| Summary: | The present work focuses on the effect of multistage solution heat treatment (MSHT) and artificial aging on two-stage stir-cast LM4 + TiB<sub>2</sub> (1, 2, and 3 wt.%) composites on the mechanical properties as compared to as-cast and single-stage solution heat-treated (SSHT) composites. Two novel tests, viz. the confirmation hardness test and the chemical analysis test, were performed to ensure the soundness of the casting and uniform distribution of TiB<sub>2</sub> within the matrix. Samples subjected to MSHT + aging at 100–200 °C displayed the highest hardness and UTS values compared to as-cast and SSHT + aging at 100–200 °C samples. Compared to as-cast alloy, peak-aged samples of 1–3 wt.% (MSHT + aging at 100 °C), hardness values improved from 107–150%, and UTS values improved from 47–68%. The presence of metastable phases (θ′-Al<sub>2</sub>Cu and θ″-Al<sub>3</sub>Cu) and of hard TiB<sub>2</sub> particles are the reason for the improvement in the properties. Peak aged LM4 + 3 wt.% TiB<sub>2</sub> composite displayed the highest hardness of 175 VHN and UTS of 251 MPa. Fracture analysis of the LM4 alloy showed dimple rupture, and its composites revealed quasi-cleavage fracture. Based on the overall results, the inclusion of TiB<sub>2</sub>, MSHT, and artificial aging treatment on the LM4 alloy significantly influenced the composites’ mechanical properties.
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