A new monoclinic approximant of the decagonal quasicrystal in Al-Co-Cu-W and Al-Fe-Cr alloys

A C-centred monoclinic phase (M) with a(M) = 3.31nm, b(M) = 1.23nm, c(M) = 2.48nm and beta = 112 degrees has been found in both Al66Co9Cu6W19 and Al12Fe2Cr alloys. It is shown by both electron diffraction and high-resolution electron microscopy (HREM) that this monoclinic phase is an ordered superstructure of the body-centred orthorhombic (O) approximant (a(O) = 1.23nm, b(O) = 1.24nm and c(O) = 3.07nm) of a decagonal quasicrystal. The lattice correspondence between these two phases is a(M) = c(O) - b(O) (a(M) = c(O)/sin beta), 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 diffraction 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.