Solidification behavior of Re- and Ru-containing Ni-based single-crystal superalloys with thermal and metallographic analysis

To clarify the solidification behavior of Re- and Ru-containing Ni-based single-crystal superalloys, four experimental alloys with varied contents of Re and Ru were investigated by differential scanning calorimetry (DSC) and metallographic techniques. To obtain the gamma'-solvus temperatures, the stepwise solution and aging heat treatments were used. DSC analysis shows that Re leads to the increase in freezing range and gamma'-solvus temperature. On the contrast, Ru only has negligible influence on the freezing range, but leads to the lower gamma'-solvus temperature. In comparison with Ru, Re leads to more severe segregation and higher eutectic fractions in as-cast microstructures. Furthermore, the castability and phase stability of Ni-based superalloys were analyzed by the results of DSC and metallographic analysis, such as freezing range, critical nucleation temperature, gamma'-solvus temperature and eutectic fractions. It shows that Re leads to the wider freezing range and lower critical nucleation temperature, indicating the worse castability of Re-containing Ni-based single-crystal superalloys. The characteristic phase transformation temperatures of Re-containing single-crystal superalloy are determined by DSC analysis. For as-cast samples, the liquidus, solidus, critical nucleation and eutectic reaction temperature are clearly shown in DSC curves. After a stepwise solution and aging heat treatments, the higher amount of gamma' phase with cubic morphology can be obtained. Finally, the precipitation temperature of gamma' phase is determined by DSC analysis for the heat-treated samples.