Grain boundary precipitation in Inconel 718 and ATI 718Plus

Hassan, Bilal; Corney, Jonathan

doi:10.1080/02670836.2017.1333222

Cited by 58 publications

(16 citation statements)

References 77 publications

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“…The precipitates exhibiting a darker contrast than the background are γ -γ coprecipitates (referred as coprecipitates from here on). On the other hand, the brighter precipitates near grain boundaries are presumably δ precipitates, which is consistent with several reports on conventionally processed (air-cooled) IN718 [12][13][14]. The coprecipitates appear to be uniformly distributed within the grains.…”

Section: Aged Microstructural Characterizationsupporting

confidence: 91%

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: Aged Microstructural Characterizationsupporting

confidence: 91%

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

Mukopadhyay

Sriram

Zenk

et al. 2021

Metals

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“…Clearly, these microstructures account for the prominent hardness increase. In contrast, Figure 4b shows prominent precipitates within the grain boundaries and the intra-grain regions as well, but these precipitates are notably different from those in Figure 4a, and are likely to be MC carbides, since all other precipitate phases (gamma prime, gamma double-prime, delta, and other carbides) have solvus temperatures below the 1163 • C HIP temperature the specimens in Figure 4b were subjected to (Table 1) [18][19][20].…”

Section: Grain Size Measurementsmentioning

confidence: 90%

Performance Characterization of Laser Powder Bed Fusion Fabricated Inconel 718 Treated with Experimental Hot Isostatic Processing Cycles

Varela

Merino

Pickett

et al. 2020

JMMP

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Inconel 718 alloy fabricated by selective laser melting (SLM) (or laser powder-bed fusion (LPBF)) has been post-process heat-treated by stress-relief anneal at 1065 °C; stress-relief anneal (1065 °C) + solution treatment (at 720 °C) + aging (at 620 °C); hot isostatic pressing (HIP) (at 1120–1200 °C); stress-relief anneal + HIP; and stress-relief anneal + HIP + solution treatment + aging. Microstructure analysis utilizing optical metallography revealed primarily equiaxed grain structures (having average diameters ranging from ~30 to 49 microns) containing annealing twins, and a high concentration of carbide precipitates in all HIP-related treatments in the grain boundaries and intragrain regions. However, no precipitates nucleated on the {111} coherent annealing twin boundaries because of their very low interfacial free energy in contrast to regular grain boundaries. The mechanical properties for the as-fabricated Inconel 718 exhibited a yield stress of 0.64 GPa, UTS of 0.98 GPa, and elongation of 26%. Following stress-relief anneal at 1065 °C, the yield stress dropped to 0.60 GPa, while the elongation increased to 43%. The associated grain structure was an irregular, somewhat elongated, recrystallized structure. This structure was preserved at a stress anneal at 1065 °C + solution treatment + aging, but grain boundary and intragrain precipitation resulted in a doubling of the yield stress to 1.3 GPa and a reduced elongation of 12.6%. The results of HIP-related post-process heat treatments involving temperatures above 1060 °C demonstrated that the yield stress and elongations could be varied from 1.07 to 1.17 GPa and 11.4% to 19%, respectively. Corresponding Rockwell C-scale hardness values also varied from 33 for the as-fabricated Inconel 718 to 53 for simple post-process HIP treatment at 1163 °C.

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“…Z-phase particles are generally massive and are typically formed on the boundaries in high-Cr martensitic steel with high strength (Suzuki et al, 2000;Kimura et al, 2002;Sawada et al, 2006). Models for the coarsening of grain boundary precipitates are constructed by combining lattice diffusion and boundary diffusion (Hori & Saji, 1981;Hassan & Corney, 2017). That is, solute atoms are transported onto the boundaries by lattice diffusion and then migrate to the targeting precipitates by grain boundary diffusion.…”

Section: Formation Of Z-phase Under Low Stressesmentioning

confidence: 99%

Analysis of the Degradation in the Creep Strength of High-Cr Martensitic Steels

Tamura¹,

Abe²

2021

JMSR

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To investigate the formation process of the Z-phase, which lowers the long-term rupture strength of high-Cr martensitic steel, the creep curves of Grades T91, T92, and P92 were analyzed along with the experimental steels of 9Cr-1W and 9Cr-4W by applying an exponential law to the temperature, stress, and time parameters. The activation energy (Q ), activation volume (V ), and Larson-Miller constant (C ) were obtained as functions of creep strain. At the beginning of creep, sub-grain boundary strengthening occurs due to dislocations that are swept out of the sub-grains, which is followed by strengthening due to the rearrangement of M23C6 and the precipitation of the Laves phase. After Q  reaches a peak, heterogeneous recovery and subsequent heterogeneous deformation begin at an early stage of transient creep in the vicinity of several of the weakest boundaries due to coarsening of the precipitates. This activity triggers an unexpected degradation in strength due to the accelerated formation of the Z-phase. Stabilization of M23C6 and the Laves phase is important for mitigating the degradation of the long-term rupture strength of high-strength martensitic steel. The stabilization of the Laves phase is especially important for the Cr-Mo systems because Fe2Mo is easily coarsened at ~600 °C as compared to Fe2W. Lowering the hardness and Si content also prevents excess hardening due to the Laves phase, which also mitigates the degradation. The online monitoring of creep curves and the QVC  analysis render it possible to detect signs of long-term degradation under targeted conditions within a relatively short period.

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Grain boundary precipitation in Inconel 718 and ATI 718Plus

Cited by 58 publications

References 77 publications

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

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

Performance Characterization of Laser Powder Bed Fusion Fabricated Inconel 718 Treated with Experimental Hot Isostatic Processing Cycles

Analysis of the Degradation in the Creep Strength of High-Cr Martensitic Steels

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