Nanoscale strength distribution in amorphous versus crystalline metals
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Low-load nanoindentation can be used to assess not only the plastic yield point, but the distribution of yield points in a material. This paper reviews measurements of the socalled nanoscale strength distribution (NSD) on two classes of materials: crystals and metallic glasses. In each case, the yield point has a significant spread (10–50% of the mean normalized stress), but the origins of the distribution are shown to be very different in the two materials classes. In crystalline materials the NSD can arise from thermal fluctuations and is attended by significant rate and temperature dependence. In metallic glasses well below their glass-transition temperature, the NSD is reflective of fluctuations in the sampled structure and is not very sensitive to rate or temperature. Computer simulations using shear transformation zone dynamics are used to separate the effects of thermal and structural fluctuations in metallic glasses, and support the latter as dominating the NSD of those materials at low temperatures. Finally, the role of the NSD as a window on structural changes due to annealing or prior deformation is discussed as a direction for future research on metallic glasses in particular.
I. INTRODUCTION
The advent of nanoindentation and other nanoscale mechanical tests has permitted the study of deformation mechanisms at the finest scales, often as fine as the carriers of plasticity themselves. For example, low-load spherical nanoindentation techniques have successfully detected subnanometer perturbations associated with “incipient plasticity” phenomena such as dislocation nucleation in crystalline materials1–7 and shear banding in metallic glasses.8–10 By applying low loads to impress a diamond tip into a test material while dynamically recording load and displacement with high resolution, deformation is stably promoted in a local region that is free from interference from prior sample deformation. What is more, due to the fine length scales of these tests, they can in principle be compared with atomistic or mechanistic simulations.11–14 A key strength of nanoindentation testing is that it permits rapid acquisition of data; hundreds of experiments can be conducted on a single sample in a matter of hours. This opens the door to new types of studies focused on statistics or spatial mapping of local mechan-
ical properties. For example, in composite microstructures an array of local nanoindentations can be used to map modulus, hardness, or other properties.15–20 Even within a single-phase sample, nanoindentation can be used to assess the statistical distribution of plastic events that may happen under nominally identical test conditions. Such measurement statistics can be used to infer details of the controlling physics that underlie deformation. In our group’s recent work, we have studied the distribution of plastic events in materials with both crystalline4,21,22 and amorphous23,24 structures, and in each case have been able to discern interesting details about the onset of plastic flow. The purpose of this pap
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