Hierarchical microstructural evolution and performance enhancement in T6-treated ADC12 alloys through coupled bismuth modification and SiC nanoparticle reinforcement

This work comprehensively evaluates the coupled influence of bismuth (Bi) modification and silicon carbide (SiC) nanoparticle reinforcement on the microstructural evolution, mechanical properties, and tribological performance of ADC12-based materials under as-cast and T6-treated conditions. ADC12 alloy, Bi-modified ADC12 alloy (0.5 wt% Bi), and ADC12–Bi–SiC nanocomposites reinforced with 1 wt% SiC nanoparticles were fabricated via ultrasonically assisted stir casting followed by T6 heat treatment. Microstructural characterization revealed that Bi primarily modified eutectic Si morphology, transforming coarse acicular Si into finer rod-like structures, whereas SiC nanoparticles promoted substantial α-Al grain refinement together with further eutectic Si refinement. Relative to the unmodified alloy, the ADC12–Bi–SiC composite exhibited reductions of approximately 60% in α-Al grain size and 72% in eutectic Si length under the as-cast condition. After T6 treatment, eutectic Si underwent pronounced spheroidization, achieving the highest roundness value (0.89) in the composite system. Quantitative strengthening analysis identified thermal-mismatch and Orowan strengthening as the predominant SiC-related contributions. The T6-treated ADC12–Bi–SiC composite exhibited the highest hardness (177.1 HV), yield strength (290.2 MPa), ultimate tensile strength (345.2 MPa), and elongation (5.3%), corresponding to improvements of approximately 81%, 101%, 82%, and 104%, respectively, relative to the as-cast ADC12 alloy. It also exhibited the lowest wear rate (2.89 × 10⁻⁴ mm³/N·m) and coefficient of friction (0.34). These findings demonstrate that complementary Bi-induced eutectic modification, SiC-assisted microstructural refinement and strengthening, and subsequent T6 treatment provide sequential control of microstructure and performance in ADC12 alloy.