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Abstract

Nickel-base superalloys are the material of choice for many high temperature applications due to their high creep resistance at elevated temperatures. The origin of the high creep resistance is the unique microstructure of Nickel-base superalloys. It consists of the γ-phase, which forms the matrix, and cuboidal L1₂-ordered γ'-precipitates. In most technologically relevant Nickelbase superalloys, the volume fraction of the γ'-precipitates is well beyond 50%. Only thin γ-phase channels separate the γ'-particles, leading to the matrix phase being the minority phase. The γ'-precipitates have a significantly higher yield strength than the matrix γ-phase and thus almost impenetrable for the dislocations unless certain dislocation or stress configurations enable their cutting events [3]. By inhibiting dislocation movement they significantly impede creep deformation.

On the other hand, at high temperatures even without an applied load the microstructure of Nickel-base superalloys is subject to changes. With increasing time of high temperature exposure the γ'-precipitates lose their cuboidal shape and are able to coalesce (Fig. 1 a) and b)). This process finally leads to a topological inversion of the microstructure with the γ'-phase becoming topologically the matrix phase surrounding the γ-phase particles [2]. This process is related to the formation of equilibrium interfacial dislocation network in the = γ/γ'-interfaces and thus the gradual loss of coherency of these interfaces [1]. In this study the role of dislocation network formation regarding stability of the characteristic superalloy’s microstructure is investigated. Then, the findings of this investigation are used to simulate the process of long-term aging using the multi-phase-field method [5].

Though during long-term aging the γ'-particles eventually coalesce and the microstructure is topologically inverted, the γ-channels stay stable for a long time. This stabilization mechanism as well as the destabilization during long-term aging is investigated using a micromechanical model based on stress fields of individual dislocations. An energy barrier preventing merging of γ'-precipitates in the coherent state of the = γ/γ'-interfaces is discovered, which is significantly lowered when the interfacial dislocation network is introduced.

During long-term aging, by creation of equilibrium interfacial dislocation networks around the γ'-precipitates in addition to the lowering of the energy barrier investigated before, the lattice misfit between the γ'- and γ-phases is accommodated leading to the loss of cuboidal shape of γ'-particles [1]. While driven by surface energy minimization, this phenomenon enables coarsening of γ'-precipitates. The results of mesoscopic simulations of these phenomena in comparison with experimental observations are shown in Fig. 1.

Acknowledgement. The authors acknowledge funding by the Deutsche Forschungsgemeinschaft (DFG) through the SFB/ Transregio 103 Superalloy Single Crystal projects B4, C4 and C5 as well as by the Korea Institute of Material Science (KIMS).

Figure 1: Long-term aging of the single-crystal NI-base superalloy ERBO-1 at 1100C for up to 250h (a, b). Phase-field simulation of long-term aging: starting microstructure c), aged microstructure d).

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[4] OpenPhase, www.openphase.de, 2014.

[5] I. Steinbach, Phase-Field Model for Microstructure Evolution at the Mesoscale. Annu. Rev. Mater. Res. 43:89-107, 2013.