ELECTRON-MICROSCOPY OF METASTABLE PHASES IN RAPIDLY SOLIDIFIED AL-CO ALLOYS

Four binary AL-Co alloys containing 2.4, 14, 20 and 25 at.%Co and a ternary AL-Co-Si alloy with 20 at.% Co and 5 at.% Si were rapidly solidified from the liquid state by melt spinning. It has been shown that the solid solubility of Co in Al can be increased up to at least 1.7at.% from the negligible value under equilibrium conditions. Two metastable crystalline intermediate phases and a D1a-type ordered phase were observed in localized regions in the rapidly solidified Al-2-4at.%Co alloy. A quasicrystalline decagonal phase has been observed in all the other rapidly solidified alloys. This decagonal phase was shown to contain dislocations, dislocation loops and also twin-like defects. The phenomenon of polytypism was also observed in this decagonal quasicrystalline phase. In extremely thin areas of the Al-14at.%Co alloy ribbons on amorphous phase was detected. Transmission electron microscopy was used to extensively characterize the phases in the as-solidified condition and also those produced during and after transformation to the equilibrium constitution.