@article {2256, title = {Misfit and the mechanism of high temperature and low stress creep of Ni-base single crystal superalloys}, journal = {Acta Materialia}, volume = {264}, year = {2024}, pages = {119576}, abstract = {

The present work proposes a new elementary deformation mechanism which governs high temperature and low stress creep of single crystal superalloys (SXs), where the misfit between the \γ- and the \γ\′-phase plays a central role. In the coherent two phase SX microstructure, there is a tendency to minimize the overall elastic strain energy. This is accomplished by the formation of dislocation networks in the \γ-phase close to the \γ/\γ\’-interfaces. The stress fields of the network dislocations and misfit stresses accommodate each other to keep the overall strain energy of the system at a minimum. Previous work has shown that dynamic recovery is associated with knitting-out reactions, where dislocations from the network shear the \γ\’-phase and annihilate with dislocations of opposite sign on the other side of the \γ\’-phase region. Due to the presence of the misfit this results in an increase of elastic strain energy which is counteracted by coupled knitting-in reactions where newly arriving \γ-channel dislocations re-establish the minimum energy configuration. Here we provide microstructural evidence for knitting-out and knitting-in reactions and show that while these reactions clearly occur, dislocation network spacings stay constant. The misfit between the \γ- and the \γ\′-phase accounts for a constant network spacing. Dislocation networks are not static but represent dynamic steady state equilibrium structures in an evolving microstructure. Knitting regular networks requires climb processes which are suggested to be rate controlling. This new view of high temperature and low stress creep mechanism of SXs allows to rationalize previous results published in the literature.

}, keywords = {Creep, Dislocations, Misfit, Superalloy single crystals, Transmission electron microscopy}, issn = {1359-6454}, doi = {https://doi.org/10.1016/j.actamat.2023.119576}, url = {https://www.sciencedirect.com/science/article/pii/S1359645423009059}, author = {A.B. Parsa and D. B{\"u}rger and T.M. Pollock and G. Eggeler} } @conference {1611, title = {Creep, Fatigue, and Oxidation Interactions During High and Very High Cycle Fatigue at Elevated Temperature of Nickel-Based Single Crystal Superalloys}, booktitle = {Superalloys 2020}, year = {2020}, abstract = {

High-temperature fatigue of Ni-based single crystal superalloys is studied at 1000 \°C in a wide range of loading conditions (\−1 ? R ? 0.8) and number of cycles (103 \− 109). Under fully reversed conditions, a competition between crack initiations from the surface\— assisted by oxidation\—and from internal metallurgical defects\—mostly large casting pores\—is observed. Increasing the testing frequency shifts the competition to a higher number of cycles. Conversely, decreasing the casting pore size or coating the specimen promotes surface initiations. When a positive mean stress is added (R ? 0), a creep deformation/damage mechanism mainly controls fatigue life, despite fracture surfaces presenting a variety of initiation types. Fatigue life can be predicted by a simple creep law if the contribution of the alternating stress is considered. A linear damage summation method that considers pure fatigue and pure creep damage is used to predict the fatigue lives, and Haigh diagrams for different alloys are presented.

}, keywords = {Creep, fatigue, High temperature, Superalloy}, isbn = {9783030518349}, doi = {10.1007/978-3-030-51834-9}, url = {http://dx.doi.org/10.1007/978-3-030-51834-9_65}, author = {Cervellon, A. and Yi, J. Z. and Corpace, F. and Hervier, Z. and Rigney, J. and Wright, P. K. and Torbet, C. J. and Cormier, J. and Jones, J. W. and Pollock, T. M.}, editor = {Tin, Sammy and Hardy, Mark and Clews, Justin and Cormier, Jonathan and Feng, Qiang and Marcin, John and O{\textquoteright}Brien, Chris and Suzuki, Akane} } @article {1396, title = {Creep resistance of bulk copper{\textendash}niobium composites: An inverse effect of multilayer length scale}, journal = {Acta Materialia}, volume = {176}, year = {2019}, pages = {189{\textendash}198}, abstract = {

Metallic multilayer systems show promising performance in extreme environments, with high stability of bi-metal interfaces down to nanometer length scales. The creep behavior of bulk, accumulative roll bonded (ARB) Copper\–Niobium (Cu\–Nb) composites has been studied at 400 \°C as a function of layer thickness, ranging from 2 \μm to 65 nm. Similar to single phase metallic systems, three regimes are observed during creep: transient, steady-state and tertiary. The mechanism controlling minimum creep rate for all conditions tested has a strong dependence on stress, consistent with dislocation-dominated creep. Unlike the conventional effect of grain size on creep resistance, this study reveals that decreasing length scale increases creep resistance.

}, keywords = {Creep, Cu-Nb, Length scale, Multilayer}, issn = {13596454}, doi = {10.1016/j.actamat.2019.06.029}, url = {https://doi.org/10.1016/j.actamat.2019.06.029}, author = {Avallone, Jaclyn T. and Nizolek, Thomas J. and Bales, Benjamin B. and Pollock, Tresa M.} }