Oxidation ahead of a crack tip in an advanced Ni-based superalloy

Kitaguchi, Hiroto; Li, H.Y.; Evans, Hugh E.; Ding, Rengen; Jones, I.P.; Baxter, Gavin; Bowen, P.

doi:10.1016/j.actamat.2012.12.017

Cited by 128 publications

(91 citation statements)

References 23 publications

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“…3). This finding is consistent with other researches' observations using more accurate characterisation techniques, like Atom Probe Tomography (APT) and Transmission Electron Microscope (TEM) [2,11], which also showed no further damage, such as oxide, oxygen or micro cracks, far ahead of the crack tips. These results prove that the depth of oxides/oxygen damage is very limited ahead of open crack front, and obviously much smaller than the damage zone measured by d.c.p.d.…”

Section: Crack Tip Damagesupporting

confidence: 82%

“…In addition intergranular fracture surface morphology was observed [5,9]. However, only very limited oxide intrusions ahead of the dwell fatigue crack tip (<10 microns) are observed on 2D metallographic sections [2,11], which is contradictory to the long range diffusion argument. Although long range oxygen diffusion was suggested [5] the measurement technique adopted could not capture the uneven advance of intergranular cracks.…”

Section: Introductioncontrasting

confidence: 40%

“…This long range oxygen/oxides damage therefore increased the baseline fatigue crack growth rate and changed the baseline fatigue crack growth mechanism from transgranular to intergranular. However, the long range diffusion theory has not been verified because no evidence of oxygen segregation far ahead of the crack tip has been observed using ATP [2] or TEM [11]. Instead, only very limited oxide intrusions were found ahead of the crack tip in these deliberately designed experiments, which suggests that long range oxygen diffusion and consequent grain boundary embrittlement does not exist.…”

Section: Damage Zone Phenomenonmentioning

confidence: 85%

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Mechanisms of dwell fatigue crack growth in an advanced nickel disc alloy RR1000

Hardy

et al. 2014

MATEC Web of Conferences

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Abstract. RR1000 is one of an advanced class of nickel-based superalloys developed for disc applications. Under one hour dwell fatigue loading, complex crack growth behaviour has been observed especially in a coarse grained version of this alloy. At a temperature of 700 • C in air an increase of nearly two orders of magnitude in crack growth rates compared to baseline fatigue crack growth rates may be seen. However for certain microstructural conditions, cracks can also demonstrate retardation following initial acceleration. When using a direct current potential difference (d.c.p.d) technique for monitoring crack growth, a damage zone of a few hundred microns is often measured ahead of a fast growing crack. Advanced characterisation techniques including SEM, ECCI and X-ray tomography have been adopted in the current study to understand the observed damage zone and retardation phenomenon. It is found that damage zones measured by d.c.p.d reflect brittle and non-uniform advance of the crack resulting from continuous dynamic or quasi-dynamic fracture of an oxide intrusion ahead of the crack tip during the dwell period. In contrast, cracking of the oxide intrusion is less frequent or even prevented during dwell periods associated with a retarded and slow growing crack. Crack tip stress relaxation plays an important role in dictating whether or not dynamic cracking of the oxide intrusion can be avoided.

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Section: Crack Tip Damagesupporting

confidence: 82%

Section: Introductioncontrasting

confidence: 40%

Section: Damage Zone Phenomenonmentioning

confidence: 85%

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Mechanisms of dwell fatigue crack growth in an advanced nickel disc alloy RR1000

Hardy

et al. 2014

MATEC Web of Conferences

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“…It has been confirmed that at high temperatures in air an oxide intrusion can form ahead of the crack tip [2,3] as a result of oxygen ingress along grain boundaries. The advance of the crack can therefore result from subsequent fracture of such oxide intrusions.…”

Section: Introductionmentioning

confidence: 92%

Thresholds of time dependent intergranular crack growth in a nickel disc alloy Alloy 720Li

