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

## TL;DR

Adding small amounts of TiO2 to Al-Zn composites improves their strength and reduces defects, making them suitable for lightweight structural uses.

## Contribution

Demonstrates that low TiO2 additions significantly enhance mechanical properties of Al-Zn composites via grain refinement and improved dispersion.

## Key findings

- 0.5 wt% TiO2 reduces void content from 0.98% to 0.62% and increases hardness by 52%.
- TiO2 additions promote grain refinement and stable dispersion within the α-Al matrix.
- Mechanical improvements include 32% higher impact strength and 23% higher tensile strength.

## Abstract

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.

0.5 wt% TiO2 refined grains reduce voids from 0.98% to 0.62% and increase hardness by 52%, impact strength by 32%, and tensile strength by 23% versus unreinforced Al–Zn.