Decagonal approximate phases in Al—Co—Cu alloys

Publisher Website
Google Scholar
Abstract
As-cast samples of Al65Co20Cu15 made without Si were annealed for different times at 1200K. The decagonal phase was never observed. Instead, a decagonal approximant with diffraction features that are virtually indistinguishable from the true decagonal phase was discovered. The pseudo-ten-fold diffraction pattern from this phase contains low intensity, periodic diffraction spots corresponding to a d-spacing of ≈ 10 nm. High resolution electron microscope images taken along the pseudo-ten-fold zone-axis show crystalline domains decorated with ten-fold symmetric clusters. Several variants of this approximant with different periodicities are observed following isothermal annealing. These presumably arise from different packings of the ten-fold symmetric clusters. A new, crystal approximant phase also forms with annealing. Unlike previous reports, we find that silicon not only enhances decagonal formation but is necessary. For Si concentrations < ≈ 3·0 at.%, the weak diffraction spots are triangularly shaped; for concentrations above 4·5 at.%, a one-dimensional quasicrystal is observed.