Elevated Temperature Fatigue Crack Growth in Alloy 718—part Ii: Effects of Environmental and Material Variables

Ghonem, H.; Nicholas, T.; Pineau, A.

doi:10.1111/j.1460-2695.1993.tb00103.x

Cited by 105 publications

(53 citation statements)

References 39 publications

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“…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]. Most of these studies were on long cracks using CT specimens, but the hold time effect has also been observed for pre-initiated cracks in Unotch specimens tested in bending [15].…”

Section: Introductionmentioning

confidence: 99%

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

Connolley¹,

Reed²,

Starink³

2003

Materials Science and Engineering: A

115

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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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Section: Introductionmentioning

confidence: 99%

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

Connolley¹,

Reed²,

Starink³

2003

Materials Science and Engineering: A

115

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“…Formation of intense slip bands and crack initiation along them is promoted by cyclic softening, with dislocations preferentially gliding along the softer band rather than in the harder surrounding regions [27,28] . IN718 is sensitive to the environment at elevated temperatures and grain boundary embrittlement by oxidation can occur [7,29,8] . Impingement of slip bands on grain boundaries produces local stress concentrations at the boundary.…”

Section: Discussionmentioning

confidence: 99%

“…Cracks were observed to propagate along slip bands or along grain boundaries. Some intergranular propagation in IN718 is expected at 600 o C in air, because of oxidation embrittlement of grain boundaries [5,8,30] . In the Batch B Unotches, it is proposed that surface crack propagation of the crack along slip bands and grain boundaries produced a longer, more tortuous crack path, resulting in partial shielding of the crack from the applied stresses and hence a lower crack propagation rate.…”

Section: Discussionmentioning

confidence: 99%

Effect of broaching on high-temperature fatigue behavior in notched specimens of INCONEL 718

Connolley

Starink

Reed

2004

Metall Mater Trans A

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Notches were machined in specimens of Inconel 718 by a broaching process, where differing broaching runs led to differing extents of sub-surface deformation and surface roughness.Fatigue tests were carried out at 600˚C with a trapezoidal loading waveform at 0.25Hz. The broaching process that led to the more severe sub-surface deformation (but lower surface roughness) showed the worst fatigue performance. Analysis of total strain amplitude in the notch root with the aid of an elasto-plastic finite element model, showed that the workhardening related to the subsurface deformation caused by the different broaching can account for the difference in fatigue lives. Differences in initiation and growth behaviour were seen for the two broached finishes as well as for broached and subsequently polished samples.These differences are discussed in terms of a change in crack growth initiation and growth mechanisms due to the presence of the work hardened layer.

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“…A segment of these studies has focused on the crack growth correlations with grain boundary (GB) phases and ' precipitates within the  matrix [1][2][3]. These precipitates are quantified in terms of primary (' p ), secondary (' s ) and tertiary (' t ) gamma prime.…”

Section: Introductionmentioning

confidence: 99%

Relative Contributions of Secondary and Tertiary γ' Precipitates to Intergranular Crack Growth Resistance in IN100 Alloy

Maciejewski¹,

Ghonem²

2012

Superalloys 2012 (Twelfth International Symposium)

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This paper examines the role of secondary and tertiary ' on intergranular crack growth rate in P/M IN100. A series of different heat treatments were carried out on the as received material to vary the ' statistics (size and volume fraction) through control of the different features of the heat treatment cycle. Dwell fatigue crack growth experiments have been performed on as received as well as heat treated specimens at 650°C and 700°C in air with dwell time loading cycles. Results on the as-received material show that the intergranular crack growth rate while temperature dependent, is independent of the hold time period. For the modified microstructures tested at 650°C, the dwell crack growth rate is shown to be sensitive to ' variations. Considering that the yield strength of the continuum is a function of ' statistics, the role of ' on intergranular cracking was illustrated by correlating the crack growth rate and the continuum yield strength. This inverse relationship is examined numerically by modelling the grain boundary fracture path as an interface surrounded by near and far field continuum regions represented by viscoplastic flow formulations. Results of the model show that an increase in the continuum yield strength is accompanied by a lower viscous strain, resulting in higher constraint on the grain boundary sliding and thus, a decrease in the crack growth rate. This observation was further examined by calculating the individual components of yield strength associated with tertiary and secondary ' precipitates. These components were correlated with the crack growth rate, showing higher sensitivity to the tertiary ' precipitate hardening effects.

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Elevated Temperature Fatigue Crack Growth in Alloy 718—part Ii: Effects of Environmental and Material Variables

Cited by 105 publications

References 39 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

Effect of broaching on high-temperature fatigue behavior in notched specimens of INCONEL 718

Relative Contributions of Secondary and Tertiary γ' Precipitates to Intergranular Crack Growth Resistance in IN100 Alloy

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Elevated Temperature Fatigue Crack Growth in Alloy 718—part Ii: Effects of Environmental and Material Variables

Cited by 105 publications

References 39 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

Effect of broaching on high-temperature fatigue behavior in notched specimens of INCONEL 718

Relative Contributions of Secondary and Tertiary &gamma;' Precipitates to Intergranular Crack Growth Resistance in IN100 Alloy

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Relative Contributions of Secondary and Tertiary γ' Precipitates to Intergranular Crack Growth Resistance in IN100 Alloy