L Journal of Alloys and Compounds 342 (2002) 38–41 www.elsevier.com / locate / jallcom An investigation on the transformation of the decagonal phase to a B2 phase in Al–Cu–Co alloy during mechanical milling a, b c d * N.K. Mukhopadhyay , G.V.S. Murthy , B.S. Murty , G.C. Weatherly a Centre of Advanced Study, Department of Metallurgical Engineering, Institute of Technology, Banaras Hindu University, Varanasi 221005, India b National Metallurgical Laboratory, Jamshedpur 831007, India c Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Kharagpur, India d Brockhouse Institute for Materials Research, McMaster University, Hamilton, Ontario L8S4 M1 Canada Abstract Al Cu Co decagonal phase, synthesized by a slow cooling technique, were mechanically milled in a high energy planetary ball mill 65 20 15 for 10, 20 and 30 h. X-ray diffraction, scanning electron microscopy and transmission electron microscopy techniques were used for characterization of phases. The phase transformation from the decagonal phase to a B2-crystalline phase (ordered CsCl type phase) during mechanical milling is reported here for the first time. Powders milled for more than 10 h contained predominantly the B2 type crystalline phase with a lattice parameter of 0.29 nm. This crystalline phase was found to be quite stable after 30 h of milling and also on subsequent annealing at 600 8C. These experimental results lend support to an earlier suggestion that the decagonal phase in Al–Cu–Co is actually less stable than the B2 phase at low temperatures.  2002 Elsevier Science B.V. All rights reserved. Keywords: Decagonal quasicrystals; B2 phase; Nanocrystals; Al–Cu–Co alloy; Mechanical milling; Ballistic diffusion 1. Introduction in an electron microscope and observed the transformation of the DQC phase first to a disordered bcc phase and then Metastable quasicrystal (QC) phases can transform to to a CsCl type ordered B2 phase in Al–Cu–Co–Si. They crystalline phases during annealing with a release of heat, proposed that at room temperature the B2 phase observed the signature of an exothermic reaction. However, stable after irradiation is the equilibrium phase. The co-existence QC phases [1] such as Al Cu Co are stable up to their of either B2 or B2 based Al Ni type vacancy ordered 65 20 15 3 2 melting / liquidus temperature and have been included in phase with the DQC phase has often been observed in the ternary equilibrium phase diagram [2]. Decagonal (D) slowly cooled sample [8] and vapor deposited thin film [9]. QC phase found in the ternary and quaternary Al/Si–Cu– Kang and Dubois [10] observed the transformation of the Co system has also been reported to relax to various DQC phase to a crystalline phase during compression microcrystalline phases or approximant phases on anneal- testing of an Al–Cu–Co–Si sample. The crystalline phase ing [3–5], suggesting that this phase may be stable in this was rhombohedral and could be indexed according to a system only at high temperatures, transforming to mi- hexagonal cell of Al Ni type with a50.41 nm and c50.5 3 2 crocrystalline phases during a suitable (low temperature) nm. Wu et al. [11] have observed the formation of processing treatment. It appears to be difficult to equili- disordered bcc phase during scratch testing of a QC brate the DQC phase and other similar phases at low material. Finally Zhang et al. [12] and Mukhopadhyay et temperature as many of the complex phases are formed by al. [13] have observed the disordered B2 phase in ion a peritectic reaction [6]. Zhang and Urban [7] have argued milled samples of the Al–Cu–Co–Si system. The phase that at low temperatures atomic diffusion is slow or even transformation was attributed to the preferential sputtering completely frozen out, and hence it is difficult to equili- of Al during ion milling, causing a change of composition brate by a simple heat treatment experiment. They studied towards Al TM and leading to the transformation, first 50 50 the issue of the stability by employing electron irradiation to the disordered bcc phase and finally to the ordered B2 phase. The aim of the present study is to establish the trans- *Corresponding author. E-mail address: mukho@banaras.ernet.in (N.K. Mukhopadhyay). formation of the decagonal phase to an ordered B2 0925-8388 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388(02)00129-9