In this paper, horizontal continuous casting Al–12%Si alloy was modified by 30% fine-grained
structural material (FSM) of the same composition as the master alloy. The effect of this FSM
master alloy on the morphologies and crystal defects of eutectic Si has been investigated.
Results show that microstructure of eutectic Si alters from coarse needle-like to fine fibrous
shape after modification. TEM observations reveal that the crystal defects such as twins and
stacking faults generate in the modified eutectic Si while no obvious defects are found in the
unmodified Si. Furthermore, hundreds of nanoparticles present in the modified eutectic Si
along with the twins and stacking faults. Finally, we determine the composition of these
nanoparticles to be Si–Si bonded nanoculsters which should be the genetic factors in terms of
metal structural heredity.