Determination of γ′ solution temperature in Re-rich Ni-base superalloy by small-angle neutron scattering

Abstract

A harmful segregation of heavy elements (e.g.W, Mo, Re) during solidification of Ni-base superalloys can only be eliminated by using a homogenizing heat treatment, which needs to be carried out in the single-phase (γ) field above the γ′ solvus temperature but below the solidus temperature. Small-angle neutron scattering (SANS) was employed forin situobservation of the dissolution of precipitates in an Re-rich superalloy. The temperature dependence of the relative volume fraction and the size distribution of smaller γ′ precipitates, and the specific surface of large inhomogeneities as well as some other parameters were determined from the two-dimensional scattering curves measured for as-cast and heat-treated samples. Overlap of the incipient melting region with the region where a certain amount of precipitates remained undissolved was observed, thus complicating a determination of the temperature at which all γ′ precipitates are already dissolved. Nevertheless, conclusions about the temperature at which the precipitates dissolve and about the temperature at which the incipient melting starts could be formulated. The total scattering probability is suggested as the measure of the overall homogeneity of the distribution of elements in the sample. The temperature dependence of this parameter indicates the optimum solution procedure.