Aluminum matrix composites (AMCs) are used in aerospace, automotive, and marine industries for their strength and wear resistance. In this study, LM13 aluminum alloy was reinforced with 10 wt.% nano-ZrO₂ by stir casting combined with cryogenic solidification. Cast iron chills of five thicknesses (5, 10, 15, 20, and 25 mm) were used to study their effect on microstructure and mechanical properties. Tensile strength rose from 145 MPa at the 5 mm chill to 200.63 MPa at the 25 mm chill, a gain of about 38.4%. Micro-Vickers hardness rose from 71.20 VHN to 99.60 VHN, a gain of about 39.9%. These values are averages of three tensile tests and five hardness indentations per condition. The gains track the finer microstructure seen with thicker chills, most likely due to a faster cooling rate; cooling rate itself was not measured, so this remains an interpretation rather than a proven mechanism. The percentage gains describe the trend across the 5–25 mm series, not a comparison against an unreinforced LM13 control, since such a control was not separately cast in this work. Among all conditions tested, the 25 mm chill gave the best overall hardness and tensile strength. Combining nano-ZrO₂ reinforcement with chill-assisted, cryogenically accelerated solidification appears promising for improving LM13 composites. Since the study followed a single-factor design, the result should be read as a demonstrated trend rather than a statistically confirmed synergistic effect; dedicated control experiments are needed to confirm this.
