Determination of the next-nearest neighbor order in β phase in Cu-Al-Ni shape memory alloys

The metastable beta phase undergoes two order-disorder transitions during quenching from high temperature in Cu-based shape memory alloys. First, it undergoes a B2 ordering at nearest neighbors and then a second one at next-nearest neighbors that has not been clearly established. Neutron powder diffraction measurements have been performed at room temperature in a fully ordered alloy (Cu-27.4 Al-3.6 Ni at. %). Rietveld analysis allows to conclude that a L2(1) order at next-nearest neighbors is present in the ordered phase. The site occupancy of the different positions has been determined. (C) 2002 American Institute of Physics.