Mechanisms of microsegregation-free solidification

Abstract

At high rates of solidification, two mechanisms can produce microsegregation-free crystalline alloys: planar growth and partitionless solidification. For growth at high velocities, but still with equilibrium partitioning of solute, capillarity can stabilize a planar liquid-solid interface. This type of stability, known as absolute stability, has been confirmed experimentally for AgCu alloys and should apply only when the net heat flow is towards the solid. Another possibility for producing microsegregation-free alloys is partitionless solidification which can occur at high velocities and arises from the kinetics of interface motion. These kinetics involve the trapping of solute by the moving interface, causing the partition coefficient to be unity. A unified model for the variation in the interface temperature and partition coefficient with interface velocity is presented. This model spans the range from slow velocities, where local equilibrium is usually valid, to high velocities where partitionless solidification occurs. Considerations necessary to predict the conditions of microsegregation-free solidification for concentrated alloys are also discussed.