Intergranular crack tip oxidation mechanism in a nickel-based superalloy

Andrieu, Éric; Molins, Régine; Ghonem, H.; Pineau, A.

doi:10.1016/0921-5093(92)90358-8

Cited by 166 publications

(133 citation statements)

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“…A considerable amount of work has been performed to characterise the effect of temperature and environment on long crack propagation rates in IN718. For temperatures up to 650 o C, it was found that the main time-dependent process influencing fatigue crack propagation was grain boundary oxidation [5,6,7]. Hold times at maximum or minimum load in the fatigue cycle were found to significantly increase crack growth rates, with the dwell allowing time for oxide embrittlement of grain boundaries to occur [8][9][10][11][12][13][14].…”

mentioning

confidence: 99%

Short crack initiation and growth at 600°C in notched specimens of Inconel718

Connolley¹,

Reed²,

Starink³

2003

Materials Science and Engineering: A

113

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The natural initiation and growth of short cracks in Inconel ® 718 U-notch specimens has been studied at 600 o C in air. U notches were introduced through broaching, and hardness traces and optical microscopy on cross sections through the U notch broaching showed that the broaching process had introduced a deformed, work hardened layer. Fatigue tests were conducted under load control using a 1-1-1-1 trapezoidal waveform, on specimens with as-broached and polished U-notches. Multi-site crack initiation occurred in the notch root. Many of the cracks initiated at bulge-like features formed by volume expansion of oxidising (Nb,Ti)C particles. In unstressed samples, oxidation of (Nb,Ti)C particles occurred readily, producing characteristic surface eruptions. Scanning electron microscopy on metallographic sections revealed some sub-surface (Nb,Ti)C oxidation and localised matrix deformation around oxidised particles. A mechanism for crack initiation by carbide expansion during oxidation is discussed. Surface short crack growth rates in the notch root of polished specimens were measured using an acetate replica technique. Observed short crack growth rates were approximately constant across a wide range of crack lengths. However, there was a transition to rapid, accelerating crack growth once cracks reached several hundred microns in length. This rapid propagation in the latter stages of the fatigue life was assisted by crack coalescence. Polishing the U-notch to remove broaching marks resulted in a pronounced increase in fatigue life. IntroductionNickel-base superalloys like Inconel ® 718 (IN718) are used extensively for turbine discs and other components in industrial and aerospace gas turbines. Since turbine discs are safety-critical components, considerable effort is expended in determining their safe operating life. The lifing procedure adopted must enable the reliable prediction of the safe life of the disc, without being overly conservative. Once engines are in service, regular inspection of the engines and refinements of lifing models may enable life-extension programs to be considered. In the traditional predicted safe life philosophy, the life is declared based on laboratory specimen and component testing. In recent years a damage-tolerant approach to lifing has been adopted, in which it is accepted that components contain defects or inhomogeneities from which cracks can initiate. Assuming an initial flaw size c o , the fatigue life is determined as the number of cycles required to propagate the crack to some critical size c c . The initial size c o can be based on the maximum defect size present in the material, or on the minimum defect size detectable by nondestructive testing (NDT) [1]. Selection of c c is based on knowledge of the fracture toughness of the material, the limit load, the maximum allowable strain or maximum permitted compliance change for a particular component. Damage-tolerant lifing requires accurate information on fatigue crack initiation and growth rates. Fatigue lives are determi...

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mentioning

confidence: 99%

Short crack initiation and growth at 600°C in notched specimens of Inconel718

Connolley¹,

Reed²,

Starink³

2003

Materials Science and Engineering: A

113

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“…The detrimental effects of environmental factors, in particular oxygen, on high-temperature fatigue crack growth resistance in nickel alloys have been clearly demonstrated by experimental results and well documented in the literature [1][2][3][4][5][6][7][8][9][10][11]. For polycrystalline nickel superalloys, environmental effects were frequently observed to modify the fracture morphology from transgranular to intergranular during crack growth, as well as to increase the crack growth rates significantly for a given stress intensity factor [5,6,8,9].…”

Section: Introductionmentioning

confidence: 72%

“…For polycrystalline nickel superalloys, environmental effects were frequently observed to modify the fracture morphology from transgranular to intergranular during crack growth, as well as to increase the crack growth rates significantly for a given stress intensity factor [5,6,8,9]. Considerable effort has been made to understand the crack tip oxidation mechanisms in order to provide a basis for the development of quantitative models that predict crack growth under operational temperatures and loading conditions.…”

