PREPEAK ON THE STRUCTURE FACTOR OF LIQUID HYPOEUTECTIC AL-FE ALLOY

Using a theta-theta X-ray diffractometer the structure of liquid hypoe utectic Al-Fe alloy at 675 degrees C and its change with different the rmal histories are investigated. Liquid Al is used as a reference syst em. It is found that a pre-peak appears on the small angle part of the structure factor of the nonsuper-heated 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-pea k can be observed on them. The appearance of a pre-peak is a mark of t he intermediate-range order (IRO). The pre-peak is taken as the correl ation between Fe atoms on the IRO length scale. Assume that the crysta lline structure is the first-order approximation of the liquid structu re. 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 gives a DO3-like structure. if the vacancy among the units is filled with f ee-like Al cells, the composition of the entity is about Al7Fe, close to that of the metastable phase AI,Fe from rapid solidification. It is speculated that there are Al6Fe-like clusters in the liquid hypoeutec tic Al-Fe alloy.