Electron Microscopic Investigations And First-principles Calculations Of Interfaces In Ni-based Single Crystal Superalloys

The superior high-temperature properties of Ni-based single crystal superalloys make them the best candidates for use in jet engines and for the blades in aerospace turbine engines. To improve the high-temperature strength, solid solution strengthening elements have been added to create advanced-generation superalloys. However, when Ni-based superalloys are composed of excessive amounts of such refractory elements, instabilities are promoted. These instabilities can lead to the formation of deleterious topologically close-packed (TCP) phases as a result of the depletion of the solid solution strengthening elements when the superalloy is exposed to elevated temperatures. Furthermore, these TCP phases can serve as damage accumulation sites that initiate or accelerate crack propagation, which ultimately cause the creep strength of the material to decrease. The platinum group element Ru plays as a symbol element in the fourth-and fifth-generation, Ni-based single crystal superalloys. It is known that the fundamental mechanisms by which Ru additions are able to suppress the formation of TCP phases and improve the high-temperature phase stability of Ni-based superalloys are still controversial. There have been only a few detailed reports describing the influence of Ru on the TCP/matrix atomic interfacial structure in single-crystal Ni-based superalloys. In this study, the γ/γ’ interfacial structure at the atomic scale and related distribution characteristics of alloying elements have been investigated by high-resolution transmission electron microscopy. The influence of Ru on the TCP/matrix interfacial structure and the related redistribution behaviors of alloying elements have been analyzed. In addition, the site preference and partition behaviors of alloying elements have also been investigated by the first principle calculation.(1) The experimental results of the investigated superalloys after creep rupture test at1100℃/137MPa indicate that the distribution behaviors of alloying elements at both sides of the γ/γ’ interface are obvious:Ni, Al, Ta and W mainly partition into the γ’ phase, while Cr, Co and Re mainly partition into the y phase. Local clustering of Re (clusters～2-3nm in size) appears at the apexes of the kink structure. During creep, the formation of the kink structure results from the interactions of heavy atoms clusters and the core structure of the interfacial dislocation. An atomic structural model of the coherent γ/γ’interface includes a step. During high temperature low stress creep of single crystal superalloys, y’rafting occurs and the γ/γ’ interfacial dislocation networks display six-fold and four-fold symmetry.(2) The study of microstructural stability during long-term thermal exposure at1100℃/800h indicates that the TCP phase in Ru-free alloy has the typical long, slender needles shaped morphology and the corresponding TCP/matrix interfacial structure exhibits irregular step-shaped with overlapping minor and major steps. However, with the addition of Ru, the morphology of TCP phases changes to short, board-plate-shaped, and the corresponding TCP/matrix interfacial structure represents regular two-atomic-layer step shape. The addition of Ru changes the distribution behaviors of alloying elements between the matrix and TCP phases, suppresses the precipitation of TCP phases and changes TCP morphology as well as the related TCP/matrix interfacial structure.(3) The influence of Ru on the site preference of alloying elements at y/y’ interface and γ’ phase has been investigated using the first-principles method based on the density functional theory. The strength mechanism between Ru and Re have been clarified that there is a strong interaction between Re atom and Ru atom through p-p hybridization.(4) The micro-mechanisms of Re atoms preferentially partitioning to TCP phase has been investigated using the first-principles method. Re is prone to occupy the W site in a less close-packed plane of the σ phase due to the bonding characteristics and interstitial spaces in the crystal structure of σ phase.