The rheology of an Al–10Cu alloy with four different level of grain refiner was studied by vane rheometry at solid fractions close to the dendrite coherency solid fraction for each composition. The focus is on the influence of grain size, crystal morphology and solid fraction on the development of strength and Reynolds’ dilatancy. The results show that the dendrite coherency solid fraction marks the onset of dilatancy in all alloys (i.e. the semi-solid alloys expand when sheared after dendrite coherency). The amount of dilatancy increases with increasing solid fraction and grain size. The influence of grain size is linked to the formation of shear bands within the samples where most of the volume change occurs. These bands are approximately 10–15 mean grains wide. The influence of crystal morphology on the development of strength after dendrite coherency and dilatancy is low, but it strongly influences the dendrite coherency solid fractions in the four alloys.