Fisk

Lim

et al. 2014

MATEC Web of Conferences

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Abstract. At high temperatures in air, introducing a dwell period at the peak stress of fatigue cycles promotes time dependent intergranular crack growth which can increase crack growth rates by upto a few orders of magnitude from the rates of transgranular fatigue crack growth in superalloys. It is expected that time dependent intergranular crack growth in nickel-based superalloys may not occur below a critical mechanical driving force, K th−IG , analogous to a fatigue threshold ( K th ) and a critical temperature, T th . In this study, dwell fatigue crack growth tests have been carefully designed and conducted on Alloy 720Li to examine such thresholds. Unlike a fatigue threshold, the threshold stress intensity factor range for intergranular crack growth is observed to be highly sensitive to microstructure, dwell time and test procedure. The near threshold crack growth behaviour is made complex by the interactions between grain boundary oxidation embrittlement and crack tip stress relaxation. In general, lower K th−IG values are associated with finer grain size and/or shorter dwell times. Often a load increasing procedure promotes stress relaxation and tends to lead to higher K th−IG . When there is limited stress relaxation at the crack tip, similar K th−IG values are measured with load increasing and load shedding procedures. They are generally higher than the fatigue threshold ( K th ) despite faster crack growth rates (da/dN ) in the stable crack growth regime. Time dependent intergranular crack growth cannot be activated below a temperature of 500 • C.

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“…[3,6,23] Furthermore, the grain boundary structure and character can affect the da/dt response in several ways, including (1) serrated grain boundary can hinder grain boundary sliding [94,95] and reduce stress concentration at the grain boundary, thereby delaying fracture of the oxide layer and improving crack growth resistance, (2) the type of reactants (e.g., carbides or Laves Phase) [88] may vary with the grain boundary character, thereby affecting the type of oxide(s) formed along the crack-tip grain boundary, and (3) the transport of oxygen vacancies [96] to the tip of the oxide layer along the oxide/grain boundary interface or through the oxide layer under the influence of the crack-tip stress field. [86,97] The results of this study suggest that one possible means of suppressing time-dependent crack growth is to increase the K th values as high as possible. For the oxides investigated, the most promising oxides are Cr 2 O 3 and Al 2 O 3 , which are both protective oxides.…”

Section: ½20mentioning

confidence: 93%

Time-Dependent Crack Growth Thresholds of Ni-Base Superalloys

Chan

2014

Metall Mater Trans A

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A micromechanical model has been developed for predicting the time-dependent crack growth threshold and its variability by considering oxide formation or cavity formation ahead of an elastic crack subjected to a sustained load at a stress intensity factor, K, at elevated temperatures in air. It is demonstrated that stress relaxation associated with a volume-expansion process such as the formation of creep cavities or oxides with a positive transformation strain can induce residual stresses at the tip of the elastic crack. The near-tip residual stresses must be overcome by the external load, thereby instigating a growth threshold, K th , for the onset of time-dependent crack growth. This micromechanical framework provides the basis for developing appropriate predictive models for the time-dependent crack growth thresholds associated with several damage processes, including (1) oxidation-assisted intergranular crack growth, (2) K-controlled creep crack growth along an intergranular path, and (3) stress corrosion cracking. The micromechanical threshold models have been utilized to predict the time-dependent crack growth thresholds of a variety of Ni-base superalloys. The material parameters that contribute to the variability of the time-dependent crack growth thresholds have been identified and related to variations of mixed oxides or creep cavities formed near the crack tip. A size scale effect is also predicted for the transformation toughening phenomenon, which is largest at or below K th but diminishes at increasing K levels above the threshold. Finally, the micromechanical models are utilized to identify means for suppressing time-dependent crack growth in Ni-base alloys.

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Oxidation ahead of a crack tip in an advanced Ni-based superalloy

Cited by 128 publications

References 23 publications

Mechanisms of dwell fatigue crack growth in an advanced nickel disc alloy RR1000

Mechanisms of dwell fatigue crack growth in an advanced nickel disc alloy RR1000

Thresholds of time dependent intergranular crack growth in a nickel disc alloy Alloy 720Li

Time-Dependent Crack Growth Thresholds of Ni-Base Superalloys

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