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In-situ Al-MgAl 2 O 4 metal matrix composite was successfully manufactured using SiO 2 with the aid of ultrasonication. MgAl 2 O 4 particles and their clusters were identified at grain boundaries and interdendritic regions within the grain envelopes. The composite showed 2-5 fold of grain size reduction with respect to the reference alloy cast at similar conditions. The composite has shown 10% increase in yield stress and 15% increase in UTS while maintaining the ductility similar to reference alloy. CTE mismatch strengthening and grain boundary strengthening are suggested to be influencing in the improvement in mechanical properties.

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A new grain refiner master alloy based on the Al-Zr-Ti system was prepared by salt-assisted synthesis. Of the Al 3 Zr particles in the master alloy, 90% were ranged between 1 lm and 13 lm. An 80% reduction in the grain size was observed with the addition of an equivalent 0.2 wt.% Zr master alloy combined with ultrasonic treatment in an Al alloy. The new master alloy demonstrated a 30% improvement in grain refinement efficiency as compared to the master alloy prepared from conventional binary master alloys.

In-situ a-Al 2 O 3 was successfully synthesized and dispersed in Al alloy using B 2 O 3 and ultrasonication-aided liquid mixing technique. Microstructure analysis identified a-Al 2 O 3 as the most common phase in the composite master alloy, whereas AlB 12 was frequently observed and AlB 2 was rarely found in the alloy. Grain refinement analysis of selected Al alloys registered a transition of columnar to equiaxial grains of aAl with the inoculation of the master alloy and ultrasonication treatment. Similarly, an improvement in the mechanical properties of A357 alloy was observed with the combination of inoculation and ultrasonication treatment.

A new grain refining master alloy containing MgAl 2 O 4 and Ti was synthesized by in situ reaction of TiO 2 particles in an Al-Mg melt. MgAl 2 O 4 particles formed were distributed in the melt by ultrasonic cavitation processing. The obtained master alloy showed considerable (50 pct) grain refining ability in a commercial A357-type Al-Si alloy. Ultrasonication contributed further to 25 pct in the grain refinement. In comparison with a commercial Al-5 pct Ti-1 pct B master alloy, the efficiency of the new master alloy is less at a lower addition rate. Nevertheless, both master alloys performed similarly at higher additions. The strength and ductility of the inoculated and ultrasonicated alloy showed at least a 10 pct and a 50 pct increase, respectively, as compared with non-grain-refined alloy and a similar mechanical performance in comparison with the alloy inoculated with Al-5 pct Ti-1 pct B master alloy.

Al/MgAl 2 O 4 in situ metal matrix composites have been synthesized using value-added silica sources (microsilica and rice husk ash) containing ∼97% SiO 2 in Al-5 wt.% Mg alloy. The thermodynamics and kinetics of MgAl 2 O 4 formation are discussed in detail. The MgO and MgAl 2 O 4 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 MgAl 2 O 4 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 MgAl 2 O 4 by the reaction 3SiO 2(s) + 2MgO (s) + 4Al (l) → 2MgAl 2 O 4(s) + 3Si (l) . The stoichiometry of MgAl 2 O 4 , 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 MgAl 2 O 4 crystallite size, D, from 24 to 36 nm was observed in the samples held for 10 h.