An electron microscopic study of a rapidly solidified Al-5 wt. % Co alloy
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An electron microscopic study of a rapidly solidified Al-5 wt. % Co alloy G. Van Tendeloo University of Antwerp (RUCA), Groenenborgerlaan 171, B-2020 Antwerpen, Belgium
Jyothi Menon and C. Suryanarayana Centre ofAdvanced Study in Metallurgy, Banaras Hindu University, Varanasi-221 005, India (Received 12 February 1987; accepted 5 May 1987) An Al-5 wt. % Co alloy has been rapidly solidified from the liquid state, resulting in the formation of a slightly supersaturated aluminum solid solution and the monoclinic Al 9 Co 2 phase. High-resolution electron microscopy and electron diffraction have been extensively employed to characterize the as-solidified as well as the annealed foils. The high-resolution micrographs of the as-solidified solid solution showed the presence of clustering and Dlatype ordering on an extremely small microscopic scale. Annealing at temperatures above 623 K resulted in the precipitation of Al 9 Co 2 in the form of platelets arranged in a Widmanstatten pattern. Orientation relationships between the Al 9 Co 2 platelets and the matrix have been established. Guinier-Preston zonelike platelets on close-packed { i l l } planes, however, have also been observed upon electron irradiation in the annealed alloys, presumably due to the enhanced diffusivity of cobalt atoms.
I. INTRODUCTION Rapid solidification of metallic melts at cooling rates exceeding about 105 K s ~' has been found useful in developing newer and stronger alloys.1"4 The improvement in properties has been associated with structural refinement and attributed to the formation of metastable phases that include supersaturated solid solutions, nonequilibrium crystalline intermediate phases, and metallic glasses. Interest in study of nonequilibrium microcrystalline alloys has greatly increased in recent years, more so after the discovery of quasicrystalline phases (see, e.g., Ref. 5). Efforts are also being made in establishing the structure property correlations in several light alloys based on aluminum,6 magnesium,7 and titanium.8 As a part of an ongoing program on development of high-strength, high-temperature aluminum alloys at Banaras Hindu University, Varanasi, we have recently undertaken work on rapidly-solidified Al-Co alloys. The solid solubility of Co in Al under equilibrium conditions is very small and < 0.02 wt. % at the eutectic temperature of 930 K.9 It decreases with decreasing temperature and is negligible at room temperature. Several intermediate phases have been found to form in Alrich Al-Co alloys, and their crystal structure data are summarized in Table I of Ref. 10. There have been only a few investigations on the structure of rapidly solidified Al-Co alloys. Garrett and Sanders11 studied the formation of coarse intermetallics in Al-5 to 20 wt. % Co alloys after melt spinning and suggested that the primary intermetallics nucleate siJ. Mater. Res. 2 (5), Sep/Oct 1987
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multaneously with the primary aluminum, with aluminum tending to nucleate on the substrate side and intermetallics on the air
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