Oxidation ahead of a crack tip in an advanced Ni-based superalloy

Oxide intrusions which formed ahead of the crack tip in an advanced nickel-based superalloy after exposure to air at 650 °C have been investigated by (scanning) transmission electron microscopy equipped with energy-dispersive X-ray spectrometry. Three different cases were considered: (i) a fast growing crack; (ii) a stationary crack exposed to a 5 h period under load; and (iii) a stationary crack exposed to a 5 h period but held at zero mechanical applied load. The intrusions formed during both hold periods were much longer than those present ahead of the (growing) dynamic crack (∼1.5 μm) and consisted of layered oxides in the thermodynamic sequence of CoO, NiO, Cr₂O₃, TiO₂ and Al₂O₃. The last three of these oxides appeared to form protective layers along the flanks and, for the stationary cracks, at the intrusion tip. Calculations of the oxygen partial pressure both across the width and along the length of the oxide intrusions have been undertaken assuming such protective conditions. Significant quantities of oxygen are thus not expected to exist in the alloy ahead of intrusions which are sealed by protective oxide layers.