Bimodal size distributions of γ' precipitates in Ni-Al-Mo. I: Small-angle neutron scattering

The evolution of the microstructure during the decomposition of two Ni-Al-Mo alloys with a bimodal size distribution of precipitates was studied using small-angle neutron scattering. Precipitate size distributions and space correlations of the bimodal precipitate ensemble were determined by model fitting of the scattering curves. The correlation effects between the precipitates were taken into account by the locally-monodisperse (LMD) hard-sphere model. This model assumes that the ensemble of precipitates have their positions correlated with their sizes, being locally monodisperse. The form factors were used according to the shape of the precipitates, spherical in the case of alloy I, Ni-8.8Al-9.6Mo (at.%) (isotropic system), and cuboidal for alloy II, Ni-9.5Al-5.4Mo (anistropic system). To characterize the microstructure of the two alloys a set of characteristic parameters (e.g. volume fraction, average radii, precipitate number density, etc.) were obtained using relations between the moments r-n of the size distribution N(r). The model proved particularly suited to characterize the isotropic system. In the case of the anisotropic system it was insensitive to changes of the size distribution N(r) during aging.