Structures of molten metals Cu and Al, alloys Cu-Al, Al-Ni, Al-Fe and, Al-Si were investigated by using high temperature X-ray diffractometer. It has been found that there are not only short-range order structures (SRO) but also medium range order structures (MRO) in Cu-Al, Al-Ni, Al-Fe alloy melts. There are only short-range order structures in the pure metals Cu and Al and Al-Si alloy melts at different temperatures. It has also been found that the presence and the disappearance of the MRO structures in molten metals are a function of temperature. Moreover, the pre-peak in the structure factor is an indication of MRO in molten metals. There is a pre-peak in each structure factor S(Q) of Al-Fe alloy melt containing 14 wt% Fe, 16 wt% Fe and 19 wt% Fe at 1550degreesc, showing that there are the medium range order structures in these alloy melts. For Al-35wt% Ni alloy, the pre-peak exists in S( Q) when the temperature is lower than 1300degreesC, and it is weakened drastically when the temperature surpasses 1300degreesC. The pre-peak occurs at values of scattering vector Q = 18.5 nm(-1) in the structural factor of Cu-12 wt %Al alloy melts at 1250degreesC. The height of the pre-peak in the melt decreases with increasing temperature. These results show that there exist not only the SRO structure but also MRO structure in the Al-TM melts, and the MRO is correspondent to the tendency of formation of chemical compound. The formation mechanism of the MRO is also studied in this work. Based on the measured results of Cu-Al alloy, a model of the MRO is presented.
