The microstructure characteristics of rapidly solidified hypereutectic Al-Si alloy were analyzed as function of annealing temperature and holding time by scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and X-ray diffractometry (XRD). 

The precipitation and coarsening of supersaturated Si element were also investigated. The results indicate that the microstructure of rapidly solidified Al-Si alloy is composed of fine block-like primary and needle-like eutectic Si phase dispersed homogeneously in the α(Al) matrix. The structural distortion of the α(Al) matrix is observed owing to large solidification rate as a result of gas atomization. While, the structural distortion is released after annealing. The studies of coarsening of supersaturated Si in the matrix suggest that the precipitated Si phase coarsens with increasing annealing temperature or prolonging holding time. 

However, the coarsening behavior is different to the classical LSW theory. The coarsening mechanism is more likely to be controlled by interface diffusion with coarsening exponent. Additionally, the annealing temperature only affects the coarsening rate constant and activation energy, but less affects the coarsening exponent.