Characterization and Modeling of Deformation Mechanisms in Ni‐Base Superalloy 718

McAllister, Donald; Lv, Duchao; Deutchman, H.; Peterson, Benjamin; Wang, Y.; Mills, Michael J.

doi:10.1002/9781119075646.ch88

Cited by 7 publications

(10 citation statements)

References 17 publications

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“…For example, the planar fault traversing from the bottom left to the top right of the micrograph shows isolated faulted coprecipitates. Isolated faults have been frequently reported for air-cooled IN718 containing non-compact coprecipitates, deformed under similar conditions [19][20][21][22][23]. Further, across this micrograph, several dislocation loop/debris like features are visible.…”

Section: Discussionsupporting

confidence: 68%

Creep Behavior of Compact γ′-γ″ Coprecipitation Strengthened IN718-Variant Superalloy

Mukopadhyay

Sriram

Zenk

et al. 2021

Metals

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The development of high-temperature heavy-duty turbine disk materials is critical for improving the overall efficiency of combined cycle power plants. An alloy development strategy to this end involves superalloys strengthened by ‘compact’ γ′-γ″ coprecipitates. Compact morphology of coprecipitates consists of a cuboidal γ′ precipitate such that γ″ discs coat its six {001} faces. The present work is an attempt to investigate the microstructure and creep behavior of a fully aged alloy exhibiting compact coprecipitates. We conducted heat treatments, detailed microstructural characterization, and creep testing at 1200 °F (649 °C) on an IN718-variant alloy. Our results indicate that aged IN718-27 samples exhibit a relatively uniform distribution of compact coprecipitates, irrespective of the cooling rate. However, the alloy ruptured at low strains during creep tests at 1200 °F (649 °C). At 100 ksi (689 MPa) load, the alloy fails around 0.1% strain, and 75 ksi (517 MPa) loading causes rupture at 0.3% strain. We also report extensive intergranular failure in all the tested samples, which is attributed to cracking along grain boundary precipitates. The results suggest that while the compact coprecipitates are indeed thermally stable during thermomechanical processing, the microstructure of the alloy needs to be optimized for better creep strength and rupture life.

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Section: Discussionsupporting

confidence: 68%

Creep Behavior of Compact γ′-γ″ Coprecipitation Strengthened IN718-Variant Superalloy

Mukopadhyay

Sriram

Zenk

et al. 2021

Metals

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“…Grain boundaries are decorated with δ-phase platelets providing Zener-Smith pinning to improve creep resistance [9][10][11]. Nanoscale, long-range ordered γ'-(Ni3(Al,Ti) L12) and γ"-(Ni3(Nb) D022) precipitates contribute to the high-temperature strength via coherency and anti-phase boundary (APB) strengthening [12,13]. Duplet and triplet co-precipitate morphologies have been widely reported for Alloy 718 [14][15][16].…”

Section: Introductionmentioning

confidence: 99%

On the early stages of precipitation during direct ageing of Alloy 718

Theska

Nomoto

Godor³

et al. 2020

Acta Materialia

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The Ni-based superalloy Alloy 718 is used in aircraft engines as high-pressure turbine discs and must endure challenging demands on high-temperature yield strength, creep-, and oxidation-resistance. Nanoscale γ'-and γ"-precipitates commonly found in duplet and triplet co-precipitate morphologies provide high-temperature strength under these harsh operating conditions. 'Direct ageing' of Alloy 718 is an attractive alternative manufacturing route known to increase the yield strength at 650 °C by at least +10%, by both retaining high dislocation densities and changing the nanoscale co-precipitate morphology. However, the detailed nucleation and growth mechanisms of the duplet and triplet co-precipitate morphologies of γ' and γ" during the direct ageing process remain unknown. We provide a correlative high-resolution microscopy approach using transmission electron microscopy, high-angle annular dark-field imaging, and atom probe microscopy to reveal the early stages of precipitation during direct ageing of Alloy 718. Quantitative stereological analyses of the γ'-and γ"-precipitate dispersions as well as their chemical compositions have allowed us to propose a model for the microstructural evolution. It is shown that fine γ'-and γ"-precipitates nucleate homogeneously and coarsen according to the Lifshitz-Slyozov-Wagner-theory. However, γ"-precipitates also nucleate heterogeneously on dislocations and experience 3 accelerated growth due to Nb pipe diffusion. Moreover, the co-precipitation reactions are largely influenced by solute availability and the potential for enrichment of Nb and rejection of Al+Ti.

