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At least 271 records · Page 15

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NASA NTRS2015

Effect of Notches on Creep-Fatigue Behavior of a P/M Nickel-Based Superalloy

A study was performed to determine and model the effect of high temperature dwells on notched low cycle fatigue (NLCF) and notch stress rupture behavior of a fine grain LSHR powder metallurgy (PM) nickel-based superalloy. It was shown that a 90 second dwell applied at the minimum stress (min dwell) was considerably more detrimental to the NLCF lives than similar dwell applied at the maximum stress (max dwell). The short min dwell NLCF lives were shown to be caused by growth of small oxide blisters which caused preferential cracking when coupled with high concentrated notch root stresses. The cyclic max dwell notch tests failed mostly by a creep accumulation, not by fatigue, with the crack origin shifting internally to a substantial distance away from the notch root. The classical von Mises plastic flow model was unable to match the experimental results while the hydrostatic stress profile generated using the Drucker-Prager plasticity flow model was consistent with the experimental findings. The max dwell NLCF and notch stress rupture tests exhibited substantial creep notch strengthening. The triaxial Bridgman effective stress parameter was able to account for the notch strengthening by collapsing the notched and uniform gage geometry test data into a singular grouping.

Dwell Notch Fatigue↗

NASA NTRS2017

Creep, Fatigue and Environmental Interactions and Their Effect on Crack Growth in Superalloys

Complex interactions of creep/fatigue/environment control dwell fatigue crack growth (DFCG) in superalloys. Crack tip stress relaxation during dwells significantly changes the crack driving force and influence DFCG. Linear Elastic Fracture Mechanics, Kmax, parameter unsuitable for correlating DFCG behavior due to extensive visco-plastic deformation. Magnitude of remaining crack tip axial stresses controls DFCG resistance due to the brittle-intergranular nature of the crack growth process. Proposed a new empirical parameter, Ksrf, which incorporates visco-plastic evolution of the magnitude of remaining crack tip stresses. Previous work performed at 704C, extend the work to 760C.

environmental interactions↗

NASA NTRS2018

Segregation and Phase Transformations Along Superlattice Intrinsic Stacking Faults in Ni-Based Superalloys

In this study, local chemical and structural changes along superlattice intrinsic stacking faults combine to represent an atomic-scale phase transformation. In order to elicit stacking fault shear, creep tests of two different single crystal Ni-base superalloys, ME501 and CMSX-4, were performed near 750 degrees Centigrade using stresses of 552 megapascals and 750 megapascals, respectively. Through high resolution Scanning Transmission Electron Microscopy (STEM) and state-of-the-art energy dispersive x-ray spectroscopy, ordered compositional changes were measured along SISFs (Superlattice Intrinsic Stacking Faults) in both alloys. For both instances, the elemental segregation and local crystal structure present along the SISFs are consistent with a nanoscale to D019 (ordering of a hexagonal close-packed crystal) phase transformation. Another notable observation is prominent Cr- and Co-rich Cottrell atmospheres and new evidence of more complex reordering processes responsible for the formation of these faults. These findings are further supported using density functional theory calculations and High Angle Annular Dark Field (HAADF) STEM image simulations.

STEM-EDS↗

NASA NTRS2019

The Effect Stacking Fault Segregation and Phase Transformations have on Creep Strength in Ni-based Superalloys

In this study, two commercially similar polycrystalline Ni-based disk superalloys (LSHR and ME3) were creep tested at 760C and 552MPa to approximately 0.3% plastic strain. LSHR consistently displayed superior creep properties at this stress/temperature regime even though the microstructural characteristics between the two alloys were comparable. High resolution structural and chemical analysis, however, revealed significant differences between the two alloys among active gamma prime shearing modes involving superlattice intrinsic and extrinsic stacking faults. In ME3, Co and Cr segregation and Ni and Al depletion were observed along the intrinsic faults - revealing a gamma prime -to- gamma phase transformation. Conversely in LSHR, an alloy with a higher W content, Co and W segregation was observed along the intrinsic faults. This observation combined with scanning transmission electron microscopy (STEM) simulations confirm a gamma prime-to-D019 phase transformation along the intrinsic faults in LSHR. Using experimental observations and density functional theory calculations, a novel local phase transformation strengthening mechanism is proposed that could be further utilized to improve the high temperature creep capabilities of Ni-base disk alloys.

