Solution Heat Treatment Optimization of Fourth-Generation Single-Crystal Nickel-Base Superalloys

The optimization of the solution heat treatment (SHT) of fourth-generation single-crystal nickel-base superalloys LDSX-6B and LDSX-6C is presented. The methodological approach to optimizing the SHT process is particularly highlighted. Differential scanning calorimetry (DSC) measurements and electron-probe microanalysis (EPMA) mapping were carried out to investigate material properties in the as-cast condition and after SHT. The DSC equipment was also adopted as a vacuum furnace to evaluate the suitability of the SHT ramp profile and to check the safety margins with regard to incipient melting during SHT. SHT trials were carried out in a laboratory-scale vacuum furnace, after which the heat-treated samples were subjected to DSC experiments, microstructural analysis, and EPMA mapping to assess the effects of SHT peak temperatures and soak periods. From the DSC and EPMA results, as-cast LDSX-6B shows a lower degree of elemental microsegregation; hence, this alloy is relatively easier to homogenize in the SHT trials. In contrast, as-cast LDSX-6C was found to have a higher degree of elemental microsegregation; therefore, it is much more difficult to homogenize and is highly prone to incipient melting. The results of this study indicate that an increase in the SHT peak temperature and/or soak period will lead to an improved compositional homogeneity in the material as expected. After SHT, both alloys retained some residual elemental microsegregation and the LDSX-6C alloy showed precipitation of topologically close-packed (TCP) phase at the dendrite cores. The most appropriate and economic SHT process may be determined based on the methodological approach presented in this study and the requirements of the materials during service.