In situ TEM investigation of oxidation behavior in second generation nickel-based single crystal superalloy CMSX-4

The high-temperature oxidation resistance of Ni-based single crystal superalloys is paramount to their service reliability. However, conventional ex-situ characterization techniques struggle to capture the transient behavior during the initial stages of oxidation, often obscuring the underlying nanoscale kinetic processes. To this end, we utilized in situ transmission electron microscopy (TEM) to directly observe the oxidation behavior of CMSX-4, a typical second-generation nickel-based single crystal superalloy, during continuous heating. The results indicate that during the gradual temperature increase, the γ phase preferentially oxidizes, while the γ’ phase, as an intermetallic compound, experiences substantial oxidation only at 600 °C. At 800 °C, the γ’ phase rapidly oxidizes, driven by the outward diffusion of Ni and Co into the γ matrix, which results in the extensive formation of NiO and CoO. Furthermore, complementary ex-situ bulk oxidation tests at 1000 °C were incorporated to reveal the macroscopic multi-stage kinetics governed by the development of a multilayered scale and the spallation induced by an intermediate refractory-rich oxide band. The selective oxidation of the two phases, along with the observed crystallographic orientation-dependent oxidation behavior and cross-scale correlations, provides valuable insights for designing high-temperature alloys with enhanced oxidation resistance.