In-situ environmental TEM study of γ′-γ phase transformation induced by oxidation in a nickel-based single crystal superalloy

Oxidation induced phase transformation from γ′ to γ phase can severely degrade creep properties of superalloys and has been studied by in-situ environmental transmission electron microscopy, energy dispersive spectrometer and electron energy loss spectroscopy in detail. The in-situ observation showed that, during oxidization, the ordered γ′ phase (L12) gradually transformed to disordered γ phase (face-centered cubic (fcc)) with the appearance of oxidation product consisted of γ-Al2O3 crystallites with a strong crystallographic texture. This directly proves that γ′-γ phase transformation are induced by aluminum oxidation with aluminum depleted in γ′ phase. Furthermore, the oxygen was demonstrated to diffuse preferentially through γ′-γ phase interface which is different from previously thought (γ matrix).