Microstructural development in Al/MgAl₂O₄in situ metal matrix composite using value-added silica sources

Al/MgAl₂O₄in situ metal matrix composites have been synthesized using value-added silica sources (microsilica and rice husk ash) containing ∼97% SiO₂ in Al-5 wt.% Mg alloy. The thermodynamics and kinetics of MgAl₂O₄ formation are discussed in detail. The MgO and MgAl₂O₄ phases were found to dominate in microsilica (MS) and rice husk ash (RHA) value-added composites, respectively, during the initial stage of holding the composites at 750 °C. A transition phase between MgO and MgAl₂O₄ was detected by the scanning electron microscopy and energy-dispersive spectroscopy (SEM-EDS) analysis of the particles extracted from the composite using 25% NaOH solution. This confirms that MgO is gradually transformed to MgAl₂O₄ by the reaction 3SiO_2(s)+2MgO_(s)+4Al_(l)→2MgAl₂O_4(s)+3Si_(l). The stoichiometry of MgAl₂O₄, n, computed by a new methodology is between 0.79 and 1.18. The reaction between the silica sources and the molten metal stopped after 55% of the silica source was consumed. A gradual increase in mean MgAl₂O₄ crystallite size, D, from 24 to 36 nm was observed in the samples held for 10 h.