Origin of the anomalous volume expansion in Al–Si alloys above liquidus

The sessile-drop method and high-temperature small-angle x-ray diffraction technique were used to characterize the density of liquid Al–Si alloys with Si contentC= 0 to 22 wt%. A distinct abnormal volume expansion of the hyper-eutectic melts was identified just above the liquidus (withinT_L–T_k) during the heating sequence (h.s.), which is irreversible during the cooling sequence (c.s.). This phenomenon is explained by the segregation of Si atoms from the supersaturated “quasi-eutectic” structure and formation of silicon clusters in the superheated melt. In addition, a pre-peak in the x-ray diffraction spectrum of Al–14% Si melt was observed at 850 °C, which is associated with the silicon clusters. The pre-peak remains when the superheated melt is cooled down to the liquidus (670 °C); this result is in accord with the density measurements.