Origin of pre-peak in structure factors of liquid Cu-Al-Ni alloys

Abstract

Molten Cu-Al-Ni alloys have been investigated using the X-ray diffraction method. A distinct pre-peak has been found around a scattering vector magnitude of 18.5 nm-1 in the structure factor. The pre-peak increases in its intensity with decreasing temperature and the addition of Ni. The appearance of a pre-peak is a mark of medium range order. The structural unit size corresponding to the pre-peak is equal in magnitude to the (111) interplanar lattice spacing of the β phase. The appearance of a pre-peak is due to the existence of a cluster with a β phase-like structure in the melt. The size of the clusters increases with decreasing temperature but their structural unit size remains constant. Shape memory alloy ribbons can be fabricated by the rapid solidification technique. At a quenching temperature corresponding to that of the lower pre-peak, the structure of the martensite obtained has a lower degree of order and the temperature of the reverse martensitic transformation is lower. At a quenching temperature corresponding to a higher pre-peak, the structure of the martensite obtained has a higher degree of order and temperature of the reverse martensitic transformation is higher. These clusters can become crystal seeds of the β phase and β phase develops from these clusters, thus resulting in a correlation between the pre-peak and martensitic transformation.