Banded microstructure formation in off-eutectic alloys

Abstract

The transition from a eutectic to a single phase has been examined through critical experimental studies in the vicinity of the transition conditions. The actual transition is shown to depend on the dynamics of the two competing phases. Close to the transition, an oscillating mode was found in which the system oscillated between the two steady-state morphologies. The transition from a eutectic to a single phase was observed not to be sharp, but to occur over a finite range of velocities in which an oscillating or a banded structure was formed, in which bands of primary phase and eutectic grew alternately in the growth direction. The effects of the imposed velocity and composition on this oscillatory mode were examined. It was found that, when the conditions were farther from the transition point, the oscillations damped out and the system selected one of the stable morphologies. The alloy composition had a significant influence on the oscillation behavior, and the oscillations were found to increase as the compositions deviated from the eutectic composition. The regime of banded structures was established for the carbon tetrabromide-hexachloroethane system, and a conceptual model is presented for the formation of bands. The results are generalized to show that the transition from one phase to the other is accompanied by a transition zone, in which the dynamic processes give rise to an oscillating microstructure between the two phases. Such a transition zone occurs in eutectic, peritectic, and monotectic systems and also during the cellular-to-planar transition of single-phase microstructures.