Under stress, many crystalline materials exhibit irreversible plastic deformation caused by the motion of lattice dislocations. In plastically deformed microcrystals, internal dislocation avalanches lead to jumps in the stress-strain curves (strain bursts), whereas in macroscopic samples plasticity appears as a smooth process. By combining three-dimensional simulations of the dynamics of interacting dislocations with statistical analysis of the corresponding deformation behavior, we determined the distribution of strain changes during dislocation avalanches and established its dependence on microcrystal size. Our results suggest that for sample dimensions on the micrometer and submicrometer scale, large strain fluctuations may make it difficult to control the resulting shape in a plastic-forming process.
The ongoing trend in miniaturization of micro-electro-mechanical systems (MEMS), medical devices (e.g. STENTs), and microelectronic devices leads to structural elements with one or more dimensions in the range of a few tens of nanometres to a few hundred microns. This requires novel techniques to determine the mechanical properties of materials at this small length scale.The most frequently used method is the nanoindentation technique. [1±4] It is usually straightforward to apply to a wide range of materials, is essentially non-destructive, and material properties like hardness and reduced elastic modulus can be deduced from such experiments. With some additional efforts attributes like yield strength, creep behaviour and fracture toughness can be determined. However, several problems arise when performing indentation experiments. The shape of the indenter tip is usually not exactly known, but it strongly influences the actual contact area and therefore the calculated values of hardness and stiffness at very shallow indentation depths. Furthermore, one has to deal with a complex stress and strain field beneath the indenter tip, which makes it difficult to directly compare indentation data to material properties obtained from tension, compression or bending tests, which all possess a well defined stress and strain distribution.To overcome the problems of indentation tests, efforts were undertaken to adapt conventional material test methods to micron-sized samples. Fleck et al. [5] performed torsion tests on copper wires and reported increasing torsional resistance when the wire diameter was reduced from 170 to 12 lm. Stölken and Evans [6] performed bending tests on nickel foils and observed increasing bending strength as the foil thickness decreased from 50 to 12.5 lm. This increase in yield strength and flow stress was attributed to an increasing strain gradient with decreasing specimen size. [5,7] Weiss et al. [8] performed tension experiments on free standing thin Cu foils and wires and reported a strong influence of the sample thickness on the fracture strain, with thinner samples exhibiting reduced plasticity.In recent times, compression tests on micron-sized samples prepared with the focused ion beam (FIB) technique have been reported in the literature. [9±12] For a variety of materials a strong size effect in flow stress was found, if the diameter of the so-called ªmicro-pillarsº was reduced from 10 to 0.2 lm. This size effect obtained by micro-compression tests can not be explained by strain gradient plasticity approaches. To elucidate the influence of a strain gradient on the deformation behaviour at the micrometer scale, micron-sized compression (no strain gradient present) and bending (strain gradient present) tests were performed. The results are compared and discussed in this paper. Furthermore, quantitative Auger electron spectroscopy (AES) was performed on the damaged zone at the surface of FIB designed miniaturized samples. The results obtained will be used in the interpretation of the mechanical...
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