a Combination of Green Compact Nanoparticle Incorpo
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TRODUCTION
METAL matrix nanocomposites (MMNCs) strengthened with nano-sized ceramic or other hard particles have shown their potential to significantly improve mechanical properties of light alloys such as those based on aluminum beyond the properties of conventional metal matrix composites (where the reinforcement particles are micron-sized).[1–5] While the ductility of the matrices deteriorates with the addition of micron-sized ceramic particles due to their tendency to crack during mechanical loading and relatively high reinforcement concentration in the matrix,[6] it could be maintained or even improved with a low addition of nanoparticles, e.g., 0.5 or 1.0 wt pct.[5,7] These enhancements in the mechanical properties by nanoparticles are associated with the obstruction of dislocation movement and also the promotion of fine grain sizes.[8–10] Also, it is crucial to achieve a uniform distribution of nanoparticles through the matrix and good bonding between the SINAN KANDEMIR, Lecturer, is with the Department of Mechanical Engineering, Izmir Institute of Technology, Izmir, Turkey. Contact e-mail: [email protected] HELEN V. ATKINSON and SARAH V. HAINSWORTH, Professors, and DAVID P. WESTON, Lecturer, are with the Department of Engineering, University of Leicester, Leicester, UK. Manuscript submitted April 1, 2014. Article published online August 14, 2014 5782—VOLUME 45A, NOVEMBER 2014
matrix and nanoparticles in order to maximize composite mechanical properties.[11–13] Although there are a number of available fabrication routes for MMNCs, an ultrasonic method,[5,14] which combines casting with ultrasonic cavitation-based dispersion of nanoparticles in molten alloys, seems an economical and promising route in terms of producing engineering components with complex shapes.[7,15] In the fabrication of MMNCs by liquid state routes, poor wettability (which can affect the bonding at the reinforcement-matrix interface) and the tendency of ceramic nanoparticles to agglomerate and cluster due to their large surface-to-volume ratio are the main barriers to obtaining a uniform dispersion of nanoparticles into the matrix.[5] However, it has been shown that good dispersion of ceramic nanoparticles in molten metals is possible with the ultrasonic method due to high intensity ultrasonic waves with localized implosive impact, namely transient cavitation, and acoustic streaming.[15–17] It has been assumed that ultrasonic cavitation could break up nanoparticle clusters due to the collapse of cavitation bubbles when they reach a critical size in the clusters.[18] Acoustic streaming which is a circulating flow is then considered to play a role for distributing nanoparticles throughout the matrix. In addition to the barriers mentioned, the incorporation of nanoparticles into the melt, i.e., a nanoparticle feeding mechanism, is a most critical consideration in the fabrication of nanocomposites by ultrasonic melt METALLURGICAL AND MATERIALS TRANSACTIONS A
treatment for effective particle distribution,[19] as nanoparticles should be fe
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