Amorphization of rapidly quenched quasicrystalline Al-transition metal alloys by the addition of Si
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I. INTRODUCTION Shechtman et al.' have recently reported a quasicrystalline phase for rapidly solidified Al 86Mn14. This alloy shows an electron diffraction pattern with apparent fivefold symmetry.1'2 It has since been found that this phase is formed in rapidly solidified Al-Mn alloys over a composition range from about 9-25 at. % Mn. 3 Bendersky etal. have reported this phase for alloys with 15-35 wt. %/Mn. As well, Bancel et al.4 and Dunlap and co-workers5~8 have reported a similar structure in a wide variety of rapidly quenched Al-transition metal alloys with the approximate stoichiometry 6:1. Previously Suzuki et al.9 reported that rapidly quenched Al-Fe-Si alloys were amorphous over a small composition range near the stoichiometry Al70Fe13Si17. This structure has been confirmed recently by Guyot and Audier10 and Dini and Dunlap. 11 The transition from a quasicrystalline to an amorphous structure, which occurs with increasing Si content in rapidly quenched Al86 _ x Fe14Si;c and Algg^Mn^Si* alloys, has already been reported.'' Several studies on rapidly quenched AlMnSi alloys that exhibit icosahedral symmetry have also been reported by, e.g., Chen and Chen,12 Eibschutz et al.,13 Warren et al.,14 Schaefer and Bendersky,15 and Elser and Henly.16 In the present work we report on the effects of Si on the structure of a variety of rapidly quenched quasicrystalline alloys of Al and 3c? transition metals of the form Al 86 _ x 7 1 14 Si x where (7 7 =Cr, Mn, Fe, Co, and Ni). The structure has been determined by x-ray diffraction measurements, and the thermal properties have been studied by differential thermal analysis measurements.
II. EXPERIMENTAL METHODS Alloys of the stoichiometry Al86 _ x T14Six were prepared with T = Cr, Mn, Fe, Co, and Ni by rapid quenching from the melt onto the surface of a single Cu roller. Details of the sample preparation have been presented elsewhere.7'8'17 The quench rate was estimated on J. Mater. Res. 1 (3), May/Jun 1986 http://journals.cambridge.org
the basis of the surface velocity of the roller. With the exception of the alloys prepared specifically for the investigation of the effects of quench rate, all alloys reported in this study were quenched at a rate of 1.5 X10 6 K s~'. Alloys were prepared with values of x from 0 up to some maximum value. For alloys with T = Cr, Mn, Fe, Co, and Ni, the maximum x prepared was 24,16,16,16, and 16, respectively. The x-ray diffraction measurements were performed in a Siemens powder diffractometer using MoATa radiation. Differential thermal analysis (DTA) measurements were performed on a Fisher 260F thermal analyzer. All measurements were made with a heating rate of 20 K/min. III. RESULTS AND DISCUSSION A. Structure of as-quenched alloys All as-quenched alloys with x = 0 showed the quasicrystalline structure reported previously by Dunlap and Dini7'8 for alloys of these stoichiometries. Shechtman et al.1 have reported that these alloys with T = Cr, Mn, or Fe are quasicrystalline with an icosahedral structure. Dunlap and Dini7'8 have shown that si
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