Shear mechanisms at 0–50% solid during equiaxed dendritic solidification of an AZ91 magnesium alloy

The rheology of equiaxed solidifying alloys containing 0–50% solid has been studied by vane rheometry. We focus on the incipient motion and subsequent flow mechanisms of quiescent solidifying microstructures. It is demonstrated that, once the growing crystals have impinged, the rheology can be interpreted within the framework of saturated compacted granular materials with little intercrystal cohesion. The material expands in response to shear and deforms primarily by Reynolds’ dilatancy-enabled crystal rearrangement. Dilatancy leads to strain localisation in dilatant shear bands shortly after crystal impingement and is proposed to cause the initiation of cracks in the expanding regions between crystals at higher solid fraction.