Short-range clustering and long-range periodic decomposition of an electron irradiated Ni-Cu alloy

Diffuse and small-angle neutron diffraction studies of the null matrix ⁶²Ni-41 at.% ⁶⁵Cu have been performed after thermal treatment and after irradiation with 3 MeV electrons at temperatures between 373K and 640K. Besides an increase in the short-range clustering, a periodic decomposition with a wavelength around 45 AA is observed after irradiation at temperatures between 373K and 480K. The decomposition structure was found to be stable against thermal annealing and irradiation, at least up to 510K. Upon thermal treatment above 600K dissolution of the long range decomposition takes place. By extrapolation of the short-range clustering data from high temperatures, the critical temperature of the coherent miscibility gap is obtained at 527K and that of the incoherent gap at 575K. The decomposition kinetics within the gap can be described satisfactorily by Cook's clustering model (1969, 1970) for large scattering wavevectors, i.e. for the short-range clustering. The small-angle peak kinetics show several specific deviations from the predictions of the common spinodal decomposition theories. The possibility cannot be excluded that the observed long-range decomposition is a radiation-induced phase transformation.