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

A harmful segregation of heavy elements (e.g. W, Mo, Re) during solidificat ion of Ni-base superalloys can only be eliminated by using a homogenizing h eat treatment, which needs to be carried out in the single-phase (gamma) fi eld above the gamma' solvus temperature but below the solidus temperature. Small-angle neutron scattering (SANS) was employed for in situ observation of the dissolution of precipitates in an Re-rich superalloy. The temperatur e dependence of the relative volume fraction and the size distribution of s maller gamma' precipitates, and the specific surface of large inhomogeneiti es as well as some other parameters were determined from the two-dimensiona l scattering curves measured for as-cast and heat-treated samples. Overlap of the incipient melting region with the region where a certain amount of p recipitates remained undissolved was observed, thus complicating a determin ation of the temperature at which all gamma' precipitates are already disso lved. Nevertheless, conclusions about the temperature at which the precipit ates dissolve and about the temperature at which the incipient melting star ts could be formulated. The total scattering probability is suggested as th e measure of the overall homogeneity of the distribution of elements in the sample. The temperature dependence of this parameter indicates the optimum solution procedure.