Relativity of medium range order in molten Cu-Sn alloys and phase diagram

Evolution of the medium range order (MRO) in Cu - Sn alloy melts with different compositions at the same superheat temperature has been studied. A prepeak, which characterises the MRO, appeared in front of the main peak. Based on the positions Q(pp) and heights H-pp of the prepeak on the structure factors of liquid Cu - Sn alloys, it was found that the Q(pp) was higher around Cu5Sn, and the highest at Cu5Sn while the H-pp was smaller, and a minimum at Cu5Sn. As proved by the first peak of the pair correlation functions and the coordination numbers, the molten structure of Cu - Sn changes from order to disorder largely when the tin concentration is in the range of 35.3 - 60 wt-%. In addition, a shoulder appears on the right side of the first peak of the pair correlation function and shifts to small r (real space).