Transient directional solidification experiments have been carried out with an Al-15wt. %Si alloy under cooling rates () from 0.2 to 54 K/s. A mixture of globular and fibre-like Si particles within the eutectic is shown to occur for cooling rates () > 9 K/s. The presence of refined globular Si particles arranging the microstructure either as primary particles or within the eutectic mixture seems to contribute for a combination of high tensile strength (σu) and elongation (δ). It is shown by correlations between eutectic (λΕ) and secondary dendrite (λ2) spacings and σu and δ, given by Hall–Petch type formulae that the tensile properties increase significantly with the decrease in these spacings. However, it is shown that for higher ranges of spacings, i.e.: λΕ−1/2 < 0.65 and λ2−1/2 < 0.18, both tensile strength and elongation remain unaltered Ver menos