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 te rnary AL-Co-Si alloy with 20 at.% Co and 5 at.% Si were rapidly solidi fied from the liquid state by melt spinning. It has been shown that th e solid solubility of Co in Al can be increased up to at least 1.7at.% from the negligible value under equilibrium conditions. Two metastabl e crystalline intermediate phases and a D1a-type ordered phase were ob served in localized regions in the rapidly solidified Al-2-4at.%Co all oy. A quasicrystalline decagonal phase has been observed in all the ot her rapidly solidified alloys. This decagonal phase was shown to conta in dislocations, dislocation loops and also twin-like defects. The phe nomenon of polytypism was also observed in this decagonal quasicrystal line phase. In extremely thin areas of the Al-14at.%Co alloy ribbons o n amorphous phase was detected. Transmission electron microscopy was u sed to extensively characterize the phases in the as-solidified condit ion and also those produced during and after transformation to the equ ilibrium constitution.