Characterization of tensile properties and microstructures in directionally solidified Al-Si alloys using linear roughness index

The tensile properties in directionally solidified Al-Si alloys and in gravity die casting of the same composition are presented. Examples of relating both the tensile and the microstructural properties of these alloys with the fracture roughness index are indicated. The roughness index was measured on vertical sections cut through the tensile fracture surface. The tensile properties examined were the yield and ultimate tensile strengths, strain at fracture, and Young's modulus. The analyzed microstructural features were porosity, dendrite area fraction, secondary dendrite arm spacing, and Si particle spacing. In almost all cases an unambiguous correlation was found between the roughness index and the tensile or the microstructural properties. A marked improvement in ductility was observed for directionally solidified samples over their gravity die casting processed counterparts. The roughness index diminished going from die cast to direction solidification, and this is likely accompanied by change in the fracture mechanisms.