Special Diffraction Effects in Al7o_xCo15Cux+yNi15_y Decagonal Alloys

Stable decagonal phases are observed to form in both as cast and rapid solidification condition in Al70Co15Nil5 and A16SCo20Cu15 alloys. The former shows diffraction spots and diffuse intensity at positions other than those of a decagonal phase in the 10-fold and 2-fold diffraction patterns. These compounds are stabilised at a specific e/a ratio. Therefore, quaternary alloys with general alloy composition A17o-,Co15Cux,,,Nils-,, were prepared with varying e/a ratio. We present here the electron metallographic characterisation of the melt-spun
ribbons of different alloy compositions. Changes observed during electron microscopic investigation include streaking and diffuse intensity around the spots and also the presence of totally diffuse rows in some cases. The consequence of the latter on the underlying structure is disorder in the decagonal phase. Owing to the presence of streaking in a 10-fold like zone in the A170Co15Cu5Nilo composition, symmetry breaking of 10-fold to a 2-fold Diagonal phase (222) occurs. Further, the presence of a type II superstructure is also observed in A168Co 15Cu 17Ni5 composition.