P HILOSOPHICAL MAGAZINE A, 1998, VOL. 78, NO. 1, 143±156 A new monoclinic approximant of the decagonal quasicrystal in Al±Co±Cu±W and Al±Fe±Cr alloys By X. Z. Liao² , H. X. S ui²  and K. H. Kuo²  § ² Beijing Laboratory of Electron Microscopy, Institute of Physics, Chinese Academy of Sciences, PO Box 2724, 100080 Beijing, PR China  Department of Materials Engineering, Dalian University of Technology, 116024 Dalian, PR China [ Received 9 January 1997 and accepted in revised form 5 November 1997] Abstract A C-centred monoclinic phase (M) with a M = 3. 31 nm, b M = 1. 23 nm, c M = 2. 48 nm and b = 112 ë has been found in both Al 66 Co 9 Cu 6 W 19 and Al 12 Fe 2 Cr alloys. It is shown by both electron diraction and high-resolution electron microscopy (HREM) that this monoclinic phase is an ordered super- structure of the body-centred orthorhombic (O) approximant (a O = 1. 23 nm, b O = 1. 24 nm and c O = 3. 07 nm) of a decagonal quasicrystal. The lattice correspondence between these two phases is a M = c O - b O ( a M = c O / sin b), b M = a O and c M = 2b O . Based on the structural model of the orthorhombic approximant, a tentative structural model of the monoclinic approximant was proposed. The simulated [010], [100] and [201] electron diraction patterns and HREM images along these three orthogonal directions are comparable with the experimentally observed results. The long-period antiphase domain structure observed in this monoclinic approximant has been analysed in connection with this structural model. § 1. Introduction In a previous paper (Sui et al. 1995) on the body-centred orthorhombic approx- imant (O) of the decagonal quasicrystal (DQC) with a O = 1. 23 nm, b O = 1. 24 nm and c O = 3. 07 nm found in the Al 12 Fe 2 Cr alloy, a C-centred monoclinic superlattice (M) with a M = c O / sin b = 3. 31 nm, b M = a O = 1. 23 nm, c M = 2b O = 2. 48 nm and b = 112 ë was also mentioned, based mainly on the appearance of streaking and weak spots, whose k index is (2n + 1)/2 (n is an integer), in the [100] electron dirac- tion pattern (EDP ) of the orthorhombic approximant. Similar streaks in the c O * direction observed in the [100] EDP of the orthorhombic phase (a = 1. 24 nm, b = 1. 26 nm and c = 3. 14 nm) in the Al 60 Mn 11 Ni 4 and Al 24 Mn 5 Zn alloys have also been interpreted as supperlattice re¯ections of a monoclinic approximant (Singh and Ranganathan 1994, A. Singh and S. Ranganathan 1996, private commu- nication). In fact, Liao (1991) has also found this monoclinic phase in an Al±Co±Cu alloy contaminated with W during melting in an arc furnace. The structural features of this monoclinic phase in a specially prepared Al 66 Co 9 Cu 6 W 19 alloy have been studied by a systematic electron diraction analysis ( § 3) and a detailed high-resolu- tion electron microscopy (HREM) observation ( § 4). Recently, a layered structural model of the body-centred orthorhombic approx- imant in the Al 12 Fe 2 Cr alloy has been proposed (Sui et al. 1997) based on its close 0141±8610/98 $12. 00 Ñ 1998 Taylor & Francis Ltd. § Email: khkuo@image.blem.ac.cn.