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 degrees 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. (c) 2007 Elsevier B.V. All rights reserved.