Nucleation of Primary Mg2Si in Al-Mg-Si Alloys
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Al-Mg2Si Metal Matrix Composites have the potential to replace the frequently used near- and hyper-eutectic Al-Si alloys. In the as-cast condition, these alloys can contain coarse dendritic primary Mg2Si particles which have adverse effects on the mechanical properties. The effects of P additions on the primary Mg2Si particles and the eutectic structures in an Al-15Mg2Si-5Si alloy were investigated by thermal analysis and microstructural examination. Test alloys with nominal phosphorous contents of 0, 5, 20, 40, 60, 80, 100 and 500 ppm P were prepared. Confirmative composition measurements were carried out by glow discharge mass spectrometry (GDMS). With increasing P addition the primary Mg2Si particles were considerably refined, and the morphology changed from coarse dendritic to fine polygonal particles. Thermal analysis revealed that the nucleation temperature for primary Mg2Si increased from 640.6 °C to 651.2 °C with addition of 5 ppm P, and reached a maximum of 656.4 °C with addition of 20 ppm P. AlP particles were confirmed to act as nuclei for primary Mg2Si particles by chemical analysis in an Electron Probe Micro Analyzer (EPMA). It is argued that for P additions up to 60 ppm, nucleation occurs on all favorable AlP particles, leading to site saturation. For higher P contents, it is suggested that the recalescence reduces the initial growth rate, allowing the formation of an Al-enriched boundary layer around the growing crystal. This retards further growth in all directions, causing the morphology change from dendritic to polygonal.