Abstract
An attempt to define the relationship among temperature gradient G in liquid ahead of the interface, freezing rate R and initial solute concentration C0, governing the cellular/dendritic transition, was made from observations of the solid/liquid interface in the unidirectionally solidified Al-Fe and Al-Zn alloys.
By increasing C0 and R or decreasing G, the solid/liquid interface morphology changes successively from broken hexagonal cell to dendritic cell and dendrite. In both alloys, the C0 vs. G/R relationship for the transition from broken hexagonal cell to dendritic cell is curved, while the C0 vs. G/R1/2 is linear. In the Al-Fe alloys, the transition occurs at smaller C0 R1/2/G than in the Al-Zn alloys. The side branches of dendrite in the Al-Fe alloys are rod-type and tend to grow parallel to heat flow direction. The high enrichment of Fe solute ahead of the interface, which leads to the formation of the FeAl3 compound at boundaries, suppresses the linear growth of the side branches. In the Al-Zn alloys, only slight enrichment of Zn solute and no side branches are observed; the branches are plate-like and the dendrite is described as cellular dendrite. The size of hexagonal cell in both alloys is proportional to the inverse of the cooling rate.
