HREM STUDY AND STRUCTURAL MODELS OF THE (1/1,2/1)-ORTHORHOMBIC AND (2/1,1/1)-ORTHORHOMBIC APPROXIMANTS IN AN AL70CO15NI10TB5 ALLOY

The (010) layers of the (1/1,2/1) and (2/1,1/1) approximants (these rational ratios of two consecutive Fibonacci numbers were used to substitute for the irrational tau in two orthogonal quasiperiodic directions) of the Al70Co15Ni10Tb5 decagonal quasicrystal were found to consist of elongated hexagonal subunits with an edge length of 0.76 nm in different tessellations by means of high-resolution electron microscopy (HREM). This explains why these two approximants inter-grow easily with a fixed orientational relationship and domains occur abundantly in them. The primitive orthorhombic unit cell of the (1/1,2/1) approximant, a = 1.26, b = 1.60 and c = 2.30 nm, consists of two differently oriented hexagons. Its structure is similar to that of Al3Pd reported recently by Matsuo and Hiraga (1994). On the other hand, the base-centred orthorhombic unit cell of the related (2/1,1/1) approximant, a = 2.00, b = 1.60, c = 1.38 nm, consists of two hexagons of a single orientation. Its pentagonal and decagonal layer structure, derived from that of Al3Pd or the (1/1,2/1) approximant, consists of subunits similar to those that exist in a decagonal quasicrystal. The simulated images based on these structures compared favourably with the observed [010] HREM images of these two approximants.