X-ray diffraction study of decagonal Al-Co-Ni as a function of composition

The evolution was studied of structural order/disorder phenomena across the wide stability field of the decagonal phase in the system Al-Co-Ni. Full single-crystal X-ray diffraction data sets were collected of 31 samples annealed at 900 degreesC. Based on reciprocal space reconstructions of the imaging-plate-scanner data, the variation in Bragg as well as in diffuse scattering is discussed in detail. The samples were also characterized metallographically, by powder X-ray diffraction, differential thermal analysis and energy-dispersive X-ray spectroscopy. The (quasi)lattice parameters vary smoothly over the stability range of decagonal Al71.5CO28.5-xNix (8 less than or equal to x less than or equal to 20). A smooth variation is also observed for the diffuse scattering related to the approximate to 8 A superstructure. These observations indicate a continuous ordering process at 900 degreesC from the basic Co-rich phase via the type II and type I superstructures to the basic Ni-rich phase.