Over-modification band in aluminum-silicon eutectic solidified structure modified with sodium

Abstract

The nature and origin of so called "over modification band" which appears in the microscopic structure of eutectic Al-Si alloy treated with an excessive amount of sodium has been investigated in detail by the method of 1) the optical and scanning electron microscope observation of structures, 2) the X-ray microprobe analysis of the distribution of sodium in solidified structures and 3) the change in the thermal analysis curves depending on the added amount of sodium.
The over modification band may be classified into two groupes. The first group of band is formed when the temperature gradient in the liquid in front of the solid-liquid interface is decreased abruptly, for example, when the melt is cast in an insulating mold. The increase in sodium concentration and the decrease in temperature gradient results in the increase in the constitutional supercooling of aluminum. The condition will be satisfied for the change from the co-operative Al-Si eutectic growth with a plane solid-liquid interface to the dendritic growth of aluminum. The dendrite of silicon is not formed due to the existence of excess amount of sodium. When the growth of aluminum dendrite reaches to certain extent, the supercooling will be reduced and the co-operative growth of the eutectic is resumed leaving behind the "band" of aluminum dendrites with coarse silicon particles in between.
The second group of band is formed due to the local segregation of sodium when the sodium addition is highly excessive. The band formed in such case is composed of aluminum dendrites, silicon particles and a ternaly compound having chemical composition close to AlSiNa.
The formation of over modification band, in the case of either groupes, is governed by the solidification rate, temperature gradient at the solidliquid interface and the sodium content of the liquid. Therefore, by the proper consideration of the solidification condition, the formation of the band may be avoided even when the sodium content is in excess.