Neutron diffraction analysis of the order in a Cu-Al-Ni shape memory alloy processed by powder metallurgy

摘要

The metastable β phase in Cu-based shape memory alloys (SMA) undergoes two order-disorder transitions during quenching from high temperature. First, it undergoes a B2 ordering at the nearest neighbors and then a second one at the next nearest neighbors that has not been clearly established. The kind of order obtained and the ordering evolution during aging at low temperature influence on the martensitic phase since it inherits the atomic order of the β phase. Neutron Powder Diffraction measurements have been performed at room temperature in a fully ordered alloy (Cu-27.4 Al-3.6 Ni at.%) obtained after quenching from 1173 K and aged during 2 hours at 473 K. The differences in the neutron scattering factors for Ni and Cu atoms promotes differences in reflections depending on the kind of order (DO_3 or L2_1). Rietveld analysis allows to conclude that a L2_1 order at next nearest neighbors is present in the ordered phase.