Pre-peak on the structure factor of liquid hypoeutectic Al-Fe alloy

Using aθ-θ X-ray diffractometer the structure of liquid hypoeutectic Al-Fe alloy at 675°C and its change with different thermal histories are investigated. Liquid Al is used as a reference system. It is found that a pre-peak appears on the small angle part of the structure factor of the nonsuperheated hypoeutectic Al-Fe alloy, but disappears after superheating whereas the structure factors of liquid Al hardly change with the different thermal histories, and no pre-peak can be observed on them. The appearance of a pre-peak is a mark of the intermediate-range order (IRO). The pre-peak is taken as the correlation between Fe atoms on the IRO length scale. Assume that the crystalline structure is the first-order approximation of the liquid structure. A model structure is constructed. The basic unit Is a cube formed by 8 Al atoms on its corner and one Fe atom occupying its center. The translation of such a unit along its fourbody diagonals by the length of a diagonal can meet the requirement of the Fe-Fe distance and glves a DO₃-like structure. If the vacancy among the units is filled with fcc-like Al cells, the composition of the entity is about Al₇,Fe, close to that of the metastable phase Al₆Fe from rapid solidification. It is speculated that there are Al₆,Fe-like clusters in the liquid hypoeutectic Al-Fe alloy.