Influence of γ′-raft orientation on creep resistance of monocrystalline Co-based superalloys

The creep behavior of a monocrystalline Co-based γ−γ^′ superalloy (Co–28.8Ni–6.2Al–1.8Ti–2.0W–2.7Mo–1.8Nb–0.9Ta, at%) is investigated for two types of rafted γ′-phase: (i) rods parallel (p-type) and (ii) plates perpendicular (n-type) to [001]. The tensile creep behavior of these two rafted microstructures (and a control unrafted microstructure) is measured at 900 ^∘C for various creep stresses up to a creep strain of 3%. At the highest stress of 305 MPa (2–4 h creep to 3% strain), microstructures with n-type rafts provide better creep resistance than p-type rafts and unrafted cuboids, consistent with n-type plates oriented perpendicular to stress being more effective at hindering dislocation motion. At an intermediate stress of 250 MPa (10–20 h to 3% creep strain), n-type rafts still provide the highest overall creep resistance, but this effect is less pronounced due to the rafts disintegrating under tensile load. At the lowest stress of 205 MPa (100–220 h to 3% creep strain), n-type rafts provide the least creep resistance, as they first disintegrate then transform into p-type rafts. Specimens starting with p-type rods and unrafted cuboids microstructures show better creep resistance, but start to exhibit onset of tertiary creep from their extensively rafted microstructure. The evolution of creep strain rate (and thus the time to reach 3% strain) for the three stresses and three microstructures studied is affected by the rafting evolution, as well as γ−γ^′ topological inversion and development of inhomogeneous γ′ spatial distribution.