The effect of geometrically necessary dislocations on grain refinement during severe plastic deformation and subsequent annealing of Al–7% Si

A comparative study has been made of room temperature severe plastic deformation of an Al–7% Si composite, containing large Si particles, and a monolithic Al–0.5% Si with the same chemical composition as that of the composite matrix. In the composite, the large local strain gradients existing between the matrix and the coarse, undeformable, Si particles produce a much higher density of geometrically necessary dislocations which are responsible for much faster grain refinement. Annealing at 200 °C produces only small variations in grain size due to the precipitation of fine Si particles while severe deformation at the same temperature leads to a much larger grain size with much coarser precipitates. The corresponding mechanical properties have been analysed in terms of the different microstructural contributions to strengthening in both alloys.