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- Krupp, Ulrich, 1968, et al.
(författare)
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Intergranular Oxidation Effects During Dwell-Time Fatigue of High-Strength Superalloys
- 2017
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Ingår i: Oxidation of Metals. - : Springer Science and Business Media LLC. - 1573-4889 .- 0030-770X. ; 88:1 (SI), s. 3-14
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Tidskriftsartikel (refereegranskat)abstract
- The present paper summarizes experimental work to identify the mechanisms of dwell-time cracking during service operation of polycrystalline nickelbase superalloys, such as Alloy 718 and AD730. By means of crack growth monitoring during various kinds of cyclic loading in vacuum and in air using the potential drop technique, it was shown that the combination of sustained tensile stress, load reversal, and oxidizing atmosphere leads to an increase in the crackpropagation rate by orders of magnitude, as compared to cyclic reference tests without dwell time and/or under vacuum conditions. By careful metallographic and theoretical analysis, the embrittling effect was attributed to stress-induced oxygen diffusion ahead of the intergranular crack tip followed by decohesion in a nanometerscale and had been termed ‘‘dynamic embrittlement.’’ More recently, atom probe tomography of the near-crack tip region revealed that the damage zone consists of Cr-rich transition oxides rather than elemental oxygen. This is in qualitative agreement with TGA measurements on Alloy 718 specimens without mechanical loading, which shows that crack propagation velocities of 50 lm/s do not allow massive Cr2O3 or NiO scale formation. By means of a quantitative analysis of the fracture surface, it became evident that grain-boundary attack depends on the grainboundarycharacter. This observation was supported by four-point bending experiments on grain-boundary-engineered samples with a high fraction of coincident site lattice boundaries and bicrystalline samples with well-defined grain-boundary misorientation relationships with respect to the loading axis. Taking the experimental results into account, semiquantitative modeling concepts have been developed to correlate crack propagation rates with the oxygen grain-boundary diffusivity, the local microstructure, and the mechanical stress states. These concepts are discussed in terms to adapt grain size and precipitate microstructure of polycrystalline superalloys.
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| 2. |
- Krupp, Ulrich, 1968, et al.
(författare)
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The potential of spinodal ferrite decomposition for increasing the very high cycle fatigue strength of duplex stainless steel
- 2016
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Ingår i: International Journal of Fatigue. - : Elsevier BV. - 0142-1123. ; 93, s. 363-371
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Tidskriftsartikel (refereegranskat)abstract
- Duplex stainless steels (DSS) have become candidate materials for structural applications, where conventional austenitic stainless steels fail due to very high cycle fatigue (VHCF) in combination with corrosive attack. It seems that DSS exhibit a fatigue limit, which can be attributed to the two-phase austenitic-ferritic microstructure. Ultrasonic VHCF testing revealed that the phase boundaries are efficient obstacles for the transmission of slip bands and microstructural fatigue cracks up to 10(9) cycles and even beyond. The barrier strength is determined by the misorientation relationship between neighbouring grains but also by the strength of the individual phases. By thermal treatment at 475 degrees C, spinodal decomposition of the ferrite phase results in the formation of Cr-rich alpha' precipitates. While during static loading these precipitates give rise to a loss in ductility (475 degrees C embrittlement), it was shown that the HCF strength can be increased and that there is also a tendency towards a beneficial effect on the VHCF behaviour. A more detailed analysis of the local plasticity sites by means of atom probe tomography (APT) revealed a dissolution of the a' precipitates within operated slip bands. The dissolution might be an indication for a local softening mechanism that limits the VHCF strengthening effect of spinodal decomposition.
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