The importance of sign and magnitude of γ/γ′ lattice misfit in superalloys—with special reference to the new γ′-hardened cobalt-base superalloys

In this work, the remarkable high-temperature strength properties of monocrystalline γ′-hardened Ni-base superalloys are reviewed with the goal of understanding better the important role played by the γ/γ′ lattice misfit δ with respect to microstructural stability and high-temperature creep and fatigue resistance. In particular, attention is focussed on the phenomenon of microstructural degradation by directional coarsening in the form of γ/γ′ rafting which also occurs in turbine blades in service. The role played by the lattice misfit (which is negative for all currently used Ni-base superalloys) is elucidated with respect to its sign, magnitude and temperature dependence. Based on a detailed discussion of high-temperature deformation involving γ/γ′ rafting, it is argued that superalloys with a positive γ/γ′ lattice misfit should exhibit better high-temperature strength properties than otherwise similar superalloys with a negative misfit. In this context, the new class of γ′-hardened Co-base superalloys is of special interest. These alloys have promising high-temperature strength properties and possess a positive lattice mismatch. It is suggested that this positive mismatch is one of the beneficial factors that contribute to the promising properties of these alloys and that this aspect should be explored further.