Hardness and Abrasion Resistance of Nanocrystalline Nickel Alloys Near the Hall-Petch Breakdown Regime
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Hardness and Abrasion Resistance of Nanocrystalline Nickel Alloys Near the Hall-Petch Breakdown Regime Christopher A. Schuh1 and T. G. Nieh2 1 Department of Materials Science and Engineering, Massachusetts Institute of Technology 77 Massachusetts Avenue, Room 8-211, Cambridge, MA 02139, USA 2 Materials Science and Technology Division, Lawrence Livermore National Laboratory 7000 East Avenue, L-350, Livermore, CA 94550, USA ABSTRACT The breakdown of classical Hall-Petch scaling remains an area of scientific interest, and will govern the limiting strength of nanocrystalline alloys for structural applications. In this work we discuss the hardness and scratch resistance of nanocrystalline nickel and nickel-tungsten solid solution alloys, assessed through nanoindentation and nano-scratch techniques. The materials have been prepared by electrodeposition, and are fully dense with grain sizes between 6 and 22 nm. In this range of grain sizes, there is some evidence for a breakdown of Hall-Petch scaling, reflected in both the hardness and abrasion data. The role of solid solution alloying on this breakdown is also discussed. INTRODUCTION The grain size of metals can now be varied through a broad range, including the submicrocrystalline and nanocrystalline regimes, owing to the development of novel processing methods. For example, severe plastic deformation techniques [1, 2] are applicable to most alloys, and can produce grain sizes below one micron. Compaction of nano-powders or nanocrystalline powders offers the possibility of finer grain sizes below 100 nm [3, 4], as do several deposition-based methods such as electrodeposition [5, 6]. The development of nanocrystalline alloys has been driven technologically, by the promise of exceptional properties, as well as scientifically, by the potential breakdown of classical scaling laws and the accompanying need for new materials physics in the nanostructured limit. For example, HallPetch strengthening (strength increasing as the inverse square-root of grain size) is expected to break down for grain sizes in the range of 5-30 nm [7] and in fact should give way to a weakening trend toward the amorphous limit. Such an inflection is not only scientifically interesting, but also sets the physical limit for strengthening of nanostructured engineering materials. Hall-Petch scaling and its possible breakdown in the nanocrystalline regime have been investigated experimentally and theoretically by many authors. The purpose of the present work is to extend the discussion of Hall-Petch breakdown to consider, firstly, the possible roles that solid-solution alloying may play with regard to this effect, and secondly, the implications of Hall-Petch breakdown for abrasion resistance in nanostructured materials. These issues have particular relevance with regard to next-generation micro electromechanical systems made by electrodeposition-based “LIGA” processes.
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EXPERIMENTAL PROCEDURES Foils of nanocrystalline nickel (n-Ni) and nickel-tungsten (n-Ni-W) were fabricated by direct cur
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