Effects of microstructure on high temperature dwell fatigue crack growth in a coarse grain PM nickel based superalloy

Li, Hangyue; Sun, Jifeng; Hardy, Mark; Evans, Hugh E.; Williams, S.; Doel, T.J.A.; Bowen, P.

doi:10.1016/j.actamat.2015.02.023

Cited by 137 publications

(92 citation statements)

References 27 publications

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“…Indeed such a threshold has been explored by Chan et al [6] through a numeric approach and variable threshold values were predicted corresponding to oxidation of different constituent phases. The formation of different constituents of oxides was confirmed and ultimately observed to dictate different crack growth behaviour in an advanced RR1000 nickel disc alloy [7]. However, it is suggested that it is the kinetics of crack tip stress relaxation that plays a significant role on the constituent and the rupture of oxide intrusions ahead of the crack tip [1,7].…”

Section: Introductionmentioning

confidence: 92%

“…Based on an earlier work [5] it is suggested that K th-IG may be related to a critical external mechanical driving force at which oxide intrusions ahead of the crack tip can no longer remain intact and start to crack. Although the fracture toughness of oxides are usually very small, < 2 MPa √ m [4], they can be protected by compressive stresses resulting from volume expansion upon formation, constraint from the matrix, and crack tip blunting due to localised creep deformation [7]. Hence more external mechanical driving force is needed for crack growth to occur.…”

Section: K Th-ig and T Thmentioning

confidence: 99%

“…near threshold regime [7]. For the investigated alloy, uneven grain size is common due to the cast and forged processing route undertaken.…”

Section: Effect Of Microstructurementioning

confidence: 99%

“…The mechanism that accounts for the intergranular crack growth at 650 • C is likely to be purely environmentally related, as the crack growth acceleration disappears in vacuum (the results will be reported elsewhere). Below a temperature at which the creep is not significant, the intergranular crack growth mechanism has been determined to be stress assisted grain boundary oxidation (SAGBO) [9,10] which involves forming oxide intrusions ahead of the crack tip and their rupture under the influence of the crack tip stress field [5,7].…”

Section: K Th-ig and T Thmentioning

confidence: 99%

“…However, it is suggested that it is the kinetics of crack tip stress relaxation that plays a significant role on the constituent and the rupture of oxide intrusions ahead of the crack tip [1,7]. It is also likely that crack growth behaviour is extremely sensitive to microstructure variations with a reduction of external stress intensity factor towards the time dependent threshold value [7].…”

Section: Introductionmentioning

confidence: 99%

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

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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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