Low temperature plasticity of an AlPdMn quasicrystal
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Low temperature plasticity of an AlPdMn quasicrystal Frédéric Mompiou, Daniel Caillard CEMES/CNRS 29, rue J. Marvig 31055 Toulouse, France
ABSTRACT TEM observations have been performed in AlPdMn single-grain quasicrystal deformed at low temperature. They show that dislocation motion has occurred by climb associated with vacancy diffusion. At room temperature, deformation also occurs by crack followed by rehealing.
INTRODUCTION Since quasicrystals have been discovered, several studies have been devoted to explain their unusual mechanical properties, such as high brittleness at low and medium temperature. It is now well established that plastic deformation at high temperature takes place by nucleation and motion of dislocations at least for icosahedral AlPdMn [1]. From the first and further experiments, substantial progress has been made in the interpretation of these defects in transmission electron microscopy. The idea that dislocations essentially move by glide has then raised, although no experimental determinations of both Burgers vectors and displacement planes have been obtained. The first evidence of climb came out recently from careful studies of dislocations in as-grown samples [2-4]. However, since these observations have been performed on dislocations induced by the constraints generated during the elaboration and cooling of the samples, further ones had to be performed on plastically deformed materials to confirm the importance of climb. In that context, we have performed TEM observations on samples deformed at low temperature (20°C and 300°C) under a high confining pressure. At such low temperatures, the planes of motion could be determined as the habit planes of the so-called “phason faults”, which have been produced during the motion of dislocations [5]. Details on the rules of contrast of “imperfect” dislocations and phason faults can be found in [6].
EXPERIMENT Single grains of icosahedral Al70.1Pd20.4Mn9.5 were grown by Y. Calvayrac in CECM (Vitry) along a 5-fold direction [1/0, 0/1, 0/0] (notation from [7]) by the Czochralski method. Two cylindrical samples were then deformed in a multi-anvil apparatus designed to generate a high differential compressive stress. A pressure of 5 and 7 GPa has been reached respectively at 20°C and 300°C. After a few percents of deformation, the samples were cut in, 20°, and 45° away from the 5-fold compression axis. They were polished mechanically and thinned down by ion
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milling. All the observations have been performed in a JEOL 2010 HC electron microscope operating at 200 kV.
RESULTS Samples deformed at 300°C Several dislocation families have been found with large corresponding densities. All the dislocations are imperfect and exhibit phason faults in their wake. Dislocations of the first family are shown in figure 1. They form typical arrangements consisting of one leading pair of dislocations (1, 2) followed by one or several isolated ones (3). From its trace direction and its variation of apparent width as a function of tilt axis, the pla
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