Superalloy↗

NASA NTRSDate not supplied

Twin nucleation and growth mechanism in Ni-based superalloys

While micro-twinning is the dominant creep deformation mechanism in Ni-based superalloys at temperatures above 700 C, many aspects of twin nucleation and growth remain unexplored. Kolbe mechanism for micro-twinning, based on thermally activated reordering, is probably the only concept currently widely accepted by the scientific community. We propose a qualitatively different mechanism for nucleation and growth of twins. The mechanism can be briefly described as follows. Penetration of a gamma prime precipitate by two 1/2(110) edge dislocations travelling on adjacent {111} glide planes triggers nucleation (at the interface of the precipitate and the matrix) and emission of Shockley partial of screw character on the glide plane of the edge dislocation, which entered the precipitate first (generating trailing high-energy anti-phase boundary (APB)). Propagation of this Shockley partial into the precipitate converts the APB into super intrinsic stacking fault (SISF). The recurring arrival of additional edge dislocations on the glide planes adjacent to the configuration described above leads to formation of super extrinsic stacking fault SESF, subsequent micro-twin formation, and growth of the twinned region. We demonstrate the proposed mechanism via molecular dynamics simulations.

Ni-superalloys↗

NASA NTRSDate not supplied

Twin nucleation and growth mechanism in Ni-based superalloys

While micro-twinning is the dominant creep deformation mechanism in Ni-based superalloys at temperatures above 700 C, many aspects of twin nucleation and growth remain unexplored. Kolbe mechanism for micro-twinning, based on thermally activated reordering, is probably the only concept currently widely accepted by the scientific community. We propose a qualitatively different mechanism for nucleation and growth of twins. The mechanism can be briefly described as follows. Penetration of a   precipitate by two 1/2(110) edge dislocations travelling on adjacent {111} glide planes triggers nucleation (at the interface of the precipitate and the matrix) and emission of Shockley partial of screw character on the glide plane of the edge dislocation, which entered the precipitate first (generating trailing high-energy anti-phase boundary (APB)). Propagation of this Shockley partial into the precipitate converts the APB into super intrinsic stacking fault (SISF). The recurring arrival of additional edge dislocations on the glide planes adjacent to the configuration described above leads to formation of super extrinsic stacking fault SESF, subsequent micro-twin formation, and growth of the twinned region. We demonstrate the proposed mechanism via molecular dynamics simulations.

Ni-based superalloys↗

NASA NTRSDate not supplied

Effect of Alloying Additions on Twinning in Ni-Based Superalloys

Micro-twinning is the dominant creep deformation mechanism in Ni-based superalloys at temperatures above 700 C. We use atomistic simulations to study two mechanisms of twin nucleation and growth that are characterized by qualitatively different rate limiting processes. In case of the mechanism proposed by Kolbe, the rate limiting process is diffusion-mediated atomic reshuffling. In case of the other mechanism, we proposed recently, the rate limiting process is nucleation of Shockley partial dislocation. We demonstrate the effects of alloying additions on functionality of these mechanisms.

Ni-based superalloys↗

NASA NTRS2024

Effect of Cr on Twinning in Ni-Based Superalloys

Micro-twinning is the major creep deformation mechanism in Ni-based superalloys at temperatures above 700 °C. Recent experiments indicate that formation and extension of micro-twins is reliant on the segregation of gamma phase formers (e.g., Co and Cr) along the twin interface in gamma prime phase. In this study we use molecular dynamics simulation to investigate the effect of site preference of Cr in Ni 3 Al gamma prime phase on deformation twinning. We demonstrate that, depending on the site preference (Cr on Ni sites, or Cr on Al sites), two drastically different deformation behaviors can be expected.

Ni-based superalloys↗

NASA NTRS2024

Effects of Alloying Elements on Twinning in Ni-Based Superalloys

Micro-twinning is the major creep deformation mechanism in Ni-based superalloys at temperatures above 700 C. Recent experiments suggest that superlattice stacking faults in   phase may serve as the precursors to twin formation. Segregation of alloying elements to these precursors may have a significant effect on formation and extension of micro-twins. Using atomistic modeling we investigate and explain the effects of Cr and Nb alloying additions on these processes. Our results help to rationalize the experimentally observed puzzling effects of elemental composition of the alloy on creep resistance.

