~ Pergamon Acta metall, mater. Vol. 43, No. 9, pp. 3539 3551, 1995 Elsevier Science Ltd 0956-7151(95)00024-0 Copyright © 1995 Acta Metallurgica Inc. Printed in Great Britain. All rights reserved 0956-7151/95 $9.50 + 0.00 A TRANSMISSION ELECTRON MICROSCOPIC STUDY OF ICOSAHEDRAL TWINS--I. RAPIDLY SOLIDIFIED A1-Mn-Fe ALLOYS ALOK SINGH and S. RANGANATHANt Centre for Advanced Study, Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India (Received 3 November 1993; in revised form 16 October 1994) AMtraet--Dendrites of the icosahedral quasicrystalline phase in an aluminium matrix were formed on rapid solidification of an A190 Mn5Fe5 alloy. A large number of these particles was found to be twinned with one of the five-fold symmetry axes as the twin axis, as established by careful tilting experiments in the transmission electron microscope. The twin relationship can be expressed in 60 different ways by a rotation around seven different axes of the icosahedron and utilising 12 different angles. In the alloys of composition Als6 Mn7 Fe7and Als0 Mn ~0 Fe~0 an icosahedral phase, AI 3 Fe and ~-AI formed at wheel surface velocities above 23.5 m/s. The icosahedral phase in these alloys is not twinned. At 12 m/s wheel velocity only A13 Fe and ~-AI formed. The relationship of the icosahedral twin to the decagonal phase has been brought out. 1. INTRODUCTION After the discovery of the icosahedral [1] and the decagonal quasicrystalline phases [2-4], the AI-Mn binary system has been of special interest. The decag- onal phase formation was found to be promoted by a manganese content more than 16% (all compo- sitions are given in at.%) and a solidification rate slower than that required for the formation of the icosahedral phase. The decagonal quasicrystalline phase was then discovered in the A1-Fe system too [5]. Though the occurrence of the icosabedral phase in binary AI-Fe alloys has been reported [6], it is not completely accepted. The quasicrystalline phases have been studied extensively in AI-Mn and A1-Fe binary systems but the AI-Mn-Fe ternary system has not received adequate attention. This ternary system is of interest because, since the AI-Mn alloys with Mn< 16% produces the icosahedral phase and AI-Fe alloys are known to produce the decagonal phase, a substitution of manganese by iron is ex- pected to produce structures intermediate between the icosahedral and the decagonal phase. Schaefer [7] has claimed a single phase decagonal quasicrystal in an Ala0Mn~sFe 2 alloy. An early report of investigation of the AI-Mn-Fe system is by Degischer [8]. In a subsequent study on this system, Phillips [9] identified the constituent phases as A13Fe, AI4Mn, AIrMn and ~-AI. Further work on this system has been done by Denholm et al. [10, 11] for determining the location of the eutectic troughs, the ternary eutectic point, and the corre- tTo whom all correspondence should be addressed. sponding variation in the composition of A13Fe and AI~(MnFe). A ternary AI6(MnFe) phase, isostruc- tural to and with lattice parameters close to, the orthorhombic phase A16Mn was reported by Hollingsworth et al. [12]. An evaluation of the AI-Mn-Fe ternary phase diagrams has recently been made by Ran [13]. For studying the quasicrystalline phases Wang et al. [14] have studied the crystallization process in rapidly quenched A16Mn and AIr(MnFe) alloys and observed that the icosahedral and ~-AI both partici- pate to form stable AI6Mn and AIr(MnFe) crystalline phases respectively. Koskenmaki et al. [15] found the microstructure of rapidly solidified AlsrMnTFe7 to consist of rosette shaped icosahedral phase grains but in numerous instances these grains were twinned with a mirror plane normal to one of the five-fold axes. Ranganathan et al. [16] studied these icosahedral twins in A190Mn~0alloy and showed that the overall symmetry of the grain becomes close to that of the decagonal phase. Singh and Ranganathan [17] have reported a preliminary study of these twins in AI-Mn-Fe alloys. This paper reports detailed investigations into twinning of the icosahedral phase. Three A1-Mn-Fe alloys have been studied here by rapid solidification using melt-spinning and twin-roller techniques. Qua- sicrystal phase formation, its solidification mor- phology, the occurrence of icosahedral twins and other related phases were examined. Twinning of the icosahedral phase has been re- ported in the A162Cu25.sFet2.5 alloy [18] in a recent independent study. A companion paper [19] reports the study of icosahedral twins in Al~Su-Fe alloys. 3539