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Hier finden Sie wissenschaftliche Publikationen aus den FraunhoferInstituten. On the mechanism of dynamic embrittlement and its effect on fatigue crack propagation in IN718 at 650°C
 Procedia Structural Integrity 2 (2016), pp.557564 ISSN: 24523216 
 European Conference on Fracture (ECF) <21, 2016, Catania> 

 English 
 Journal Article, Conference Paper, Electronic Publication 
 Fraunhofer IWM () 
Abstract
1N718 is a commonly used nickelbase alloy for high temperature applications, e.g., in gas and steam turbines. At elevated temperatures, this and other superalloys are prone to the failure mechanism "dynamic embrittlement". Dynamic embrittlement can be considered as a kind of stress corrosion cracking, driven by tensilestresscontrolled oxygen grain boundary diffusion. Oxygen embrittles the grain boundaries by weakening the grain boundary cohesion resulting in fast and brittle intercrystalline crack propagation. In order to reveal the mechanism of dynamic embrittlement, hightemperature fatigue crack propagation tests were carried out at 650 degrees C applying various dwell times and testing frequencies. Most of the tests were performed in laboratory air, but some experiments were run in vacuum as well, in order to eliminate environmental effects and, hence, to define the reference fatigue crack propagation behaviour. The observations show that at low stress intensity factor ranges Delta KI, continuous crack growth occurs. At intermediate values of Delta KI, no crack propagation takes place during the dwell part of the cycle. Rather, the crack extends during unloading and reloading between subsequent hold times. The time necessary to grow the crack under sustained load during the dwell time was found to decrease with increasing stress intensity factor. Therefore, at high values of Delta KI, there is a contribution of the crack propagation at constant stress, since the incubation time is shorter than the dwell time. A mechanismbased model was developed for the range of test parameters, where intergranular and transgranular areas exist side by side in the fracture surface. The total crack growth per cycle is calculated by a linear combination of the intergranular and the transgranular contribution using the corresponding area fractions as weighting factors. It is shown that simulation calculations based on this model approach correspond very reasonably to the experimental observations. Hence, the model provides a quantitative mechanismenrelated description of the effect of dynamic embrittlement on fatigue crack propagation rate.