Analysis of Fe-, Cu-, Mg- and Sr-Based Phases in Al–Si Alloys: Role of P Addition

The present work was performed on experimental Al–Si near eutectic cast alloys, with different additives mainly Fe, Mg, Mn, Cr, Sr and P. The alloys were solidified at 0.8°C/s, very close to equilibrium conditions. Precipitated phases, primarily Fe-, Cu-, Mg-, and Sr-based intermetallics, were examined. Although the phases reported in the present work were documented previously, the range of chemical composition of each phase was confirmed using an electron probe microanalyzer (EPMA) equipped with wavelength dispersive spectroscopy (WDS) and electron dispersive X-ray spectroscopy (EDS) facilities. Some of these alloys were cast in the form of hardness test pieces using a steel permanent mold preheated at 425 °C that provided a microstructure with an average dendrite arm spacing (DAS) of ~22 µm. The test pieces were solution heat treated at 500 ± 2 °C for times up to 24 h, followed by artificial aging at 155 °C for 5 h (T6 treatment). Hardness tests were carried out on the heat-treated test pieces. It was found that addition of Sr decreased the hardness. The role of P addition (AlP) on the nucleation of primary Si particles has been discussed. Although the addition of P in terms of 30 ppm leads to refining of the primary Si particles, the P-Sr interaction has a marked negative effect of the modification of the eutectic Si particles.