Abstract
For some polycrystalline metals with grain sizes in the nano regime, experiments have suggested a deviation away from the Hall-Petch relation relating yield stress to average grain size [1]. The debate continues whether or not such deviations are a result of intrinsically different material properties of nanocrystalline (nc) systems, or due simply to inherent difficulties in the preparation of fully dense nc-samples and in their microstructural characterization. Nevertheless, it suggests that the traditional work hardening mechanism of pile-up of dislocations originating from Frank-Read sources may no longer be valid at the nanometer scale. In-situ deformation testing in the transmission electron microscope (TEM), performed on Cu and Ni3Al nc samples, reveals a limited dislocation activity in grains below 50nm [2,3]. However, due to the presence of large internal stresses which make grain boundaries (GB) in TEM images difficult to observe, and also possible artifacts induced by thin-film geometry such as dislocations emitted from the surface [4], in-situ tensile tests did not until now, bring convincing evidence for abundant dislocation activity. Mechanical testing also revealed the issue of the “GB state” by means of a property dependence on thermal history and internal strains. It is shown that a substantial strengthening can be obtained by a short heat treatment. The cause of the strengthening is possibly associated with a reduction in internal strains and/or dislocation content produced by the annealing [5]. The effect of strengthening has been measured both on nc materials obtained by grain refinement techniques and those obtained by consolidation of clusters.
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Van Swygenhoven, H., Derlet, P.M., Hasnaoui, A., Samaras, M. (2003). Impact of Grain Boundaries on Structural and Mechanical Properties. In: Tsakalakos, T., Ovid’ko, I.A., Vasudevan, A.K. (eds) Nanostructures: Synthesis, Functional Properties and Applications. NATO Science Series, vol 128. Springer, Dordrecht. https://doi.org/10.1007/978-94-007-1019-1_8
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DOI: https://doi.org/10.1007/978-94-007-1019-1_8
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