Plasticity in Nanomaterials
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Plasticity in Nanomaterials Guo-Dong Zhan, Joshua D. Kuntz, Julin Wan, and Amiya K. Mukherjee Department of Chemical Engineering and Materials Science University of California, One Shields Avenue, Davis, CA 95616 ABSTRACT There have been many predictions of the reinforcing effects of carbon nanotubes in various composite matrices but large improvements in properties have not yet been convincingly demonstrated. In the present study, we have successfully realized this possibility in reinforcing nanocrystalline alumina. Fully dense single-wall carbon nanotubes (SWCN)/Al2O3 nanocomposites with nanocrystalline alumina matrix have been fabricated at sintering temperatures as low as 1150oC by spark-plasma-sintering (SPS). A fracture toughness of 9.7 MPam1/2, nearly three times that of pure nanocrystalline alumina, has been achieved in the 10 vol.% SWCN/Al2O3 nanocomposite. Moreover, high-strain-rate superplasticity has been achieved in Al2O3/ZrO2/MgAl2O4 nanocomposite with truly nanocrystalline grain size of 100 nm. Compression superplastic tests were conducted in the temperature range of 1300-1450°C at strain rates 10-3-10-1 s-1. The results generated a stress exponent of ~ 2 and an activation energy of ~ 620 kJ/mol. INTRODUCTION The fabrication of nanocrystalline materials is an exciting area of materials research because such bulk materials with grain sizes less than 100 nm exhibit novel properties as compared with their microcrystalline counterparts, such as optical transparency and enhanced superplasticity. However, the brittleness of nanocrystalline ceramics has limited their potential and promise for use in structural applications. Carbon nanotubes, especially single-wall carbon nanotubes (SWCN), should be ideal reinforcing fibers for composites [1]. Theoretical and experimental studies [2,3] showed that carbon nanotubes with very high aspect ratios (length-todiameter ratio of 1,000 or more) have exceptional mechanical characteristics. SWCN are among the stiffest fibers known, with a measured Young’s modulus of ~1.5 TPa [4]. However, to date, the utilization of the extraordinary mechanical properties of carbon nanotubes in composites has not been successfully realized, e.g., in alumina based systems only a 24% increase in toughness has been obtained so far. In the present study, we have successfully realized the potential of carbon nanotubes in significantly reinforcing ceramics for the first time. Superplasticity is another exciting area for nanocrystalline ceramic materials. High strain rate superplasticity (HSRS) is usually referred to as the demonstration of high ductility at strain rates around 10-2 s-1 or greater [5,6] . HSRS used to be a phenomenon found exclusively in finegrained metals and metal matrix composites. For ceramics, the typical superplastic strain rate is in the range of 10-5-10-4 s-1. Ceramic HSRS was not observed until very recently [7,8]. Kim et al. [7] reported a composite ceramic material consisting of tetragonal ZrO2, MgAl2O4 and αAl2O3 phases that exhibit superplasticity at strain
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