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Cast particulate composites, containing in-situ generated reinforcing particles of alumina, have been developed by solidification of slurry obtained by dispersion of externally added manganese dioxide particles (MnO 2 ) in molten aluminum, and alumina is formed by reaction of manganese dioxide with molten aluminum. The chemical reaction also releases manganese into molten aluminum. Magnesium is added to the melt in order to help wetting of alumina particles by molten aluminum and to retain the particles inside the melt. The present work aims to understand the influence of key parameters such as processing temperature, time, and the amount of MnO 2 particles added on the microstructure and mechanical properties of the resulting cast in-situ composites. The sequence of addition of MnO 2 particles and magnesium has significant influence on the microstructure and mechanical properties. Increasing processing temperature and time increases the extent of reduction of MnO 2 particles, generating more alumina particles as well as releasing more manganese to the matrix alloy. Alumina helps to nucleate finer and sometimes blocky MnAl 6 in the matrix of the composite and thereby results in relatively higher ductility and increased strength in the composite as compared to the base alloy of similar composition. Even in the presence of relatively higher porosity of 8 to 9 vol pct, one observes a percent elongation not below 7 to 8 pct, which is considerably higher than those observed in cast Al(Mg)-Al 2 O 3 composite synthesized by externally added alumina particles.

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In-situ particle-reinforced aluminum alloy-based cast composites have been synthesized by solidification of the slurry obtained by dispersion of externally added titanium dioxide (TiO 2 ) particles in molten aluminum at different processing temperatures. Alumina particles (Al 2 O 3 ) form in situ through chemical reaction of TiO 2 particles with molten aluminum. Simultaneously, the chemical reaction also releases titanium, which dissolves into molten aluminum and results in the formation of intermetallic phase Ti(Al 1-x ,Fe x ) 3 during solidification. Increasing the processing temperature increases (1) the amount of elongated as well as blocky intermetallic phase Ti(Al 1-x ,Fe x ) 3 , (2) the proportion of alumina particles in the reinforcing oxides, and (3) the porosity content in the resulting cast in-situ composite. The difference in particle content and porosity between the top and the bottom of the cast ingot increases with increasing processing temperature. The hardness of the cast in-situ composite is significantly more than that of the matrix alloy due to the presence of reinforcing particles, but the hardness is greatly impaired by the presence of porosity at the top of the cast ingot. The percent elongation of the cast in-situ composite decreases with increasing processing temperature possibly due to increasing porosity as well as an increasing amount of elongated intermetallic phase, which affects the percent elongation of the matrix alloy. The tensile and yield stresses of the cast in-situ composite decreases with increasing processing temperature again due to increasing porosity, which affects the ultimate tensile stress more than the yield stress. In the cast in-situ composite containing 3.31 Ϯ 0.77 vol pct of porosity, the Brinell hardness is about 6 times its yield stress. The estimated yield stress of the cast in-situ composite at zero porosity as given by the linear least-squares fit appears to increase with particle content at a significantly higher rate than that predicted by the shear-lag model.

Cast in-situ Al(Ti)-Al2O3(TiO2) composites, synthesized by dispersing titanium dioxide (TiO2) particles in molten aluminium, which reduces these particles, partially or fully, forming alumina (Al2O3) and releasing titanium to the matrix alloy, may provide materials for lightweight components in automobiles and aircrafts. Wear tests, conducted at different normal loads and at constant sliding velocity of 1.05 m/s using a pin-on-disc wear testing m/c, under dry sliding conditions, indicate that the cumulative volume loss and wear rate of in-situ composites are significantly lower than those observed in either the commercial aluminium or Al-Ti base alloys, under similar load and sliding conditions. At a given particle content, the wear rate increases with increasing porosity content presumably due to its combined effect on real area of contact as well as subsurface cracking. The wear rate of in-situ composites having relatively lower porosity decreases with increasing particle content, but, at relatively higher porosity, decreases a little or remains unchanged with increasing particle content.