Dependence of small-angle neutron scattering contrast on the difference in thermal expansions of phases in two-phase alloys

Theoretical expressions describing small-angle neutron scattering (SANS) contrast dependence on temperature in the region where no phase-composition changes occur were derived for two-phase Ni superalloys. The theory is based on the difference in thermal expansion of the two primary phases, γ and γ′. The simulations show that the scattering contrast temperature evolution is significant enough to be considered in in situ SANS experiments with superalloys at elevated temperatures. The simulations performed show that the magnitude of the scattering contrast at room temperature is firmly connected with the particular shape of the scattering contrast temperature dependence. This fact can be used for determination of the scattering contrast without a knowledge of the compositions of the individual phases. The theoretical expressions derived for scattering contrast were proven experimentally on an Ni–Fe-base alloy, DT706. The evolution of lattice parameters of both the matrix and the precipitate phases was obtained from an in situ wide-angle neutron diffraction experiment. The theoretical scattering contrast dependence was then successfully fitted to the measured SANS integral intensity.