Section: Introductionmentioning

confidence: 99%

Prediction of crack growth in a nickel-based superalloy under fatigue-oxidation conditions

Tong

Hardy

2010

Engineering Fracture Mechanics

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• This is the author's version of a work that was accepted for publication in the journal Engineering Fracture Mechanics. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication.A definitive version was subsequently pub- AbstractPrediction of oxidation-assisted crack growth has been carried out for a nickel-based superalloy at elevated temperature based on finite element analyses of oxygen diffusion, coupled with viscoplastic deformation, near a fatigue crack tip. The material constitutive behaviour, implemented in the finite element code ABAQUS via a user-defined material subroutine (UMAT), was described by a unified viscoplastic model with non-linear kinematic and isotropic hardening rules. Diffusion of oxygen was assumed to be controlled by two parameters, the oxygen diffusivity and deformation-assisted oxygen mobility. Low frequencies and superimposed hold periods at peak loads significantly enhanced oxygen concentration near the crack tip. Evaluations of near-tip deformation and oxygen concentration were performed, which led to the construction of a failure envelop for crack growth based on the consideration of both oxygen concentration and accumulated inelastic strain near the crack tip. The failure envelop was then utilised to predict crack growth rates in a compact tension (CT) specimen under fatigue-oxidation conditions for selected loading ranges, frequencies and dwell periods. The predictions from the fatigue-oxidation failure envelop compared well with the experimental results for triangular and dwell loading waveforms, with marked improvements 2 achieved over those predicted from the viscoplastic model alone. The fatigue-oxidation predictions also agree well with the experimental results for slow-fast loading waveforms, but not for fast-slow waveforms where the effect of oxidation is much reduced.

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“…Alloy 718 is, as many other nickel based alloys such as alloy 600 or X750 are, sensitive to an oxidation assisted crack growth mechanism. Indeed, many studies have emphasized the detrimental effect of oxygen and more generally of environment on the mechanical behavior of alloy 718 at high temperatures [1][2][3][4][5][6][7][8][9]. The present paper is aimed at studying the effect of two oxidation conditions close to those encountered industrially: -1000°C in air: similar to hot rolling conditions.…”

Section: Introductionmentioning

confidence: 99%

“…-650°C in air: similar to "in service" conditions of turbomachine disks. Under such conditions, the material is known to be sensitive to oxidation assisted crack propagation [3][4][5][6][7][8][9][10][11][12][13]. The core of the present study is to assess the harmfulness of intergranular oxides penetrations (IOP) on the mechanical properties of alloy 718.…”

Section: Introductionmentioning

confidence: 99%

High Temperature Intergranular Oxidation of Alloy 718

Garat¹,

Deleume²,

Cloue³

et al. 2005

Superalloys 718, 625, 706 and Various Derivatives (2005)

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Alloy 718 samples were oxidized in air under experimental conditions close to those experienced during high-temperature heat treatments (shaping: about 1000°C) or in service (turbomachine disks: 650°C). The main objective of the study was to evaluate the impact of these treatments on oxygen penetration into the grain boundaries and, hence, to assess the harmfulness of these penetrations in terms of defect initiation and propagation. The method relies mainly on the SIMS-based imaging technique. This technique, which supplements those applied in previous studies, uses a series of basic maps to locate within the volume the oxygen in free or combined state, in relation with the microstructural features (grain boundaries, grains, delta phase, etc). The results point out to intergranular oxidation for both exposure conditions without clearly demonstrating the existence of atomic oxygen penetrations ahead of the intergranular oxidation front. The affected depths seem sufficient both to form preferential initiation sites (oxidation at 1000°C) and to assist intergranular propagation of defects and/or cracks (oxidation at 650°C).

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Intergranular crack tip oxidation mechanism in a nickel-based superalloy

Cited by 166 publications

References 11 publications

Short crack initiation and growth at 600°C in notched specimens of Inconel718

Short crack initiation and growth at 600°C in notched specimens of Inconel718

Prediction of crack growth in a nickel-based superalloy under fatigue-oxidation conditions

High Temperature Intergranular Oxidation of Alloy 718

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