Grain Boundary Character Distributions in Nanocrystalline Metals Produced by Different Processing Routes
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HIGH strength, wear resistance, and fatigue tolerance make nanocrystalline metals attractive structural materials.[1] For example, pure nanostructured Ti is being tested as a replacement for less biocompatible Ti-6Al-4V in medical implants.[2] Other current or near-term applications include more environmentally benign industrial hard coatings,[3] and alternatives to depleted uranium munitions.[4] To facilitate these advances, processing scientists have developed many techniques to synthesize nanocrystalline metals in a variety of forms and compositions. Bulk parts can be manufactured by the top-down refining of a coarsegrained alloy into a nanocrystalline one by severe plastic DAVID B. BOBER, Graduate Student, is with the Department of Mechanical and Aerospace Engineering, University of California, Irvine, CA 92697, and also Livermore Graduate Scholar with the Lawrence Livermore National Laboratory, Livermore, CA 94550. AMIRHOSSEIN KHALAJHEDAYATI, Graduate Student, is with the Department of Chemical Engineering and Materials Science, University of California, Irvine, CA 92697. MUKUL KUMAR, Group Leader, is with the Lawrence Livermore National Laboratory. TIMOTHY J. RUPERT, Assistant Professor, is with the Department of Mechanical and Aerospace Engineering, University of California, and also with the Department of Chemical Engineering and Materials Science, University of California. Contact e-mail: [email protected] The submitted manuscript has been authored by a contractor of the U.S. Government under contract number DE-AC52-07NA27344. Accordingly the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for U.S. Government purposes. Manuscript submitted May 26, 2015. Article published online December 28, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
deformation (SPD).[5] Severely deformed powders produced by ball milling can be consolidated into bulk forms, or bulk sections may be made directly through accumulative roll bonding (ARB), equal channel angular pressing (ECAP), or high pressure torsion.[6–8] A variety of physical, chemical, and electrochemical deposition techniques can be used to produce nanocrystalline coatings and even thin sheets.[9–11] The current understanding of nanocrystalline metals has been primarily built around average grain size, d, driven by the past success of the Hall–Petch relation.[12] At fine grain sizes where the Hall–Petch relationship breaks down, it has been replaced by new scaling rules that again relate strength to grain size.[13] The transition from one scaling rule to another occurs at critical grain sizes where the dominant deformation mechanisms change. The first grain size threshold is 100 nm, below which dislocations nucleate at grain boundaries, sweeping through entire grains without interacting with each other and forming tangles.[14] At even smaller grain sizes, around 10 nm, grain boundary sliding and rotation supplant dislocations as carriers of plasticity, eventual
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