Dislocation Dynamics in Icosahedral Al-Pd-Mn Single Quasicrystals
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Dislocation Dynamics in Icosahedral Al-Pd-Mn Single Quasicrystals Ulrich Messerschmidt1, Martin Bartsch1, Bert Geyer1, Lars Ledig1, Michael Feuerbacher2, Markus Wollgarten2*, and Knut Urban2 1 Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, Halle(Saale), D-06120, Germany 2 Institut für Festkörperforschung, Forschungszentrum Jülich, Jülich, D-52425, Germany *Present address: Institut für Angewandte Physik, ETH Zürich, CH-8093 Zürich, Switzerland ABSTRACT The paper reviews results from in situ straining experiments on Al-Pd-Mn single quasicrystals in a high-voltage electron microscope. Slip planes were determined from the orientation and width of slip traces. Dislocations are generated by a specific cross slip mechanism. On some slip traces, dislocations move at two distinctly different velocities. A stress exponent was determined on a single dislocation by observing its displacement under decreasing load. The in situ experiments reveal the behaviour of individual dislocations in a temperature range where the deformation of bulk specimens is strongly affected by recovery. INTRODUCTION It was first shown by a drastic increase of the dislocation density that icosahedral Al-Pd-Mn single quasicrystals deform plastically by a dislocation mechanism [1]. Later on, dislocation motion was directly observed during in situ straining experiments in a high-voltage electron microscope (HVEM) [2]. The macroscopic data on the activation parameters of plastic deformation were first interpreted solely in terms of thermally activated processes controlling the dislocation mobility [3]. According to the present understanding, the deformation of Al-Pd-Mn quasicrystals at high temperatures showing a yield point and a flow range is only possible if the work-hardening is balanced by recovery processes [4,5]. Then, the macroscopic deformation data reflect the processes controlling both the dislocation mobility and the evolution of the dislocation microstructure. Since it is difficult to separate these influences, independent information is desired on the dynamic dislocation properties. It can be obtained from in situ straining experiments. Therefore, additional in situ experiments were performed in the HVEM with the aim to obtain information on the processes of dislocation generation and motion. The present paper summarizes first results. EXPERIMENTAL Microtensile specimens with tensile directions parallel to twofold and fivefold axes were prepared from icosahedral Al-Pd-Mn single quasicrystals by cutting using a wire saw, grinding, dimpling and electrolytic jet polishing. They were fixed to the grips of a double-tilting high-temperature straining stage for the HVEM [6] via holes and tungsten pins. The specimens were loaded in small load increments. The changes in the dislocation structure were recorded on photographic film or on video cassettes. Up to now, experiments were successful between 675 °C and 765 °C. Above this range the specimens degraded quickly, below it they broke in a brittle way. The activated slip planes were identif
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