THE AL-CO DECAGONAL PHASE

Three Al-Co binary alloys containing 14, 20, and 25 at percent Co are rapidly solidified from the liquid state by melt spinning. A quasicrystalline decagonal phase is observed in all three alloys. Rosette, nodular, and cellular-dendritic morphologies are obtained. Twin-like features, dislocations, dislocation loops, and planar faults may also be observed in the as-quenched alloys. The periodicity for the decagonal phase is found to vary from 0. 8 to 1. 6 nm suggesting that a polytypism-like phenomenon exists in these alloys. The quasicrystalline phase in all the alloys transformed into the equilibrium constitution consisting of fine equiaxed grains. This happened through the formation of fine holes to accommodate the density differences between the quasicrystalline and the crystalline states. Details about the occurrence, morphology, and transformation behaviour of the decagonal phase are discussed.