The crystallization and growth of AlB 2 single crystal flakes in aluminum
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The crystallization and growth of AlB2 single crystal flakes in aluminum C. Deppisch, G. Liu, and A. Hall Department of Materials Science and Engineering, University of Illinois, Urbana, Illinois 61801
Y. Xu Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801
A. Zangvil Department of Materials Science and Engineering, and Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801
J. K. Shang and J. Economy Department of Materials Science and Engineering, University of Illinois, Urbana, Illinois 61801 (Received 12 September 1996; accepted 3 March 1998)
An in situ high temperature heat treatment was used to investigate the crystallization and growth behavior of AlB2 flakes in aluminum. Aluminum samples containing 1.8% boron were heated above the liquidus and then rapidly cooled through the Al(L) 1 AlB12 region to avoid the formation of AlB12 crystals. Subsequently, a homogeneous distribution of high aspect ratio AlB2 flakes crystallized upon holding below the peritectic transition temperature. Growth rate in the (a) and (c) dimensions increased during elevated hold temperatures below the peritectic transition temperature. Surprisingly, faster cooling rates from above the liquidus to room temperature resulted in thinner, wider flakes. Similar to graphite this phenomenon is believed to result from a need to accommodate a changing misfit strain energy between the solidifying aluminum and the growing AlB2 flakes.
I. INTRODUCTION
Continuous fiber reinforced composites with an aluminum matrix exhibit excellent mechanical properties in the longitudinal direction because of the high specific strengths and moduli of their fibers. However, these composites have not proven to be cost effective due to the high cost of reinforcements and the need to protect the fiber surface from attack by aluminum, resulting in multiple processing steps.1,2 In recent years, this has led to considerable interest in discontinuous reinforced aluminum matrix composites such as whiskerand particle-reinforced composites, because of their improved mechanical properties, relatively low cost, and ease of machining and fabrication.1– 6 It has been documented in the literature that, in theory, a high modulus, high aspect ratio [width (W )y thickness (T)] flake or whisker reinforcement should yield higher strengths than a low aspect ratio particulate reinforcement of equal modulus, when both are uniformly distributed throughout the matrix.5 –12 Numerous materials crystallize as flakes such as Al–Si eutectic,13 –15 graphite eutectic,16,17 TiB2 ,18 and AlB2 .19 Although the growth mechanisms of high aspect ratio Al–Si eutectics, TiB2 flakes, and graphite flakes have been well established, similar conditions for the AlB2 flakes have not yet been determined. This is intriguing because past research indicates that high aspect ratio J. Mater. Res., Vol. 13, No. 12, Dec 1998
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flakes in epoxy display far better mech
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