The present paper summarizes experimental work to identify the mechanisms of dwell-time cracking during service operation of polycrystalline nickel-base 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 crack propagation 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 nanometer scale 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 µm/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 grain-boundary character. 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
Hydrogen gas pressure is an important test parameter when considering materials for high-pressure hydrogen applications. A large set of data on the effect of hydrogen gas pressure on mechanical properties in gaseous hydrogen experiments was reviewed. The data were analyzed by converting pressures into fugacities (f) and by fitting the data using an f|n| power law. For 95% of the data sets, |n| was smaller than 0.37, which was discussed in the context of (i) rate-limiting steps in the hydrogen reaction chain and (ii) statistical aspects. This analysis might contribute to defining the appropriate test fugacities (pressures) to qualify materials for gaseous hydrogen applications.
IN718 is a standard nickel-base alloy for high temperature usage, e.g., as forged gas turbine disks. At elevated temperatures IN718 is prone to cracking by dynamic embrittlement. Dynamic embrittlement is caused by oxygen diffusion along the grain boundaries, resulting in a time-dependent intergranular decohesion. The result is a fast brittle intercrystalline crack propagation. In contrast, vacuum testing shows a drastic reduction of the crack propagation rate and a ductile, transcrystalline fracture morphology. Objective of this investigation is to gain a better understanding of the dynamic embrittlement process. In the present study the crack growth rate has been measured with an alternate current potential drop system (ACPD) and a far-field microscope during dwell-time testing at 650°C, a positive stress ratio and two different load schemes.
At elevated temperatures, the nickel-base superalloy IN718 is prone to the failure type 'dynamic embrittlement'. Dynamic embrittlement is assumed to be driven by tensile stress-controlled oxygen grain boundary diffusion. Oxygen embrittles the grain boundaries in front of a crack and results in a fast brittle intercrystalline crack propagation. In order to reveal the mechanism of dynamic embrittlement, high temperature fatigue crack propagation tests were carried out at 650 °C in vacuum and air applying various dwell times and testing frequencies. The crack growth was monitored by the ACPD technique and a far-field microscope. The observations show that at low stress intensity factor ranges, crack propagation mainly occurs in the unloading and loading parts of the cycle and only minor crack propagation takes place during the dwell time. With increasing dwell time, the contribution of the crack propagation at constant stress increases. A mechanism-based model was developed on the basis of these findings which allows for a quantitative description of the effect of dynamic embrittlement on fatigue crack propagation rate. The results of respective simulations correspond very satisfactorily to the experimental data. Hence, the model is suitable for lifetime assessment.
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