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The important factors limiting the fourth-generation Ni-based single-crystal superalloys (Ni-SXs) are the increases in cost and density. A cheaper and lighter potential alternative element of Ru, Co's comprehensive effects were studied. The effects of Co and inhomogeneity between dendrite core (DC) and interdendritic region (ID) on creep performance at 1100 °C and 137 MPa in a 4th generation Ni-SXs were systematically investigated. The results show that the Ni-SXs with 9 wt% Co has the longest creep lifetime. Co additions alleviate the overall element segregations between DC and ID, especially Re, Cr, and W. And reverse partitioning behavior of Co also changes the partitioning ratio of elements between γ and γ′ phases, resulting in the inhibition of TCP phases precipitation for most creep time to avoid excessive consumption of solid solution strengthening elements and local stress concentration. The decrease of the lattice constant of γ reduces lattice misfit between γ and γ′ with the addition of Co, leading to the enlargement of dislocation network spacing and less resistance for movement or shearing of dislocation. The decrease of γ′ solvus temperature with Co addition promotes the dissolution of γ′ at high working temperature, resulting in severe topological inversion of γ/γ′ microstructures at ID. In addition, the main creep cracks of all Ni-SXs appear at ID, so the inhomogeneity between DC and ID is concerned. • The precipitation of TCP phases are inhibited with Co addition, which is a basis of low-cost and low-density alloy design. • The comprehensive effects of Co on high temperature creep properties are investigated and an optimal Co content exists. • The increase of Co affected the γ′ phase content, lattice mismatch, element distribution and TCP phase. • The effect of Co on microstructures evolution varies at different creep stages. • The inhomogeneity between dendrite core and interdendritic region causes creep cracks originate from interdendritic region. [ABSTRACT FROM AUTHOR]

To model the centrifugal loads and complex geometries experienced by turbine blades during service, the nickel-based single crystal superalloy DD6 samples with a groove structure are designed and tested under the coupling conditions of high-speed rotating of 20,000 r/min and high temperature of 980 ℃. Scanning electron microscopy and transmission electron microscopy are used to analyze the microstructural evolution of γ-γ' phases and dislocation morphologies of 400 h and 1000 h samples. The results indicate that microstructural degradation of the γ-γ' phase attributes to the larger centrifugal tensile stress S 11 in the applied physical stress compositions until deformation occurs. Our observations suggest that a/2 < 101 > -type dislocations initiate plastic deformation by slipping within the γ phase. However, their movement is constrained at the interface between the γ and γ' phases, resulting in their tendency to aggregate at this boundary. After the formation of stable interfacial dislocation networks, these dislocations then react to generate either a< 010 > -type dislocations or a/2 < 112 > -type dislocations, which cut into the γ' phase. However, under high stress conditions, the a/2 < 101 > -type dislocations are primarily cut directly in the form of dislocation pairs. • The γ-γ' phase degradation and dislocation evolution of DD6 superalloy was investigated. • Creep tests are conducted under high-speed rotating and high temperature condition. • Microstructural degradation of the γ-γ' phase attributes to the larger centrifugal tensile stress S 11. • Interfacial dislocation networks and superdislocations accelerate the separation and disappearance of the γ phase. [ABSTRACT FROM AUTHOR]

Amorphous Al 2 O 3 coating was prepared on Ni-based single crystal superalloy via atomic layer deposition (ALD) for high temperature oxidation protection. The oxygen inward transport and outward-cation diffusion were hindered by amorphous Al 2 O 3 coating, and the coated alloy showed high oxidation resistance. The low oxygen partial pressure at the alloy/coating interface altered the alloy oxidation behavior, allowing the priority oxidation of protective Al and Cr elements. The oxidation process was controlled by the growth of Cr 2 O 3 and Al 2 O 3. The coating exhibited high spallation resistance. Degradation of the coating, the formation of NiAl 2 O 4 phase in amorphous Al 2 O 3 layer, was observed. • Amorphous Al 2 O 3 coating was proposed and prepared on Ni-based single crystal superalloy. • The amorphous Al 2 O 3 coating improved oxidation resistance and exhibited high adhesion. • The amorphous Al 2 O 3 coating altered the oxidation behavior of the alloy. • The effect mechanism of amorphous Al 2 O 3 coating on the alloy oxidation was discussed. [ABSTRACT FROM AUTHOR]

Creep properties of a 4th generation nickel-based single crystal superalloy (Ni-SX) are investigated at 1150 °C and 1100 °C. Fracture mechanism is revealed as competitive microcrack initiation induced by topologically close-packed (TCP) phase formation and creep inconsistency on dendritic structures showing the strong temperature-and-time dependent relations. At 1150 °C, alloy displays the high nucleation rate of TCP phases. Dense TCP particles dissolve the surrounding γ/γ' microstructures forming large depleted zone where is easily stress-concentrated. Dislocations are found to emit from the interface between TCP and depleted zone that promotes dislocation shearing and microcrack initiation in the large depleted zone, as well as the fracture of alloy. On contrary, alloy crept at 1100 °C suffers to relatively restricted TCP nucleation. But the longer creep time gives to larger size of TCP precipitates. Compared to alloy crept at 1150 °C, the depleted zone is much smaller that is not prone for stress concentration. For this creep inconsistency induced by dendritic segregation plays the main role in determining the creep properties. The larger local strain rate forms microcracks primarily in interdendritic region causing the final fracture. • Creep behaviors of a new 4th generation Ni-SX are compared at 1150 °C and 1100 °C. • Temperature-and-time dependent TCP precipitation and creep inconsistency affect the creep. • Depending on temperature, the two factors display competitive behaviors. • Higher temperature of 1150 °C enlarges the influence of TCP precipitation. • Creep inconsistency on dendritic structures dominants the creep at 1100 °C. [ABSTRACT FROM AUTHOR]