STUDY OF THE DECAGONAL PHASE-STABILITY IN THE AL-CU-CO-(SI) ALLOY SYSTEM

The composition range of decagonal phase in Al-Cu-Co alloys was studied in samples equilibrated at 800, 900 and 1000-degrees-C. The study was carried out by optical metallography, scanning electron microscopy, energy-dispersive X-ray analysis and powder X-ray diffraction. At 800-1000-degrees-C the decagonal phase is in equilibrium with the liquid, (Al, Cu)13Co4 and Al(Co, Cu) binary phases. None of the equilibrium compositions of the decagonal phase at 1000-degrees-C project onto the single decagonal phase region of the 800-degrees-C isothermal section. In as-cast alloys as well as in those annealed at elevated temperatures the decagonal phase may be supersaturated at room temperature. The effect of partitioning in solidification and of Si addition to the ternary alloys is discussed. The distortions of the decagonal Structure reported in the literature may be related to the supersaturation of the decagonal phase.