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The cyclic oxidation behavior of fourth-generation nickel-based single crystal superalloys with 2–4 wt% Mo content were studied at 900 ℃ and 1100 ℃. At 900 ℃, with the increase of Mo content, the oxide layer accumulates more growth stress, resulting in the interface between the outer oxide layer and the inner oxide layer more prone to crack initiation. At 1100 ℃, the oxidation layer accumulates more growth stress and thermal stress, so the oxidation layer occurs large scale spalling, which is more severe in 4Mo alloy. The results show that the increase of Mo content of alloy promotes the oxidation of containing W-Re alloy, which promotes the volatilization of oxides and the spalling of oxide, decreases the protection of oxide layer, and deteriorates the oxidation resistance of the alloy. • The increase of Mo content of alloy promotes the oxidation of containing W-Re alloy. • With the increase of Mo content, the volatilization of oxides and the spalling of oxide of alloy was promoted. • Mo has more severe harm to the oxidation resistance of the alloy at higher temperature. • The alloy with 2 wt% Mo addition has the highest oxidation resistance. [ABSTRACT FROM AUTHOR]

The chemical and structural evolution within dislocation networks are investigated in a nickel-based single crystal superalloy crept under 1373K and 137MPa. The local region within dislocation networks can be divided to two separated parts: gridlines which compose the arrangements of dislocation networks and mesh regions as intervals between gridlines. With evolution of dislocation networks, precipitation of nano-sized γ′ phase (γ′ n) are observed in mesh region, while gridlines always display the FCC feature of γ phase. The formative mechanism of γ′ n is revealed as increased segregations of Cr, Co and Re along gridlines which enhance the contents of γ′-rich elements such as Al in mesh region to promote the local phase transition from FCC to highly ordered L1 2. However, growth of γ′ n requires massive diffusion of γ′-rich elements that is confined by the chemical barrier along gridlines with strong segregations of Cr, Co and Re. The mesh region of γ′ n and gridlines of γ reinforce each other forming potential γ/γ′ substructures to stabilize the dislocation networks during creep. Denser dislocation networks formed in dendrite core under larger local lattice misfit are accompanied with facilitated formation of γ/γ′ substructures which derive higher creep resistance than interdendritic region. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Nickel-based single crystal superalloys (Ni-SXs) are of great interests in aircraft industry due to their excellent high temperature creep strength deriving from the rigid gamma/gamma prime (γ/γ′) microstructure and the single crystal nature. The creep properties of Ni-SXs depend strongly on the degradation rate of the original rigid γ/γ′ microstructure with the dislocation movements in the microstructure. Since the applied temperature and stress strongly affect the dislocation movements, the different creep mechanisms are displayed to Ni-SXs during low temperature (∼750 °C), mid-temperature (∼950 °C) and high temperature (∼1100 °C) creep. The creep deformation during low temperature creep is mainly caused by the shearing of γ′ phase by a limited number of dislocations, while high temperature creep mainly occurs to the accumulation of creep strains with the increased dislocation activities in γ phase. To enhance the creep properties of alloy, a variety of strategies of the microstructural optimization need to be considered. One is to increase the volume fraction of γ′ phase to enhance the precipitation strengthening effects. But excessive amounts of γ′ phase are disadvantageous to low temperature strength of alloy. The coarsening of γ′ phase and the rafting process strongly correlates to dislocation movements. It is of importance to discuss the specific mechanisms of coarsening and rafting as well as their influence to the creep properties of alloy. The lattice misfit between the γ and γ′ phase plays an important role in influencing the coarsening of γ′ phase and the creep properties of alloy. A proper lattice misfit helps to form the cubic γ′ phase which is advantageous to alloy. But a too large lattice misfit can promote the coarsening of γ′ phase that damages the structural stability of alloy at high temperatures. The increased additions of refractory elements in alloy promote the formation of topologically close-packed (TCP) phase. TCP phase facilitates the nucleation and propagation of micro-cracks around it that damages the creep properties of alloy. This review aims to summarize some aspects of microstructural evolution during creep of alloy with considering their effects to creep properties. To guide the design of Ni-SXs in future, some perspectives about the microstructural optimization are properly provided. • The creep properties of Nickel-based superalloys are greatly decided by the microstructural evolution during creep. • The creep mechanisms are determined by the intrinsic dislocation activities occurred in the microstructure. • The coarsening of γ′ phase is emphasized, especially the rafting process. • Lattice misfit strongly affects the formation of dislocation networks and coarsening of γ′ phase. • The formation of TCP phase which is detrimental to creep properties need to be strictly restricted in superalloy design. [ABSTRACT FROM AUTHOR]