Morphology of solid-liquid interface and redistribution of solute in aluminum alloys

Abstract

The transition of solid-liquid interface morphology and redistributing behavior of solutes during unidirectional freezing of aluminum alloys were investigated with respect to temperature gradient (G), freezing rate (R), and initial concentration of the solute (C₀). The specimens used were Al-Fe, Al-Zn, and Al-Mn alloys having distribution coefficients (k) of less than 1 and Al-Cr alloys having the coefficient (k) of more than 1.
The critical condition for transition of interface morphology from smooth plane to irregular cell having depressions, was governed by the ratio of G/RC₀. It was found that the ratio for transition was extremely high in Al-Fe alloy and was low in Al-Mn and Al-Cr alloys.
The increase in concentration of solute at local segregation was not confirmed with the transition of interface morphology. The solute which had been condensed during freezing on the solid-liquid interface tended to increase the area of local interfacial segregation region according to the degree of constitutional undercooling.
Cell boundaries parallel to the growth direction were formed by grooves of cell nodes. The concentration of the solute, which had been in liquid state on decantation, was equal to that of the nodes on the interface. The variation in the concentration was not observed at places far away from the solid-liquid interface. However, discontinuous change of concentration was observed on completely solidified grooves. In Al-Fe alloys, the concentration of completely solidified solute on cell boundaries was higher than that on grooves due to the crystallization of FeAl₃. Whereas, reverse results were obtained in Al-Zn and Al-Mn alloys due to the dissolution of the solute. In Al-Cr alloy, the concentation of solute on the grooves was lower than that at cell center and extreme variation of concentration was not observed even when grooves were completely solidified.