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The mechanical properties of composites are strongly influenced by reinforcing particles. Many composites exhibit steep internal stress gradients that exist across the particle-matrix interfaces due to constraints from the reinforcing particulates, which could be dependent on the geometry of reinforcements or the particulate shape. The present study showed that the differences in the mechanical properties of the two composites A (with angular reinforcements) and B (with spherical reinforcements) are related not only to commonly known particulate parameters, such as their size, size distributions and volume fractions, but also to the particulate shape. The addition of both angular and spherical reinforcements significantly increased the elastic modulus, 0.2% proof stress, and ultimate tensile stress, but decreased the ductility compared with the unreinforced alloy. Composite B with spherical reinforcements had better ductility than composite A, although it had higher reinforcement volume fraction. The highest yield stress and ultimate tensile stress for all materials were achieved at the peak aged (PA) condition as compared to the under-aged (UA) and over-aged (OA) conditions. TEM studies revealed a dual precipitate morphology consisting of β′ and β″ at the PA condition.