Precipitate phase stability and compositional dependence on alloying additions in γ–γ′–δ–η Ni-base superalloys

Antonov, Stoichko; Detrois, Martin; Helmink, Randolph C.; Tin, Sammy

doi:10.1016/j.jallcom.2014.11.155

Cited by 90 publications

(15 citation statements)

References 27 publications

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“…M 23 C 6 carbides are usually formed from MC carbides decomposing and releasing C atoms, which react with the Cr atoms in the γ matrix and form M 23 C 6 precipitates surrounded by the γ′ film [35]. It has been found that, as Ni atoms diffuse more effortlessly through the γ′ film towards the M 23 Ti to form η phase at the interface between the M 23 C 6 and MC carbides [36,37]. As shown in Figure 6, MC decomposition into M 23 C 6 for both alloys is predicted to occur around 650°C and so M 23 C 6 is not expected to be present at 800ºC, which is consistent with the experimental observations.…”

Section: Discussionmentioning

confidence: 99%

“…Historically, the Al/Ti ratio has been carefully controlled in Ni-base superalloys to maintain the phase stability and avoid η precipitation [22]. In addition to the formation of the η phase, excessive alloying concentrations of Nb, Ta and Ti can promote other secondary phases such as the D0 a structured δ (Ni 3 Nb), which along with η (Ni 6 AlNb), can be detrimental to the forging properties of the alloys if present in excess [23][24][25][26]. Alloying additions of Nb, Ta and Ti have also shown to retard MC carbide decomposition into M 23 C 6 as they tend to stabilize primary MC carbides.…”

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confidence: 99%

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σ and η Phase formation in advanced polycrystalline Ni-base superalloys

Antonov

Huo

Feng

et al. 2017

Materials Science and Engineering: A

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In polycrystalline Ni-base superalloys, grain boundary precipitation of secondary phases can be significant due to the effects they pose on the mechanical properties. As new alloying concepts for polycrystalline Ni-base superalloys are being developed to extend their temperature capability, the effect of increasing levels of Nb alloying additions on long term phase stability and the formation of topologically close packed (TCP) phases needs to be studied. Elevated levels of Nb can result in increased matrix supersaturation and promote the precipitation of secondary phases. Long term thermal exposures on two experimental powder processed Ni-base superalloys containing various levels of Nb were completed to assess the stability and precipitation of TCP phases. It was found that additions of Nb promoted the precipitation of η-Ni 6 AlNb along the grain boundaries in powder processed, polycrystalline Ni-base superalloys, while reduced Nb levels favored the precipitation of blocky Cr and Morich  phase precipitates along the grain boundary. Evaluation of the thermodynamic stability of these two phases in both alloys using Thermo-calc showed that while  phase predictions are fairly accurate, predictions of the η phase are limited. IntroductionHigh strength along with considerable fatigue resistance when exposed to corrosive environments at elevated temperatures are the properties that make nickel-based superalloys the material of choice for use in the "hot section" of modern gas turbine engines and power generation applications [1-9]. The unique microstructure consisting of the ordered L1 2 intermetallic precipitates γ′ (Ni 3 Al) distributed coherently within a disordered FCC A1 matrix γ (Ni), provides substantial order and coherency type strengthening by restricting dislocation motion during plastic deformation. Hence, much of the superior high temperature properties of Ni-base superalloys can be attributed to microstructural strengthening. Additions of refractory alloying elements, such as W, Mo, Cr and Co, in modern polycrystalline Ni-base superalloys, provide strength to the γ matrix phase through solid solution strengthening. Additionally, concentrations of γ formers Al, Ti, Nb and Ta are utilized to produce an optimized volume fraction of the γ strengthening phase. Although modest improvements in the properties of the alloy can be attained by increasing the volume fraction of the γ′ precipitates, these changes are often accompanied by debits in processing capabilities, environmental resistance and overall alloy cost. Additions of Nb, Ta and Ti at relatively low alloying concentrations will preferentially partition to the γ′ phase and serve to increase the anti-phase boundary energy as well as to strengthen the precipitate′s resistance to deformation [10]. When present at elevated concentrations, residual levels of γ′ forming solutes may remain within the γ phase and subsequently serve as potent solid solution strengtheners [11][12][13][14].

