AbstractThe influence of microstructure and processing method on the initiation and growth of fatigue cracks in an aluminum‐magnesium‐silicon alloy has been studied with extruded and squeeze‐cast material cycled in strain control. Using the replication technique, initiation and early growth of short cracks was found to be dependent upon the applied strain level. At low strains decohesion at the particle/matrix interface occurred whereas at high strain levels both particle cracking and decohesion were observed. The micromechanisms of subsequent short crack growth were also influenced by strain amplitude. In the case of the squeeze‐cast material cycled at low total strain amplitudes subsequent growth took place from particle to particle along the soft precipitate‐free interdendritic regions, in the direction of maximum shear. Notable deflection occurred at triple points. Short cracks in the extruded material cycled at all total strain amplitudes were found to initiate by debonding of particles in banded zones. Early crack growth at low strain amplitudes over the first 20μ along the surface was significantly impeded by subgrains and some second phase particles, whose effect was much greater than for the squeeze‐cast material due to a finer microstructure (the average subgrain size was 6 μ) and absence of soft interdendritic regions. The fracture surface was also rougher which presented a greater degree of resistance to propagation. In both types of material, Stage I crack growth extended to a depth of about 250‐350 μ. During this portion of life short fatigue crack growth behaviour was observed. Long crack behaviour coincided with the crack switching to Stage II growth. Short crack propagation behaviour and the crack depth at which Stage I was complete could be expressed using a crack growth model based on the strength of slip bands emanating from the crack tip and the strength of barriers which were in th