L Journal of Alloys and Compounds 342 (2002) 246–250 www.elsevier.com / locate / jallcom On the phase transitions of the quasicrystalline phases in the Al–Cu–Fe–Co alloy a, b c d a * S.H. Kim , B.H. Kim , S.M. Lee ,W.T. Kim , D.H. Kim a Center for Noncrystalline Materials, Yonsei University, Seoul 120-749, South Korea b Korea Aerospace Industry, Changwon 641-120, South Korea c Korea Institute of Industrial Technology, Inchon 404-254, South Korea d Department of Physics, Chongju University, Chongju 360-764, South Korea Abstract Microstructural change with Co content together with heat treatment was studied in rapidly solidified Al–Cu–Fe–Co alloys using X-ray diffractometry and transmission electron microscopy. With an increase in Co content in the as-cast Al Cu Fe Co alloys, the relative 65 20 152x x amount of the icosahedral phase drastically decreased and that of the decagonal phase increased. In the as-melt spun alloys, effects of Co content are critically changed at around 5 at% Co. For the less Co containing alloys (below 5 at%), the icosahedral phase proved to be the major phase, while the 5 at% Co containing alloy showed the coexistence of the icosahedral and decagonal phases. The 8 at% containing alloy showed the monolithic decagonal phase in the as-melt spun state. Unlike as-cast alloys, all the as-melt spun alloys showed no trace of the l-phase. However, after heat treatment of ribbons, l-phase newly appeared in the Al Cu Fe Co and Al Cu Fe Co alloys. 65 20 10 3 65 20 10 5 The l-phase showed orientation relationships with surrounding icosahedral or decagonal phases.  2002 Elsevier Science B.V. All rights reserved. Keywords: Quasicrystals; Liquid quenching; Scanning and transmission electron microscopy 1. Introduction icosahedral phase [5,6]. Partial substitution of Fe by Co or Cr possibly leads to the formation of the decagonal phase, Following the pioneering discovery by Shechtman et al. and the typical microstructure was reported to be a mixture [1], quasicrystals have been reported in many alloy sys- of the icosahedral and decagonal phases in the as-cast tems, including Al-, Ti-, Zr-, Mg-, Cd-based alloys [2]. Al Cu Co Fe alloy, for instance [7,8]. In this study, 65 22 6.5 6.5 Among them, the Al–Cu–Fe and Al–Cu–Co quasicrystals we report the effects of Fe replacement by Co on the have been most widely studied, not only because of their comparative formation and structural variation between the thermodynamic stability and easy fabrication with rela- icosahedral and decagonal quasicrystalline phases in the tively inexpensive elements, but also because of their Al–Cu–Fe system. Phase selection and transition in the scientific importance and possibility of industrial applica- Al–Cu–Fe–Co systems with different Co contents were tions [3,4]. The compositional ranges for the stable systematically investigated for as-melt spun and heat quasicrystalline phases are very similar between the Al– treatment conditions. Cu–Fe and Al–Cu–Co systems, but the resultant micro- structures are different, typically icosahedral and decagon- al, respectively. It has been reported that in the icosahedral forming 2. Experimental procedures Al–Cu–Fe alloys, the addition of a fourth element has resulted in various microstructures. For example, the The alloys with nominal compositions of addition of Si more than 8 at% encourages the formation Al Cu Fe Co ( x 50, 3, 5, 8 and 15) were fabricated 65 20 152x x of the 1 / 1 cubic approximant, while Be addition drastical- by melt-spinning method using high purity (99.99%) ly enhances the formability and structural perfection of the elements of Al, Cu, Fe, and Co. Heat treatment for the ribbon specimens was carried out in vacuum sealed quartz *Corresponding author. tubes. X-Ray diffraction (XRD) and transmission electron 0925-8388 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388(02)00185-8