Strong, Ductile Magnesium-Zinc Nanocomposites
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TRODUCTION
RECENT work has shown a significant enhancement in metal matrix composite strength with the addition of nanoelements (e.g., nanoparticles).[1–4] Applications that have been targeted to date include structural components in the automotive and aerospace industries.[3] Creating new lightweight, high-strength materials could result in significant weight reductions and energy savings.[5] Currently restricting wide usage of these materials is the cost of materials synthesis and processing. Most metal matrix nanocomposites (MMNCs) to date are made with expensive powder metallurgy, ball milling, deposition, and infiltration techniques.[3,4,6–9] A robust solidification processing route has the potential for greatly reducing the cost of MMNC production, making them more attractive materials for a wide range of applications. However, producing MMNCs can be difficult owing to the high specific surface area and the poor wetting of ceramic nanoparticles by molten metals. Effective dispersion and stabilization of nanoelements in liquids is extremely challenging. Agglomeration and clustering commonly occur,[10] resulting in poor physical properties. Current solidification processing methods for MMNCs are limited in size and geometric complexity, preventing designers from achieving the design flexibility desired for complex structures (e.g., engine blocks). Recently, Lan et al.[11] and Yang et al.[12,13] developed a MICHAEL DE CICCO, Graduate Research Assistant, HIROMI KONISHI, GUOPING CAO, and HONG SEOK CHOI, Research Associates, and LIH-SHENG TURNG and XIAOCHUN LI, Professors, Department of Mechanical Engineering, JOHN H. PEREPEZKO and SINDO KOU, Professors, Department of Materials Science and Engineering, and RODERIC LAKES, Professor, Department of Engineering Physics, are with the University of Wisconsin–Madison, Madison, WI 53706. Contact e-mail: xcli@engr. wisc.edu Manuscript submitted November 16, 2008. Article published online October 14, 2009 3038—VOLUME 40A, DECEMBER 2009
new technique that combined solidification processing (e.g., casting) with an ultrasonic cavitation based dispersion of nanoparticles in metal melts. Nanoparticle reinforced magnesium and aluminum alloys were successfully fabricated. Experimental results show a nearly uniform distribution and good dispersion of the SiC nanoparticles within the metal matrix, resulting in significantly improved mechanical strength while maintaining useful ductility.[11–13] It was reported[14] that ultrasonic cavitation can produce transient (in the order of nanoseconds) micro ‘‘hot spots’’ that can have temperatures of about 5000 C, pressures above 100 MPa, and heating and cooling rates above 1010 C/s. The locally extreme conditions induced by the ultrasound can effectively disperse nanoparticles into molten metals due to the strong impact during cavitation and enhanced nanoparticle wettability. Additionally, the nanoparticle dispersion is maintained during solidification even after the ultrasonic vibration is removed.[12] While MMNCs have shown great strength improvement,
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