The effect of Mg on the microstructure and mechanical behaviour of two Al-Si-Mg casting alloys

The microstructure and tensile behavior of two Al-7 pct Si-Mg casting alloys, with magnesium contents of 0.4 and 0.7 pct, have been studied. Different microstructures were produced by varying the solidification rate and by modification with strontium. An extraction technique was used to determine the maximum size of the eutectic silicon flakes and particles. The eutectic Si particles in the unmodified alloys and, to a lesser extent, in the Sr-modified alloys are larger in the alloys with higher Mg content. Large Fe-rich pi-phase (Al9FeMg3Si5) particles are formed in the 0.7 pct ME alloys together with some smaller beta-phase (Al5FeSi) plates; in contrast, only beta-phase plates are observed in the 0.4 pct Mg alloys. The yield stress increases with the Mg content, although, at 0.7 pct Mg, it is less than expected, possibly because some of the Mg is lost to pi-phase intermetallics. The tensile ductility is less in the higher Mg alloys, especially in the Sr-modified alloys, compared with the lower Mg alloys. The loss of ductility of the unmodified alloy seems to be caused by the larger Si particles, while the presence of large pi-phase intermetallic particles accounts for the loss in ductility of the Sr-modified alloy

EBSD Study of the Influence of a High Magnetic Field on the Microstructure and Orientation of the Al-Si Eutectic During Directional Solidification

International audienceThe effect of a high magnetic field on the morphology of the Al-Si eutectic was investigated using EBSD technology. The results revealed that the application of the magnetic field modified the morphology of the Al-Si eutectic significantly. Indeed, the magnetic field destroyed the coupled growth of the Al-Si eutectic and caused the formation of the divorced alpha-Al and Si dendrites at low growth speeds (a parts per thousand currency sign1 mu m/s). The magnetic field was also found to refine the eutectic grains and reduce the eutectic spacing at the initial growth stage. Moreover, the magnetic field caused the occurrence of the columnar-to-equiaxed transition of the alpha-Al phase in the Al-Si eutectic. The abovementioned effects were enhanced as the magnetic field increased. This result should be attributed to the magnetic field restraining the interdiffusion of Si and Al atoms in liquid ahead of the liquid/solid interface and the thermoelectric magnetic force acting on the eutectic lamellae under the magnetic field