Isothermal aging of a γ′-strengthened Co–Al–W alloy coated with vacuum plasma-sprayed MCrAlY bond coats

• γ′-Strengthened Co–Al–W superalloy was coated with different MCrAlY bond coats.
• Aging in air at 900 °C shows good compatibility of superalloy and coatings.
• γ′ phase in the superalloy is dissolved due to diffusion of Cr from the bond coat.
• Several W-rich phases that form in the interdiffusion zone are identified.
• Nanoindentation shows superior mechanical properties of the superalloy.

Cobalt-based superalloys with a γ/γ′ microstructure were discovered in 2006 and are currently being investigated as an alternative to nickel-based superalloys for high-temperature, high-load applications in gas turbine blades. They promise a better castability combined with a similar creep strength. Superalloy turbine blades are commonly coated with oxidation resistant bond coats. For this reason their compatibility needs to be studied. Co–9Al–9W specimens with a γ/γ′ microstructure were coated with either a nickel-based or cobalt-based MCrAlY bond coat using vacuum plasma spraying. After aging at 900 °C in air for up to 500 h no decomposition of the γ′ phase was found in the bulk superalloy. The interdiffusion zone shows several different W-rich topologically close-packed phases arising from the dissolution of the γ′ phase in this region. The W-rich phases are identified to be μ phase for both bond coats and R phase for the nickel-based bond coat only. Their total volume is higher for the nickel-based bond coat. Therefore the cobalt-based bond coat is better suited for the Co-based superalloy substrate. Room temperature hardness and Young's modulus were measured using nanoindentation in the initial state and after heat treatment. A significantly higher Young's modulus was found for the cobalt-based bond coat.