Effects of aluminised-coating on microstructure and properties of Ni–Co-base superalloys

A Ni–Co-base superalloy was subjected to three different pack aluminising procedures (1000 °C for 3.5 h, 890 °C for 8 h and 620 °C for 10 h) to produce the Al-rich coating with simultaneously improved oxidation and wear resistance. All samples showed a multi-layer coating made up of (Ni, Fe, Co)Al phase, (Ni, Fe, Co) ₃ Al phase and transition layer. The AT620 sample possessed the lowest oxidation rate but the highest double-edge-notched (DEN) strain during high-temperature stress rupture. The high oxidation resistance might be associated with the creation of Al ₂ O ₃ layer (∼5.6 μm), contributing to an inhibition of oxygen permeation. However, a premature failure was caused by the strong deformation incompatibility between the substrate and alumina layer. The AT890 samples had higher oxidation and creep-induced-crack resistance than those of the AT1000 samples, which was the result of the effective Al inter-diffusions in the transition layer suppressing the undesirable Cr-rich σ phase.