Effect of ruthenium on the precipitation of topologically close packed phases in Ni-based superalloys of 3rd and 4th generation

Published on Aug 15, 2015in Acta Materialia7.656
· DOI :10.1016/J.ACTAMAT.2015.05.033
Kamil Matuszewski6
Estimated H-index: 6
(FAU: University of Erlangen-Nuremberg),
Ralf Rettig15
Estimated H-index: 15
(FAU: University of Erlangen-Nuremberg)
+ 8 AuthorsRobert F. Singer53
Estimated H-index: 53
(FAU: University of Erlangen-Nuremberg)
Sources
Abstract
Abstract The precipitation of topologically close packed (TCP) phases is detrimental for the high temperature strength of high refractory Ni-based superalloys. The beneficial influence of Ru with respect to this so called instability is nowadays well accepted. In the present paper the precipitation of topologically close packed (TCP) phases is studied quantitatively in two experimental alloys (one Ru-free and one with addition of Ru) to clarify the mechanism of the Ru effect. It is confirmed that the TCP phase precipitates undergo sequential phase transformation with the tetragonal σ-phase precipitating first. Ru retards the phase transformation and leads to decreased equilibrium volume fraction of TCP phases. The results clearly indicate that Ru decreases the driving force for TCP phase precipitation. Investigations of crystallography and chemistry of the TCP/matrix interface point to an additional effect by increase of misfit strain energy.
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Two Ni-based superalloys (one Ru-free and one containing 1 at.% Ru) were comparatively studied by Atom Probe Tomography and complimentary techniques. Ru addition impedes precipitation of the σ phase at 950 °C. Ru partitions nearly equally to the γ and σ phase. Neither reverse elemental partitioning nor destabilization of the γ′ phase is detected when adding Ru. We propose an increase in the γ/σ lattice misfit caused by Ru as a major reason for impeded nucleation of the σ phase.
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A new approach to clarify the ruthenium effect on the precipitation of topologically close packed (TCP) phases is described in the paper. It is based on focused ion beam – scanning electron microscopy (FIB – SEM) dual beam methodology as well as three-dimensional imaging. The high-temperature capabilities of nickel base superalloys can be improved by alloying with refractory elements. With excessive refractory element content or excessive exposure to high temperature, brittle TCP phases precipit...
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The suppression of topologically close-packed phases (TCP) in nickel-based single-crystal superalloys by the addition of ruthenium is modeled with a fully multicomponent mesoscale precipitation model based on the numerical Kampmann-Wagnermethod using thermodynamic CALPHAD-calculations. For the application of the model in ruthenium-containing superalloys, mobility assessments of the binary systems nickel-ruthenium (NiRu) and nickel-rhenium (Ni-Re) were performed to create a new mobility database....
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Ruthenium (Ru) suppresses the precipitation of deleterious topologically close-packed (TCP) phases in high refractory content single-crystal Ni-base superalloys. The effectiveness of Ru as a TCP suppressant appears to be the net effect of its limited solubility in the TCP phase, a lower density of structural growth ledges for atomic attachment at the TCP/matrix interface, and destabilization of the γ′ phase at elevated temperatures. These characteristics combine to limit the growth rates of prec...
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