抄録
The nature and the causes of the banded structure appearing in the eutectic Al–Si alloy were investigated by optical and scanning electron microscopy and by X-ray microprobe analysis.
The banded structure may be classified into two groups: The first group is due to an abrupt change of the solidification rate in an insulated mold. This band must be formed by the concentration variation due to sudden changes in growth rate.
The second group is due to the local segregation of a ternary element such as sodium, when it is added to a binary eutectic system.
The formation of the banded structure is governed by the solidification rate, temperature gradient at the solid-liquid interface and the build-up of the liquid layer rich in the ternary additions rejected from the eutectic solid. If the solidification condition is proper, the formation of the banded structure can be avoided even when the sodium content is in excess.
