High-performance bulk Ti-Cu-Ni-Sn-Ta nanocomposites based on a dendrite-eutectic microstructure
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G. He Light Materials Group, National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan
Y. Li Department of Materials Science, National University of Singapore, Singapore
J. Eckertb) Department of Materials and Geo-Sciences, Physical Metallurgy Division, Darmstadt University of Technology, D-64287 Darmstadt, Germany
W.K. Luo and E. Ma Department of Materials Science and Engineering, The Johns Hopkins University, Baltimore, Maryland 21218 (Received 25 February 2004; accepted 16 March 2004)
Using a Ti–Cu–Ni–Sn–Ta alloy as an example, we demonstrate a strategy for the in situ formation of nanocomposite microstructures that can lead to simultaneous high strength and ductility. Our approach employs copper mold casting for the production of bulk alloys from the melt, and the solidification microstructure is designed to be composed of micrometer-sized ductile dendrites uniformly distributed inside a matrix of nanoscale eutectic reaction products. The nanostructured matrix is achieved at a relatively deep eutectic, which facilitates the formation of an ultrafine eutectic microstructure over a range of cooling rates. The multi-component recipe stabilizes a ductile solid solution as the toughening phase and helps to reduce the eutectic spacing down to nanoscale. The multi-phase microstructure (including phase distributions, morphologies, and interfaces) has been examined in detail using transmission electron microscopy (TEM) and high-resolution TEM. The metastable eutectic reaction and the nanoscale spacing achieved are explained using thermodynamic and solidification modeling. The benefits expected from the microstructure design are illustrated using the high strength and large plasticity observed in mechanical property tests. Our nanocomposite design strategy is expected to be applicable to many alloy systems and constitutes another example of tailoring the microstructure on nanoscale for extraordinary properties.
I. INTRODUCTION
Nanostructures have long been used in alloys for strengthening purposes, such as extremely fine second phases in precipitation and dispersion hardening.1 In recent years, a large number of nanostructured alloys and a)
Address all correspondence to this author. e-mail: [email protected] b) This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http:// www.mrs.org/publications/jmr/policy.html. DOI: 10.1557/JMR.2004.0332 J. Mater. Res., Vol. 19, No. 9, Sep 2004
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composites have been developed in which the matrix, or the majority phase, is itself a nanostructured alloy.2 While such nanophase alloys and composites offer extraordinary properties such as high strength, they usually have to be made through multi-step processing, e.g., via powder metallurgy of nanocrystalline powders,3 or crystallization from amorphous materials.4,5 The former route often suffers from porosity and contamination problems in the
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