Dendrite cell size and ductility of Al—Si—Mg casting alloys: Spear and Gardner revisited

Spear and Gardner's classical results † indicate that the tensile ductility of Al-7% Si-0.4% Mg casting alloy increases with decreasing dendrite cell size. However, new experiments show that this relationship is not always followed. Spear and Gardner's results are re-examined in the light of recent observations regarding the fracture mode and the effects of Si particle size and shape on the ductility of this alloy. It is shown that Spear and Gardner's measurements of tensile ductility follow a stepwise dependence on the scale of the dendritic structure, in agreement with more recent results. This behaviour can be related to the Si particle size and morphology which determine the rate of particle cracking with the applied strain for any given dendrite cell size and to changes in the particle distribution brought about by the heat treatment. The latter causes a transition in the fracture mode, from intergranular for the finest cells, when the ductility is highest, to transgranular along the dendrite cell boundaries at intermediate and large cell sizes, when the ductility is moderate or low.