The Effect of Ruthenium on Micro-Pores in a Single-Crystal Superalloy and Consequences on Thermomechanical Fatigue

Abstract The present study investigates the impact of ruthenium (Ru) on the formation of micro-pores in a nickel-based single-crystal alloy, and their consequences on microstructural alterations during out-of-phase thermomechanical fatigue. In particular, the focus of this investigation was on two experimental nickel-based single-crystal superalloys, one Ru-free and one Ru-doped. More eutectics can be found in the as-cast Ru-doped alloy than the as-cast Ru-free alloy. Differential scanning calorimetry was employed to ascertain the freezing range of the as-cast Ru-free alloy, which was found to be 6 °C larger compared to the as-cast Ru-doped alloy. Less eutectics and larger freezing range indicate a reduced feeding ability during solidification, resulting in an increased number of solidification pores. This observation was confirmed by three-dimensional X-ray computed tomography, which revealed the presence of larger and more irregular pores in the Ru-free alloy. A thorough examination of the microstructural characteristics in the proximity of large and irregular pores in both alloys has been conducted, unveiling the existence of deformation twins, recrystallized grains, and the formation of M 6 C carbides. In the vicinity of smaller spherical pores, only deformation twins were observed. The presence of fewer large irregular pores in the Ru-doped alloy results in less pronounced microstructural alterations, thereby reducing potential crack initiation sites during out-of-phase thermomechanical fatigue. Furthermore, the segregation of ruthenium at stacking faults was observed by atom probe tomography in the Ru-doped alloy. This can potentially reduce the stacking fault energy and enhance the out-of-phase thermomechanical fatigue performance of the alloy.