Growth structures in aluminium-silicon alloys I. The coupled zone

Abstract

Directional solidification studies using a constant temperature gradient of 125°C/cm in the liquid are described and used to define the relationship between structure, Si concentration (11–20 wt%) and growth velocity (1–1500 μm/s). It is shown that the angular Si eutectic structure occurs over a wider range of growth velocities than previously suggested. Quench modification of the eutectic structure occurs over a range of growth velocities depending on the Si concentration. At lower velocities in this range, branching of the {100} angular Si side plates within the plate plane begins the transition to a fibrous structure. This is completed with the transition of the flake {undercooled} eutectic to a fibrous form. Twins are shown not to be essential for the growth of the fibrous structure and the transition is attributed to a change from a layer growth to a continuous growth mechanism. The concept of the coupled zone is discussed and the observations are used to define the coupled zone boundaries.