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Section: Discussionmentioning

confidence: 99%

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confidence: 99%

σ and η Phase formation in advanced polycrystalline Ni-base superalloys

Antonov

Huo

Feng

et al. 2017

Materials Science and Engineering: A

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“…Control of the characteristic partitioning behavior in this class of alloys is particularly important as this will contribute to the formation of secondary precipitate phases such as δ and η [16]. Moreover, elements such as Nb, Ta, and Ti may potentially partition to the γ and/or γ′ phase in varying degrees resulting in different levels of strengthening.…”

Section: Discussionmentioning

confidence: 99%

“…This may potentially be used to enhance the strength and temperature capability of the alloy, but excessive alloying with Nb, Ta and Ti can promote the formation of undesirable secondary phases such as δ (Ni 3 Nb) and/or η (Ni 6 AlNb) within the microstructure when the solubility limits are exceeded [13][14][15]. Although the effect of individual alloying additions on δ and/ or η phase precipitate formation has been previously investigated [16,17], the characteristic partitioning behavior of the elements between the γ or γ′ phases in high Nb content Ni-base superalloys is not well understood. In order to develop high strength polycrystalline Ni-base superalloys with improved temperature capabilities, a fundamental understanding of the alloying solubility limits and equilibrium phase compositions is required.…”

Section: Introductionmentioning

confidence: 99%

Comparison of thermodynamic database models and APT data for strength modeling in high Nb content γ–γ′ Ni-base superalloys

et al. 2015

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In response to the increasing need for higher operating temperatures in advanced gas turbine engines, new alloying concepts are required to develop novel nickel-base superalloys with enhanced temperature capabilities. Recent studies have shown that polycrystalline Ni-base superalloys containing elevated levels of Nb additions exhibit superior properties at elevated temperatures when compared to existing commercial Ni-base superalloys. In order to design, develop and fully exploit this innovative class of superalloys, an understanding of how alloying elements partition to each phase is essential. Using atom probe tomography (APT), compositions of the constituent phases were measured in four high Nb content γ-γ′ Ni-base superalloys and the results were compared to thermodynamic database models from Thermo-Calc. Results were also used in predicting the solid solution strength behavior of the four alloys. The differences in phase composition predictions from thermodynamic models resulted in dissimilarities between the generated strength behavior curves and those from the experimental work.

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“…It is especially crucial to control the evolution of the different phases at high temperature in order to tailor the mechanical properties. [6][7][8][9] The aim of increasing the operation temperature of the Ni-based superalloys, which allows engine manufacturers to improve fuel efficiency and, thus, results in reduced emissions, forces the development of materials stable at higher temperatures. The last few decades the scientific community has put great effort into the development of Ni-based alloys with operation temperatures above 650°C while keeping the good processing C. SOLI´S, J. MUNKE, and R. GILLES are with the Heinz MaierLeibnitz Zentrum (MLZ), TU Mu¨nchen, Lichtenbergstr.…”

Section: Introductionmentioning

confidence: 99%

In Situ Characterization at Elevated Temperatures of a New Ni-Based Superalloy VDM-780 Premium

Solís

Munke

Bergner

et al. 2018

Metall Mater Trans A

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A new Ni-base superalloy VDM-780 Premium was developed for higher service temperatures (above 650°C), keeping the good processing characteristics of alloy 718. This article presents, for the first time, the morphology and the microstructure characterization of this newly developed superalloy VDM-780 by means of scanning electron microscopy (SEM) and neutron and X-ray diffraction (XRD), after three different aging treatments performed for setting up different microstructures. Results show the presence of the c-matrix, c¢-hardening phase, and a high-temperature phase whose structure is compatible with d and g phases but whose exact crystal structure or the possibility of two different high-temperature phases still remains open. Rietveld refinements have allowed phase identification, determination of the lattice constants, and the weight fractions of constituting phases and shown that the presence of the different phases, amount, and morphology highly depend on the aging treatments. No traces of the c¢¢ phase are observed regardless of the heat treatment. In situ neutron diffraction (ND) studies at high temperature have allowed the determination of the solvus temperatures of the different phases present in each material after the corresponding aging treatment as well as the study of the evolution of their lattice parameters with temperature. The Vickers hardness (HV) of the three different samples was measured, and the results are correlated with the amount and particle size of the c¢-hardening phase of each sample.

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Precipitate phase stability and compositional dependence on alloying additions in γ–γ′–δ–η Ni-base superalloys

Cited by 90 publications

References 27 publications

σ and η Phase formation in advanced polycrystalline Ni-base superalloys

σ and η Phase formation in advanced polycrystalline Ni-base superalloys

Comparison of thermodynamic database models and APT data for strength modeling in high Nb content γ–γ′ Ni-base superalloys

In Situ Characterization at Elevated Temperatures of a New Ni-Based Superalloy VDM-780 Premium

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