1992 年 56 巻 1 号 p. 96-103
In the previous paper, the authors reported the refinement of primary silicon crystals in the hypereutectic Al-22 mass%Si alloy ingots produced by the Duplex Casting process with the combination of Al-12 mass%Si alloy as the first melt and Al-32 mass%Si alloy as the second melt. In this study, three possible origins of the refined primary silicon crystals were examined: (1) detachment of eutectic silicon crystals during the remelting process of solidified shell of the first melt, (2) nucleation in the second melt during the mixing process of the first and second melts, and (3) nucleation after complete mixing.
In order to examine the first origin, two combinations of alloy compositions were selected for the Duplex Casting process: Al-33 mass%Cu and Al-32 mass%Si; and (b) Al and Al-44 mass%Si. As a result, the primary silicon crystals were refined even though the first melt was free from silicon in either case. Therefore, the first origin was concluded to be not dominant in the refinement of primary crystals.
The combination of Al-51 mass%Ge alloy and Al-32 mass%Si alloy was selected to examine the second and third origins. The germanium concentration in the refined primary crystals, which was measured by EPMA, was lower than the possible minimum concentration predicted on the assumption that the primary crystals came from the third origin. Therefore the second origin, i.e. nucleation in the second melt, was confirmed as the predominant one.