Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Transient directional solidification experiments have been carried out with an Al-15wt. %Si alloy under cooling rates (http://www.w3.org/1999/xlink" xlink:href="tphl_a_1192297_ilm0001.gif") from 0.2 to 54K/s. A mixture of globular and fibre-like Si particles within the eutectic is shown to occur for cooling rates (http://www.w3.org/1999/xlink" xlink:href="tphl_a_1192297_ilm0002.gif")>9K/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 (sigma(u)) and elongation (). It is shown by correlations between eutectic () and secondary dendrite ((2)) spacings and sigma(u) and , 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.: (-1/2)<0.65 and (-1/2)(2)<0.18, both tensile strength and elongation remain unaltered.966228237FAPESP-Sao Paulo Research Foundation, Brazil [2015/11863-5, 2014/50502-5]CNPq (The Brazilian Research Council)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)