Effect of low TiO₂ additions on the microstructure and mechanical properties of stir-cast Al-Zn composites

Low-weight ceramic reinforcement offers an effective route to enhance aluminum-based metal matrix composites while preserving process scalability and cost efficiency. In this study, Al-2 wt% Zn composites reinforced with 0-0.5 wt% TiO₂ particles were fabricated via conventional stir casting, and their microstructural evolution and mechanical performance were systematically investigated. Microstructural characterization (OM, SEM-EDS, and XRD) reveals that TiO₂ additions promote significant grain refinement, improved phase homogeneity, and stable dispersion of TiO₂/oxide-bearing regions within a continuous α-Al matrix, while Zn remains predominantly in solid solution. A progressive reduction in void content (from 0.98% to 0.62%) further indicates improved structural integrity with increasing TiO₂ fraction. Mechanical testing demonstrates a substantial enhancement in performance with increasing reinforcement content. At 0.5 wt% TiO₂, the composite exhibits improvements of approximately 52% in hardness, 32% in impact strength, and 23% in tensile strength compared to the unreinforced alloy due to Hall-Petch grain-boundary strengthening, Orowan-type particle strengthening, and effective load transfer between matrix and reinforcement. Fractographic analysis confirms a transition from predominantly ductile shear fracture in the matrix alloy to a mixed-mode fracture mechanism governed by particle-assisted void nucleation and crack deflection in the reinforced composites. Overall, the results demonstrate that very low TiO₂ additions can deliver significant mechanical enhancement without the processing challenges associated with high reinforcement loadings, making Al-Zn-TiO₂ composites potential candidates for lightweight structural applications in automotive and aerospace sectors.