Inhibited coarsening of a sprayformed and extruded hypereutectic aluminium-silicon alloy in the semi-solid state

The microstructural evolution of a sprayformed and extruded hypereutectic aluminium-30% silicon-5% copper-2% magnesium alloy heated into the semi-solid state has been investigated. Liquid is formed initially by a quaternary eutectic reaction and then by a ternary melt reaction. These reactions occur relatively quickly. However, the binary Al-Si eutectic melt reaction takes a significant time – around several hours depending on the temperature. The coarsening rate constants (K) for the growth of the silicon particles are approximately 3 to 4 orders of magnitude lower than those for the majority of metal sprayformed alloys. This may be associated with difficulties in addition or removal of atoms from the low index silicon facets. Where growth does occur, agglomeration of silicon particles may play a large role, especially at higher liquid contents. Electron backscatter diffraction (EBSD) gives evidence of agglomeration, and furthermore shows that high angle silicon-silicon boundaries are not wetted with liquid.