Scientific Advisory Board - Erich Schmid Institute

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$dissertation global and local fracture properties of metal matrix ...$

ERICH SCHMID INSTITUTE OF MATERIALS SCIENCE one characteristic of single crystalline microcantilevers is the so called Bauschinger effect, which is reflected by a nonlinear unloading behavior in the stress-strain curves (fig. 11b). this is attributed to the partial dissolution of the gnd pile-up upon unloading induced by pile-up stresses. this reverse motion of gnds during unloading was for the first time clearly confirmed by in situ µLaue experiments (fig. 10) in terms of a reduction of the gnd density, which is proportional to the full Width at Half maximum (fWHm, fig. 11c). the higher pile-up stresses in bicrystalline cantilevers led to a greater reduction of the gnd density. Hence, a more pronounced Bauschinger effect was observed. Because of marginal pile-up stresses in the single crystalline cantilever with the slit along the neutral axis, the gnd density was only slightly reduced and a Bauschinger effect was almost missing. A significant contribution of the pile-up stress to the size-effect was demonstrated by comparing the stress level of the three different cantilever geometries at comparable strain values. it was clearly evidenced that the stress level was increased for higher magnitudes of the pile-up stress. these experiments show for the first time very clearly that the dislocation pile-up is, besides the standard strain gradient plasticity, one of the dominant mechanisms in microcantilevers. Fig . 11 a) Schematic diagrams of three different microcantilever geometries: single crystalline, bicrystalline, single crystalline with slit along the neutral axis. b) Stress-strain curves of the three cantilever geometries reveal different unloading curves and varying hardening behavior. c) The µLaue data describe the evolution of the GND density (FWHM) during loading and unloading. Fatigue of microbeams due to the high strength of nanocrystalline and ultrafine grained materials this new class of materials is a candidate for heavily loaded components. Besides the strength also the page 26 <strong>Scientific</strong> RepoRt 2012
ERICH SCHMID INSTITUTE OF MATERIALS SCIENCE fatigue properties are of essential importance. cyclic loading can induce softening and crack generation, these are the most important fatigue controlling phenomena in these types of materials. due to the very local processes it is difficult to study these mechanisms in macrosized samples. fatigue experiments of ultra-fine grained cu microbeams have been subjected to fatigue and low cycle fatigue loading. the coarsening and the initiation of crack like defects could be analyzed in detail. for the first time it could be shown that micrometer sized samples offer the possibilities to study these phenomena in detail. examples of this observation are presented in fig. 12 Fig . 12 Illustration of the micro beam fatigue experiment performed on ultrafine grained Cu. (a) shows the in situ loading of the sample, (b1-b4) illustrates the damage evolution with increasing loading cycles, and (c and d) visualizes the coarsening of microstructure by orientation micrographs and BSE images, the original grain size is visible in the undeformed and less deformed areas of the sample. In (d) the neutral axis is indicated as a blue line. <strong>Scientific</strong> RepoRt 2012 page 27
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Scientific Advisory Board - Erich Schmid Institute

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