Effect of composition on the streaking and diffuse intensity in decagonal phase in Al70−XCo15CuX+YNi15−Y system

We report here the transmission electron microscopic examination of alloys of three compositions in the system Al70−XCo15CuX+YNi15−Y. They have been chosen on the basis of: (a) e/a ratio lying between Al65Co15Cu20 (ACC) and Al70Co15Ni15 (ACN) and (b) a possible observation of change in order from ACN to ACC, the end members. The selected area diffraction studies of the melt-spun ribbons of the above compositions have revealed the presence of streaking around some spots and also diffuse rows. These have led to the transformation of a basic P105/mmc structure to that of P10/mmm having a periodicity of ∼8 Å. Such a transformation has been explained based on the 6D model earlier proposed by us. The chemical ordering in the layers is shown to accomplish the transformation. However, streaking arises owing to the presence of planar faults with a fault vector having periodic and quasiperiodic components.