Size Effects on Yield Instabilities in Nickel
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0976-EE09-05
Size Effects on Yield Instabilities in Nickel Megan J. Cordill1, Neville R. Moody2, and William W. Gerberich1 1 Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN, 55455 2 Sandia National Laboratories, Livermore, CA, 94550 ABSTRACT Dislocation events are seen as excursions, or pop-in events, in the load-displacement trace of nanoindentation experiments. When indenting single crystal metals these events occur frequently during quasi-static and dynamic loading. A single crystal of Ni (111) has been indented quasi-statically using three different loading rates (10, 100, and 1000 µN/s) as well as with three different radii diamond indenter tips (1000 nm cone, 300 nm Berkovich, and 50 nm cube corner) to examine the occurrences of excursions. As expected, excursions at higher loads have larger displacements, and that initial loading follows Hertzian behavior up to the point of yield. Also, as the tip size is reduced the excursion loads are reduced. The excursion events depend mostly on the statistical distribution of surface sources and substructure dislocation arrangements. INTRODUCTION Ever since Gane [1] and Pethica [2] had measured plastic instabilities in the SEM, researchers for the last two decades [3, 4] have been examining either displacement excursions (load control) or load drops (displacement control). Such phenomena, ill understood, occur in both bulk single crystals [3] and in thin films [4]. It is proposed here that insight into the plastic deformation mechanisms associated with such plastic instabilities will provide one of the keys to length scale effects necessary for understanding nanostructures. Early on it was suggested that such instabilities were oxide films fracturing [5]. This was later rejected as the lone possibility since such instabilities were found in gold [6] and sapphire [7]. It was then proposed that such instabilities were the first dislocations nucleated under the tip and the combined role of dislocation nucleated slip bands and film fracture led to large instabilities [8]. More recent direct evidence using in situ displacement controlled nanoindentation inside a transmission electron microscope has confirmed that dislocation events occur at much smaller forces than those observed at the first excursion detected in load control [9]. Guided mostly by experimental evidence in bulk single crystals of Fe-3%Si, W, Cu, Ni, Au [6, 8, 10] or thin films of Cu and Al [3, 11, 12], the schematic load-depth curves in Figures 1a and 1b describe the general observations of yielding during indentation loading. In Figure 1a, the indenter follows the Hertzian curve at the left until it encounters a defect which triggers a yield excursion under constant load conditions. The stresses immediately accelerate dislocations to near-terminal velocities allowing a large plastic cavity to form with increased contact area. In turn, this sufficiently lowers the stress, τarrest, to values much lower than the internal stress, τµ, represented by the dislocations created.
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