Decagonal approximate phases in Al-Co-Cu alloys

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.