| The formation of quasicrsatals in rapidly solidified Al-Li alloys have been investigated by detailed transmission electron microscopy, including a combination of bright and dark field imaging, selected area diffraction pattern analysis and energy dispersive X-ray microanalysis. Al-Cu-Li, Al-Cu-Li-Mg and 2024-2Li alloys rapidly solidified by melt spinning consist of various types of solidification microstructure, ranging form a fine columnar grain structure near the chilled ,surface to a coarse cellular structure near the unchilled surface, depending upon the wheel rotation speed during melt spinning and the local cooling rate through the thickness of the ribbon. Fine scale δ` precipitates during melt spinning in Al-Cu-Li, Al-Cu-Li-Mg and 2024-2Li alloys. The intercellular particles in as-melt spun Al-Cu-Li alloys are the T_8, Θ or T_2 icosahedral phases, depending upon the alloy composition, but in as-melt spun Al-Cu-Li-Mg and 2024-2Li alloys are always the icosahedral phase. Addition of Mg is effective in stabilizing the icosahedral phase in Al- Cu-Li alloys. The icosahedral phase forms as a second solidification product and is located at primary α-Al cell or dendrite boundaries, with strong dependence of icosahedral particle size upon the melt spinning wheel speed and local cooling rate. The composition of the icosahedral phase in rapidly solidified Al-Li alloys is not stoichiometric. Thus, the composition of the icosahedral phase in as-rapidly solidified Al-Cu-Li-Mg and 2024-2Li alloys can be expressed as (Al, Si)_6 (Cu, Fe, Mn)(Li, Mg)₃, assuming Si, Mg and (Mn, Fe) replacing Al, Li and Cu respectively in the Al_6CuLi₃ icosahedral structure. |
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