In-situ fatigue tests monitored by Synchrotron Radiation X-ray microtomography were carried out in order to visualize the three dimensional (3D) shape and evolution of short cracks in the bulk of a cast Al alloy. After the in-situ fatigue test the sample has been infiltrated with liquid Gallium (Ga) in order to visualize the grain structure of the material. Irregularities of the crack advance along the crack front can clearly be correlated to the gain structure of the material.