The aim of this thesis was to improve the understanding of the microstructural and fracture mechanisms of three different as cast AlSiCu alloys produced with two different cooling velocities. Standardised fracture mechanical tests at room temperature following ASTM 1820, with CT-samples were used to analyse the alloys. Samples of all alloys, cooling velocities as well as two different sample directions were tested. The different cooling velocities were achieved by casting different sample thicknesses. Samples and data of already conducted tensile tests were used as base and comparative materials. To understand the fracture processes, additionally to the measurement of the fracture toughness, the microstructure and the equivalent fracture surface was compared. To verify the results of the fracture mechanical tests a comparison of J and KIc testing, CTOD measurements and an analysis of the crack path has been done. The microstructures, fracture surfaces and the crack path were analysed with a light microscope and a scanning electron microscope.