When a crystal deforms plastically, phenomena such as dislocation storage, multiplication, motion, pinning, and nucleation occur over the submicron-to-nanometer scale. Here we report measurements of plastic yielding for single crystals of micrometer-sized dimensions for three different types of metals. We find that within the tests, the overall sample dimensions artificially limit the length scales available for plastic processes. The results show dramatic size effects at surprisingly large sample dimensions. These results emphasize that at the micrometer scale, one must define both the external geometry and internal structure to characterize the strength of a material.

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Under stress, crystals irreversibly deform through complex dislocation processes that intermittently change the microscopic material shape through isolated slip events. These underlying processes can be revealed in the statistics of the discrete changes. Through ultraprecise nanoscale measurements on nickel microcrystals, we directly determined the size of discrete slip events. The sizes ranged over nearly three orders of magnitude and exhibited a shock-and-aftershock, earthquake-like behavior over time. Analysis of the events reveals power-law scaling between the number of events and their magnitude, or scale-free flow. We show that dislocated crystals are a model system for studying scale-free behavior as observed in many macroscopic systems. In analogy to plate tectonics, smooth macroscopic-scale crystalline glide arises from the spatial and time averages of disruptive earthquake-like events at the nanometer scale.

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In this paper we present a mechanical test methodology to explore specimen size effects in Ni3Al, where the overall test sample dimensions artificially limit the volume for substructure evolution and hence the availability of jogs/kinks along individual dislocation lines. The test methodology consists of using Focused Ion Beam milling to micromachine cylindrical compression samples that have diameters ranging from 5 to 20 microns into the surface of a bulk sample, which is followed by nanoindentation using a flat-ended tip to measure the mechanical properties of the microsamples in uniaxial compression. The initial test results show that there is a strong increase in the flow stress with decreasing sample size, although misfit between the flat indenter tip and the top surface of the compression samples complicates complete interpretation of the mechanical test results at this time.

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