Micromechanisms Responsible in Deformation of Nickel-Base Superalloy Single Crystals under Multiaxial Loading at High Temperatures
Final Report Abstract
Creep behavior was evaluated in the high temperature regime (at 1020 °C), applying low nominal stresses (at 160 MPa) along the [001]. Specimens with plain bar (standard tests) and semicircular notch geometries were compared to determine the effect of a quasi-isostatic tensile stress state (induced by the notch) on the creep behavior. It could be determined that notch strengthening takes place under the conditions evaluated here, although the stress gradient was kept over time due to the strong stress intensity factor Kt exerted by the circular notch. This gradient was also associated with a microstructural gradient and a micromechanical gradient. However, the effect of the quasi-static tensile stress state was corroborated through the underdeveloped rafting and dislocation network in the core of the notch root as compared with a plain bar specimen under the same nominal stress with otherwise the same loading conditions and duration. The effect of mechanical grinding was observed at the subsurface of the notch root and a homogeneous redistribution of plastic deformation could be analyzed as far as 200 µm from the notch root surface. The presence of slip bands extending from the notch root towards the skeletal point. The stress concentration factor caused by the semicircular notch led to an interesting mixture of notch strengthening with the presence of deformation mechanisms related to otherwise strong deformation localization, which usually leads to catastrophic failure, in this case being present more homogeneously distributed. The high temperature is probably responsible for a fast stress redistribution, which hinders a strong deformation localization. The notch geometry is also responsible for a shape of stress redistribution, which differs to somewhat milder notches. A comparison with such notches, e.g., as studied by the collaboration partners at Ruhr-University Bochum, shows that close to the notch root surface of milder notches the stress level is lower than in the center [Cao et al., 2018]. For notches in this project, the stress state, if redistributed with respect to the elastic stress state at the beginning of the experiment, higher stress levels close to the notch root surface are kept for as long as 189 h (equivalent to over 5% plastic deformation). Additionally, during the experimental characterization of the crept samples, an interesting and unforeseen observation was made: needle-like elongated M23C6 carbides were found to nucleate and grow in the γ phase within the dendritic cores, both in the gauge length and in the head of the creep specimens. Namely, the LEK 94 alloy does not nominally incorporate carbon, yet in its patent a tolerance of up to 0.1 at.% is given. While, the carbon found in the as-received material is mainly bound to primary MC carbides in the interdendritic regions (as is well known to happen in these alloys), in the course of high temperature exposure, carbon seems to travel to the dendritic cores, steadily forming the long needle-shaped M23C6.
Publications
- Transmission electron microscopy study of the microstructural evolution during high-temperature and low-stress (011)[011] shear creep deformation of the superalloy single crystal LEK 94, Journal of Materials Research 32 (2017) 4491-4502
L. Agudo Jácome, G. Göbenli and G. Eggeler
(See online at https://doi.org/10.1557/jmr.2017.336) - On Shear Testing of Single Crystal Ni-Base Superalloys, Metallurgical and Materials Transactions A 49A (2018) 3915-3962
G. Eggeler, N. Wieczorek, F. Fox, S. Berglund, D. Bürger, A. Dlouhy, P. Wollgramm, K. Neuking, J. Schreuer, L. Agudo Jácome, S. Gao, A. Hartmaier and G. Laplanche
(See online at https://doi.org/10.1007/s11661-018-4726-9) - Three-dimensional reconstruction and quantification of dislocation substructures from transmission electron microscopy stereo pairs, Ultramicroscopy 195 (2018) 157-170
L. Agudo Jácome, K. Pöthkow, O. Paetsch and H.-C. Hege
(See online at https://doi.org/10.1016/j.ultramic.2018.08.015)
