Characterization and formation of rod-shaped (Al,Si)3Ti particles in an Al-7Si-0.35Mg-0.12Ti (Wt Pct) alloy

In this study, the rod-shaped particles in an Al-7Si-0.35Mg-0.12Ti (wt pct) casting alloy have been characterized using transmission electron microscopy. It is found that these particles invariably contain Ti, Al, and Si and that they have the structure of the equilibrium phase (Al,Si)3Ti. A near-rational orientation relationship is observed between the (Al,Si)3Ti particles and the a-Al matrix phase. For this orientation relationship, the long axes of the (Al,Si)3Ti rods are invariably parallel to the moire´ planes defined by the intersection of closest-packed planes of the (Al,Si)3Ti and a-Al phases. In contrast to the (Al,Si)3Ti or Al3Ti particles form directly from the melt act as heterogeneous nucleation sites for aluminum grains and thus grain-refined Al-Si foundry alloys, the (Al,Si)3Ti particles are found to form during solution treatment at temperatures above 673 K (400 C). Their formation occurs in the center of aluminum grains and/or dendrites which is Ti enriched due to partitioning during solidification. The low diffusivity of Ti in a-Al allows the particles to form in the Ti-enriched areas near the center of grains as the Ti concentration is not able to be homogenized during typical solution treatment times.