Highly supersaturated Al-Mn solid solution alloys and their decomposition by heat-treatment

Abstract

Aluminum alloys, containing larger amounts of manganese above the maximum equilibrium solubility, were solidified by cooling the molten alloys in a special copper mould. The structures and properties of the solidified alloys and their decomposition process by annealing at various temperatures were examined by means of optical and transmission electron microscopes, X-ray diffraction analysis, microhardness test, and electric resistivity measurement.The results obtained were summarized as follows.(1) Solid solution containing a considerably large amount of Mn was easily obtained with aluminum alloys. The degree of supersaturation depended upon the cooling rate from molten state. The structure was not completely homogeneous; i.e., the solute concentration on the cell boundaries was higher than that in the interior of the cell. The difference in the above concentrations was greater with higher content of Mn or lower cooling rate.(2) Discontinuous precipitation was observed on the cell structure when annealed at some temperatures. When annealed at above 550°C, Al6Mn was found to be as a stable second phase and it was replaced by G phase at annealing temperatures below 550°C. The G phase was more easily formed when the degree of supersaturation was relatively low.