Effect of boron on the microstructure of near-eutectic Al–Si alloys

The effect of boron on the microstructure of a near-eutectic Al–Si alloy (ZL109) was investigated by scanning electron microscopy (SEM) and electron beam microprobe analysis (EPMA). It was found that α-Al dendrites and eutectic clusters were significantly refined by the addition of boron. Another interesting discovery is that the near-eutectic alloy exhibited hypereutectic structure characteristics when the level of boron added exceeds 0.3%, i.e., primary Si is precipitated in the eutectic microstructure. A new type of nucleation substrate for the primary Si is found, AlxCamBnSi. This appears to be the main reason for the precipitation of primary Si.