Origin of improved ductility in semi-liquid die-cast Al-7%Si-0.4%Mg alloys

Tensile tests and detailed microstructure observation were performed for Al-7%Si-0.4%Mg alloy castings with systematically controlled microstructure. The semi-liquid die-cast allay showed improved ductility compared to the cast alloys exhibiting ordinary dendrite structure. The microstructure of the alloy was characterized by colonies consisting of single or several globular dendrite cells. The EBSP analysis and slip line observation have revealed that the misorientation among these colonies are large and so they can be recognized as "effective grains". Presence of refined effective grains reduces stress concentration at the grain boundaries and prevents the localized crack formation during tensile deformation. This is considered to be the origin of the improved ductility in the semi-liquid die-cast alloy.