On the nature of the quasicrystalline phases obtained in alloys of Al 86 Mn 14 under slow solidification rates
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I. INTRODUCTION The icosahedral quasicrystalline phase of Al-Mn and its quasicrystalline related phases have been widely studied in the past. l~* This icosahedral phase has been found to solidify in conjunction with other quasicrystalline phases3: the so-called T phases. Furthermore, AlMn alloys cooled at slower solidification rates have given rise to quasicrystalline phases that resemble the icosahedral phase in structure.4'5 In this article we present a study of the quasicrystalline structure obtained from an alloy of Al-Mn cooled at slow solidification rates. Quasicrystalline phases similar in structure to the so-called T" phases reported by Berdensky et al.5 were found. The analysis of pseudofivefold diffraction patterns obtained from quasicrystalline grains from the alloy studied in the present work suggests that these phases could be obtained from the icosahedral quasicrystalline phase when particular stress environments are acting on the icosahedral grain. II. EXPERIMENTAL PROCEDURE A furnace with a 200 g capacity was used to prepare the Al86Mn14 alloy, along with an alumina crucible and Al and Mn metals that had 99.9% purity. The fusion was made with the appropriate quantities of alloy metals, fundent, and slagser. After degasification of the liquid alloy and withdrawal of the slags, the alloy was held at approximately 1573 K for 10 min. The liquid alloy was subsequently poured into a copper mold with a polished surface. The mold was held at liquid nitrogen temperature (77 K ) . After the quenching we obtained an ingot of 2 in. in diameter and 0.25 in. in thickness. The J. Mater. Res. 3 (1), Jan/Feb 1988
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specimens were prepared from the contact area of the alloy and the polished surface in the mold. These specimens were finally electropolished for observation in the electron microscope. The solution used in the electropolishing was obtained with 40% acetic acid, 30% phosphoric acid, 20% nitric acid, and 10% water. The observations were carried out in a JEOL 100CX microscope.
III. EXPERIMENTAL RESULTS The quasicrystalline phases obtained in these rapidly solidified alloys (Al-Mn) have been characterized using selected area diffraction patterns (SAD). Thus, for example, Fig. 1. illustrates the most important electron diffraction patterns observed in this phase. The angles between each one of the different "zone axes" are similar to angles recently reported in the literature.6 It is important to mention that the diffraction patterns shown in Fig. 1 were obtained from a faulted quasicrystalline area similar to the T" phase.5 However, pseudofivefold patterns and also threefold diffraction patterns have been found to be close in characteristics to those reported lately for Al 4 Mn. 5 Conventional methods for proving that a quasicrystalline phase was not produced by multiple twinning have been carried out. Thus, for example, Fig. 2(a) shows a bright-field image (BF) of the grain. On the other hand, Figs. 2(b) and 2(c) show a SAD pattern and its corresponding microdiffraction patter
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