Filtern
Dokumenttyp
- Vortrag (5) (entfernen)
Sprache
- Englisch (5)
Referierte Publikation
- nein (5) (entfernen)
Schlagworte
- Dislocation (5) (entfernen)
Organisationseinheit der BAM
Eingeladener Vortrag
- nein (5)
The presentation shows how the three-dimensional quantification of dislocations and their characteristic features, e.g. Burgers vector, line direction, dislocation density, is carried out at the transmission electron microscope in scanning mode (STEM) at Division 5.1 at BAM. Exemplarily, the methods are shown for Ni-base superalloy single crystals, for which a short introduction is given using further TEM techniques. Additional examples on low angle grain boundaries, nucleation of oxides at dislocations and interaction of dislocations and carbides are shown.
The content of the presentation was addapted, aiming at scientists who work within the DFG Priority Programme 1713 "Strong coupling of thermo-chemical and thermo-mechanical states in applied materials".
The realistic description of plastic deformation caused by dislocations demands the representative measurement of their features, e.g., Burgers vector, slip plane, line direction and density. In the case of bulk deformation of technical alloys, reliable data must be ensured for large regions. Nonetheless, the (thin) filiform nature of dislocations and also the heterogeneity in some microstructures additionally require flexible analysis techniques that resolve the details of their interactions. Strong and clear channeling contrasts, faint extinction contours and the absence of chromatic aberration make scanning transmission electron microscopy (STEM) an ideal imaging mode in wide and thick regions of TEM foils, as opposed to conventional (C)TEM. It is the purpose of this contribution to show how STEM can be applied for quantitative measurement of dislocation features. Furthermore, a new tool will be presented, which enables the three-dimensional (3D) reconstruction, visualization and quantification of dislocation densities and directions from stereo-pairs. The application of these techniques will be shown on a monocrystalline Ni-base superalloy, an important class of structural materials that has been implemented in the first row blades of gas turbines. The examples are extracted from specimens subjected to creep deformation at high temperature and low stress under various macroscopic deformation geometries. The spacial and angular accuracy are discussed as well as possible sources of error.
The demand for improved castability and low angle grain boundary (LAGB) resistance has led to the addition of low contents of e.g., B, Hf, Zr or C, into large industrial gas turbine components made of Ni-base superalloy single crystals (SXs). Due to the long-term application of Ni-base superalloy SX components in the temperature regime > 1000 °C, the formation of carbides is highly probable, which could jeopardize mechanical properties, such as high cycle fatigue. In the present contribution, the effect of internal and external stresses on the nucleation and growth characteristics of M23C6 carbides is investigated. Creep experiments are performed on the Ni-base superalloy SX LEK 94, which shows a low C concentration (= 0.1 at. %), at 1020 °C under parallel and circularly notched tensile specimens at a nominal stress of 160 MPa in the crystallographic direction [001]. The carbides are then characterized via scanning (S) and transmission (T) electron microscopy (EM). Nucleation is enhanced in the dendritic cores, often as coalesced colonies, extending over micrometers within M-rich (M: Cr, Re, W, Mo) γ channels. Lath shapes with facets on {100} (parallel to growth direction) and {111} are common. These facets exist since early stages (Fig.1a) and later develop misfit dislocations (Fig.1b), preserving the orientation relationship {100}γ || {100}M23C6. Fig. 1c shows a region from the creep gage, where carbides interact with superdislocations in the γ’ phase. Possible mechanisms are discussed.
Dislocations are the carriers of plastic deformation. As such, their characterization offers important information on the properties they affect. In this contribution, a new tool is presented, which is incorporated in Amira ZIB Edition and allows for three-dimensional (3D) imaging and quantification of dislocations substructures from thick regions an electron-transparent specimen. In the tool, the dislocation segments are traced on diffraction contrast images that are obtained in the transmission electron microscope (TEM). The uncertainties related to the experimental setup and to the proposed method are discussed on the base of a tilt series.
The current trend towards cyclic, “flexible” operation of fossil-fueled power plants constitutes a major issue regarding lifetime and operational safety of the respective installations and their components, as was outlined in our complementary contribution (Part 1). The present contribution reports on the investigation of the microstructure evolution in cyclically loaded ferritic-martensitic steels and its representation in a physically-based micromechanical model.
For this purpose, specimens of P92 steel grade from the mechanical test programme outlined in our companion contribution (Part 1) were analyzed by scanning electron microscopy (SEM), including backscattered diffraction (EBSD) mapping, and transmission electron microscopy (TEM). A novel method was implemented to improve angular resolution of EBSD scans. Additionally, a correlative microscopy approach was developed and used to correlate EBSD and TEM measurements on the same locations of thick regions of electron transparent specimens. By applying these techniques, a detailed quantitative microstructure description of the as-received material condition, namely in terms of subgrain morphology and dislocation density/distributions, was established. Comparisons of as-received and cyclically loaded conditions from tests interrupted at different stages of lifetime indicate a rapid redistribution of in-grain dislocations with a strong interaction between mobile dislocations and low angle grain boundaries (LABs).
The proposed micromechanical model is formulated in a viscoplastic self-consistent (VPSC) scheme, which is a mean-field approach that allows us to include the crystal details at the level of slip systems while avoiding the considerable computational costs of full-field approaches (such as the classical crystal plasticity finite element analysis). Being physically-based, the model uses dislocation densities and includes the interaction between dislocations, e.g. annihilation of mobile dislocations, and evolution of microstructure, e.g. the grain coarsening. Particularly, the constitutive laws for dislocation evolution and interaction between dislocations and low angle boundaries are calibrated based on two-dimensional discrete dislocation dynamic (2D DDD) simulations, which are performed at a micro-/meso-scale. The results of the beforementioned EBSD experiments are considered as a direct input, involving e.g. the amount of geometrically necessary dislocations, average misorientations and grain characteristics.