Ni-based superalloys↗

NASA NTRSDate not supplied

Ab-Initio Study of Stacking Fault Segregation Behavior in Ni-based Superalloys at High Temperatures

Ni-based superalloys demonstrate extraordinary creep properties compared to traditional metallic systems due to the formation of L12 (γ’) precipitates. Despite experimental evidence of alloying element segregation to the stacking faults, prior ab-initio efforts indicated repulsion of Nb from superlattice intrinsic stacking faults (SISFs) . Due to the importance of this element in key strengthening mechanisms, we developed a new technique to account for multi-component segregation behavior at temperature combining ab-initio molecular dynamics (AIMD) and Monte-Carlo (MC) simulations. Using this technique, we replicated experimentally observed segregation behavior of Nb, Co, and Ti to the SISF in L12 (Figure 1). Furthermore, we investigated the co-segregation behaviors of individual pairs of alloying elements to determine the interaction effects which lead to this segregation behavior in the full alloy system. By simulating the reduced-chemistry models using our approach, we reconciled previous ab-initio calculations demonstrating repulsion of Nb from the SISF and the experimentally observed segregation behavior.

Molecular Dynamics↗

NASA NTRS2024

Effects of alloying elements on twinning in Ni-based superalloys

Micro-twinning is the major creep deformation mechanism in Ni-based superalloys at temperatures above 700 °C. Recent experiments suggest that superlattice stacking faults in y' phase may serve as the precursors to twin formation. Segregation of alloying elements to these precursors may have a significant effect on formation and extension of micro-twins. Using atomistic modeling we investigate and explain the effects of Nb and Cr alloying additions on these processes. The simulation shows that Nb increases the creep resistance which is mostly associated with impeding the reordering of the high energy double complex stacking fault. Cr, on the other hand, promotes twin growth, degrading the high temperature creep properties. These results can help to understand the effects of elemental composition of the alloy on creep resistance.

Ni-based superalloys↗

NASA NTRSDate not supplied

Effect of Cr Twinning in Ni-Based Superalloys

Ni-based superalloys↗

NASA NTRSDate not supplied

Effects of Alloying Elements on Twinning in Ni-Based Superalloys

Ni-based superalloys↗

DOE OSTI2025

Elucidating the corrosion mechanism of Ni-based superalloys in the presence of uranium-containing chloride molten salt

The United States Department of Energy (DOE) is committed to the advancement of nuclear reactor technology through initiatives such as the Advanced Reactor Development Program (ARDP), in an effort to diversify the United States energy portfolio towards more sustainable energy options. The ARDP includes demonstration by industry partners of molten chloride fast reactors (MCFRs). Construction of MCFRs requires qualified nuclear structural materials. Unfortunately, there are no current materials that are fully qualified by the Nuclear Regulatory Commission for the construction of molten salt reactors, including MCFRs. Adapting current structural material qualifications requires expansion of our current knowledgebase on the property-performance relationships regarding corrosion performance. In this investigation, we assess microstructural changes in a Ni-based superalloy after exposure to a UCl3¬-containing chloride salt eutectic mixture through a correlated multi-modal approach combining several advanced characterization techniques, including scanning electron microscopy/focused ion beam (SEM/FIB) and transmission electron microscopy (TEM). SEM/FIB analysis will illustrate changes in elemental composition, microstructure, and isotopic information acquired from energy x-ray dispersive spectroscopy (EDS), electron backscatter diffraction (EBSD), and secondary ion mass spectroscopy (SIMS), respectively. This information will then aid in identifying localized regions to elucidate the corrosion mechanism with TEM through a combination of electron diffraction, electron energy loss spectroscopy (EELS), and additional EDS. The findings from this investigation will further expand our assessment of the corrosion performance of structural materials in molten salt chloride systems, aiding to developing fully qualified materials for construction of MCFRs.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Compatibility of Wrought Superalloys with Supercritical CO2

Supercritical CO2 (sCO2) power cycles, particularly direct-fired cycles, have the possibility of revolutionizing clean fossil energy with peak temperatures above 700 °C and wrought precipitation strengthened alloys like Haynes 282™ for structural components. At temperatures <650 °C, it would be desirable to use less expensive alloys, however, steels are known to be susceptible to carburization. Laboratory 300 bar sCO2 autoclave results were collected on a range of alloys including less expensive Ni-based alloys like 825 compared to advanced austenitic steels like alloy 709 at 600 °C. Both alloys 825 and 709 formed thin, protective Cr-rich oxides after 1,000 h. Alloy 825 also was exposed for 1,000 h in sCO2 at 800 °C and compared to a range of Ni-based alloys. Comparing alloys 625, 825, and 282, the mass gain increased with increasing alloy Ti content under these conditions. High Al superalloys did not perform significantly better under these conditions at 800 °C.

