The 4^th generation Ni-base single crystal superalloys, which contain large amounts of refractory metals for strengthening and platinum group metals, e.g., Ru, for TCP-phase prevention, show excellent high-temperature strengths. However, these alloying elements seem to decrease high-temperature oxidation resistance, and the improvement of high-temperature oxidation resistance is one of the important issues for practical use of these superalloys.
  In this study, Ni-base superalloys with various amounts of Hf, Ta, Re and Ru were examined in isothermal and cyclic exposures at 1100°C to investigate the effect on the oxide growth rate and resistance to scale spallation. Structures of the oxide for the alloys were analyzed by XRD and SEM, and it is clarified that the existence of Hf and Ta contributes to the improvement of adhesiveness between the oxide and the matrix.