Al2O3 nanoparticles induced simultaneous refinement and modification of primary and eutectic Si particles in hypereutectic Al–20Si alloy

It is well known that the mechanical properties of hypereutectic Al–Si alloys are affected by the size, volume fraction, and distribution of primary and eutectic Si particles. However, it is very difficult to simultaneously refine and modify Si particles in hypereutectic Al–Si alloys by conventional means. This study investigates an effect of nanoparticles on Si particles during solidification in hypereutectic Al–Si alloys. Various contents of γ-Al2O3 nanoparticles were added in hypereutectic Al–20Si alloy melt and further dispersed through an ultrasonic cavitation based technique. The cast hypereutectic Al–20Si alloy with the nanoparticle addition showed a significant enhancement in both strengths and ductility. The ductility of the cast hypereutectic Al–20Si alloy was increased from 0.37% to 1.72% with an addition of 0.5 wt% γ-Al2O3 nanoparticles. Yield strength and ultimate tensile strength of the nanocomposite also showed an improvement of about 6% and 26%, respectively. Study suggests that γ-Al2O3 nanoparticles effectively induced simultaneous refinement of primary Si and modification of eutectic Si, resulting in superior ductility enhancement that is much higher than that conventional methods can offer. Microstructural analysis with optical and scanning electron microscope (SEM) revealed that the primary Si particles were refined from large star shapes with small features to polygon or blocky shapes with smooth edges and corners. Moreover, the large plate eutectic Si particles were also modified into the fine coralline-like ones. The results could have great potential for numerous applications.