Pint, Bruce↗

DOE OSTI2022

Effect of microstructure on fatigue crack propagation in additive manufactured nickel-based superalloy Haynes 282: an experiment and crystal plasticity study

Haynes 282 is a γ' precipitation-strengthened nickel-based superalloy known for its exceptional high-temperature creep resistance and excellent fabricability. Recent advancements in powder-bed fusion-based additive manufacturing (PBF-AM) have enabled the fabrication of Haynes 282 with accurate, site-specific control of the grain orientation and morphology at the microscale. This ability opens up new avenues for microstructure design, to improve the material’s fatigue crack resistance and service life. This paper investigates the fatigue crack growth behavior of hybrid microstructure Haynes 282 fabricated via PBF-AM. Previous experiments revealed a higher crack propagation rate in the coarse columnar-grained microstructural regions when compared against fine-grained areas. Here, a strain gradient crystal plasticity model was adapted to study the fracture-related mechanical fields at the crack tip in the two microstructures. The simulation results showed a consistent influence of grain structure and texture on crack propagation, as was seen in the experiment. The model analysis revealed higher crack propagation driving force along crack direction in the coarse-grained sharply textured microstructure and higher driving force for crack kinking in fine-grained more diffusely textured microstructure, which is ascribed to the combined effect of yield stress, hardening rate, texture and grain morphology. The presented modeling approach will facilitate the development of the AM-based accurate microstructure design by deepening the fundamental study in AM-specific microstructure-properties relations.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Solving Recent Challenges for Wrought Ni-Base Superalloys

This paper reviews the status of technology in design and manufacture of new wrought 21 polycrystalline Ni-base superalloys for critical engineering applications. There is a strong 22 motivation to develop new alloys that are capable of operating at higher temperatures to 23 realise improvements in thermal efficiency, which are necessary to achieve environmental 24 targets for reduced emissions of harmful green-house gases. From the aerospace sector, 25 the development of new powder metallurgy and ingot metallurgy alloys are discussed for 26 disk rotor and static applications. New compositions for powder metallurgy contain about 27 50-55% of gamma prime (γ') strengthening precipitates to ensure components operate 28 successfully at temperatures up to 788°C (1450°F). In contrast, new compositions for ingot 29 metallurgy aim to occupy a design space in temperature capability between Alloy 718 and 30 current powder alloys that are in-service, and show levels of γ' of about 30-44%. The focus 31 in developing these alloys was design for manufacturability. To complement the aerospace 32 developments, a review of work to understand the suitability of candidate alloys for multiple 33 applications in Advanced-Ultra Supercritical (AUSC) power plants has been undertaken by 34 Detrois, Jablonski and Hawk from the National Energy Technology Laboratory. In these 35 power plants, steam temperatures are required to reach 700-760°C. The common thread is 36 to develop alloys that demonstrate a combination of high temperature properties, which are 37 reliant on both the alloy composition and microstructure and can be produced readily at the 38 right price. For the AUSC applications, the emphasis is on high temperature strength, long 39 term creep life, phase stability, oxidation resistance and robust welding for fabrications. 40 Whereas for powder disk rotors in aircraft engines, the priority is enhanced resistance to 41 time dependent crack growth, phase stability and resistance to environmental damage, while 42 extending the current strength levels, which are shown by existing alloys, to higher 43 temperatures.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Creep Behavior of a High-$\gamma^{\prime}$ Ni-Based Superalloy Fabricated via Electron Beam Melting

Additive manufacturing enables the fabrication of complex engineering components previously inaccessible through traditional processes. Nickel-base superalloys with large γ' volume fraction are typically considered non-weldable and therefore exhibit a propensity for cracking during the fusion process. These crack-prone materials, however, are of great importance in gas turbine engines due to their excellent high temperature creep resistance. In this study we investigate the creep behavior of IN738LC produced by the electron beam melting process. Here, we find that with appropriate post-build heat treatment the creep response of material oriented in the build direction exhibits deformation and rupture behavior comparable to that of conventionally cast IN738 & IN738LC. In the transverse direction properties fall below the expected cast behavior, however, we argue this is likely due to differences in grain scale and crystallographic texture. It may be possible to coarsen the grain morphology with appropriate process-parameter optimization in order to reduce the severity of intergranular fracture in the transverse direction. These results illustrate that high temperature properties exhibited by additively manufactured IN738LC are suitable for the hot section of gas turbine engines.

36 MATERIALS SCIENCE↗