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“…Co-precipitation of γ′ and γ″ has been previously confirmed via TEM in SA Alloy 718 (Cozar & Pineau, 1973; Miller et al, 1999). More recent TEM studies also provide evidence for co-existing monolithic, duplet, and triplet variants in Alloy 718 (McAllister et al, 2016; Detor et al, 2018). This finding is similar to the results presented in Figures 4a and 4b, but may be limited in resolution (McAllister et al, 2016).…”

Section: Discussionmentioning

confidence: 93%

“…More recent TEM studies also provide evidence for co-existing monolithic, duplet, and triplet variants in Alloy 718 (McAllister et al, 2016; Detor et al, 2018). This finding is similar to the results presented in Figures 4a and 4b, but may be limited in resolution (McAllister et al, 2016). Additionally, Detor et al proposed a dependency between the thermo-mechanical treatment, chemical composition, and resulting particle morphology (Detor et al, 2018).…”

Section: Discussionmentioning

confidence: 93%

“…Nb, Al, and Ti in Alloy 718 promote the precipitation of both γ′ and γ″ particles, commonly occurring as duplets and triplets (Miller et al, 1999; Geng et al, 2007; Detor et al, 2018; Drexler et al, 2018). The stacking sequence of such triplets is of particular interest, as the corresponding interfaces created provide significant strengthening (McAllister et al, 2016; Detor et al, 2018). Therefore, detailed studies require not only the reproducible determination of the size, volume fraction and number densities of these nanoscale particles, but also a quantification of precipitate/matrix and precipitate/precipitate interfaces.…”

Section: Introductionmentioning

confidence: 99%

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Atom Probe Microscopy of Strengthening Effects in Alloy 718

2019

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Polycrystalline Ni-based superalloys for aerospace and power generation applications are often precipitation hardened to achieve strengthening at elevated temperatures. Here, atom probe microscopy has become an essential tool to study the complex morphology of nanoscale precipitates. This study focuses on Alloy 718, which is hardened by semi-coherent, ordered γ′ (Ni3(Al, Ti)) and γ″ (Ni3(Nb)) particles. According to previous research, these particles often occur as duplets or triplets with a stacking sequence dependent on prior processing. This creates various interfaces with a strong impact on the mechanical properties, highlighting the importance of quantitative studies which are challenging with electron microscopy. We present atom probe data reconstruction and analysis approaches particularly suited for precipitation hardened superalloys. While voltage atom probe allows for an accurate reconstruction, the acquired data volume is often limited. Laser-assisted atom probe provides statistically significant data, but the loss of crystallographic information requires correlation with voltage-mode datasets. We further describe an advanced iso-surface method where initially arbitrarily chosen concentration thresholds of Al + Ti for γ′ and Nb for γ″ particles are optimized. Recognizing the importance of the precipitate stacking order, the different types of precipitate interfaces are quantified, and these methods may be applicable to other engineering alloys.

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Characterization and Modeling of Deformation Mechanisms in Ni‐Base Superalloy 718

Cited by 7 publications

References 17 publications

Creep Behavior of Compact γ′-γ″ Coprecipitation Strengthened IN718-Variant Superalloy

Creep Behavior of Compact γ′-γ″ Coprecipitation Strengthened IN718-Variant Superalloy

On the early stages of precipitation during direct ageing of Alloy 718

Atom Probe Microscopy of Strengthening Effects in Alloy 718

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