Metal matrix composites (MMCs) combine a stiff but brittle phase, typically a ceramic, with a more ductile metal matrix. The correct fractional combination of materials can result in a material with improved stiffness, creep resistance, yield stress, and wear resistance relative to the monolithic matrix. The use of MMCs in recent years has become more widespread due to a growing understanding of the dependence of composite properties on a number of factors (e.g., interface properties, metallurgy of the matrix, and stress partitioning between the constituent phases) and appreciation of the problems that can occur in their usage. The purpose of this work was to investigate microstructural evolution in ingot metallurgy AA2618 due to the addition of 10 and 15 vol. % angular alumina (Al₂O₃) particles.

The primary investigative techniques employed were microhardness measurements, differential scanning calorimetry (DSC), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and transmission electron microscopy (TEM). In addition, other metallographic and data analysis techniques were used.

The results of this study showed that the addition of Al₂O₃ particles did not alter the aging sequence of AA2618, but it altered certain aspects of the precipitation reaction. It caused the suppression of Guinier-Preston-Bagaryatskii (GPB) zone nucleation, acceleration of the artificial aging response, lowering of peak hardness value, and nonuniform distribution of precipitate and dispersoid phases. However, it did not affect the growth mechanisms for S′ and θ′ formation. The growth parameters obtained for the unreinforced alloy and its composites were not significantly different.

Magnesium accumulation around the reinforcing Al₂O₃ particles was very pronounced. Mg-rich intermetallic particles (suggested to be MgAl2O4 spinel) were observed existing in isolation and embedded in Al₂O₃ particles. The presence of these particles was considered to be responsible for the low peak hardness obtained for the composites. Also, other intermetallic particles (such as aluminosilicates and Fe-rich particles) were observed.

Aluminide (Al_xFeNi) particles, which usually occur in AA2618, were determined to possess a variety of chemical formulae. Also, the Al xFeNi phase was determined to be more consistently indexed on the basis a C-centered monoclinic crystal structure with a = 0.867 nm;​ b = 0.900 nm;​ c = 0.859 nm;​ and β = 83.50° rather than the primitive monoclinic structure reported in the literature.