A DIFFUSE NEUTRON SCATTERING STUDY OF CLUSTERING KINETICS IN Cu-Ni ALLOYS

Diffuse scattering of thermal neutrons was used to investigate the kinetics of clustering in Cu-Ni alloys. In order to optimize the experimental conditions the isotopes 6SC~ and 62Ni were alloyed. The time evolution of the diffuse scattered intensity at 400 oC has been measured for eight Cu-Ni alloys, varying in composition between 30 and 80 at. % Ni. The relaxation of the so called null matrix. containing 56.5 at. O/, Ni, has also been investigated at 320, 340, 425 and 450 OC. Using Cook's model from all these measurements information has been deduced about diffusion at low temperatures and about thermodynamic properties of the Cu-Ni system. It turns out that Cook's model is not sufficiently detailed for an accurate description of the initial stages of these relaxations. Introduction. - The kinetics of clustering and short range order (s.r.0.) in stable solid solutions were discussed from a theoretical point of view by Cook (I). His model is based on the lattice diffusion theory for spinodal decomposition as developed by Cook, De Fontaine and Hilliard 123. The model gave good agreement with data on the kinetics of s.r.0. as measured by one of the authors (3,4). At that time no experimental data on clustering were available. One of the important results of Cook's theory was that he demonstrated the impossibility of describing cluster- ing kinetics, even approximately, by a single relaxation time in contrast to short-range order kinetics. Important theoretical contributions have been given by Khachaturyan (5) and Yamauchi (6), but in this paper we will mainly limit ourselves to a comparison with Cook's theory. For binary alloys he derived the following expres-