Effect of Dispersoids on the Microstructure Evolution in Al-Mg-Si Alloys

An Al-Mg-Si alloy with a high level of Cr is investigated via Electron Probe Micro Analysis (EPMA) and Scanning Transmission Electron Microscopy (STEM). EPMA is conducted on the same area of a sample after numerous heat treatments in a vacuum furnace to study the evolution of Mg, Si, Cr, and Fe from the segregated structure formed on casting. Mg and Si are found to segregate toward the grain boundaries and remained segregated up to 550 degrees C. Cr segregates away from the grain boundaries. Regions of lower Cr separated from high Cr regions by sharp transitions are observed. To investigate the effect of segregation on dispersoid precipitation, samples are heated to a number of different temperatures and examined using STEM. The evolution of dispersoid area fraction and effective diameter is measured as a function of position within a grain. The dispersoid area fraction decreases, while the size initially decreases and then increases toward the grain center. Both -Al(FeCr)Si and -AlCrSi dispersoids exist with a variety of morphologies. The -AlCrSi dispersoids are found to have a larger effective diameter. The change in dispersoids fraction, size, and morphology with position has important implications for the pinning effectiveness of the dispersoids against recrystallization.