Crack initiation mechanisms in AA6082 fatigued in the VHCF-regime

Over the last decades research activities in the field of very high cycle fatigue (VHCF) have strongly increased. It has been shown important to distinguish between single phase inclusion free type I and multiphase type II materials. Although, considerable efforts were made, the basic understanding of the dominating deformation/damage mechanisms is still deficient. To study the VHCF deformation mechanism of a typical type II material, the aluminium alloy AA6082 was used. The material was investigated in the as received condition and after different heat treatments. Microstructural changes triggered by the different heat treatments also shift the damage mechanisms from internal crack initiation to initiation at the surface. In the as-received condition, a non-defect crack initiation site was observed. By detailed investigations using EBSD, FIB-techniques and nanonindentation, it turned out that peripheral clusters of precipitates adjacent to the featureless crack initiation site seem to play an important role, whereas a super-grain based mechanism, as suggested by Davidson, could not be confirmed.