Filtern
Dokumenttyp
Sprache
- Englisch (4)
Schlagworte
- EBSD (3)
- TEM (3)
- SEM (2)
- Backscattered electrons (1)
- High temperature corrosion (1)
- Iron (1)
- Micromechanical model (1)
- Nano-crystals (1)
- Oxidation (1)
- P92 (1)
Organisationseinheit der BAM
Paper des Monats
- ja (1)
The current competitive situation on electricity markets forces conventional power plants into cyclic operation regimes with frequent load shifts and starts/shutdowns. In the present work, the cyclic mechanical behavior of ferritic-martensitic 9-12 % Cr steels under isothermal and thermomechanical loading was investigated for the example of grade P92 material. A continuous softening was observed under all loading conditions. The introduction of hold periods to the applied cycles reduced material lifetime, with most prominent effects at technologically relevant small strain levels. The microstructural characterization reveals a coarsening of the original “martensitic” lath-type microstructure to a structure with polygonal subgrains and reduced dislocation density. The microstructural data forms the input for a physically-based modelling approach.
The early stages of corrosion of Fe-Cr-model alloys (2 and 9 % Cr) were investigated after exposure at 650 °C in0.5 % SO2containing gas by electron backscattered diffraction (EBSD) and transmission electron microscopy(TEM). The impact of the grain orientation of the base alloy on the orientation relations of the corrosion productsis presented. After 2 min–5 min exposure the formation of a multi-layered corrosion zone was discovered. Aclear orientation relationship between ferrite and the (Fe,Cr)3O4 spinel could be demonstrated. The obtainedresults show the importance of the grain orientation on oxidation resistance.
Oxidation of a Fe –13Cr alloy under water vapor at 600°C produced a zone of nano-sized precipitation underneath the outside scale formed by iron oxides and Fe–Cr spinel. The majority of the spinel layer shows a fixed orientation relationship to the ferritic matrix: {1 0 0}α || {1 0 0}sp & <0 1 1>α || <0 0 1>sp . However, also the discovered precipitated particles are characterized by the same crystallographic orientation relationship to the respective ferritic parent grain. The habit of the precipitates is best described by a lath morphology with their main axis parallel to <1 0 0> of ferrite. Energy dispersive X-ray spectroscopy (EDX) and electron backscatter diffraction (EBSD) in scanning electron microscope (SEM) have been applied to characterize the oxide layer in micrometer scale. The clearly smaller precipitates were subsequently investigated by transmission electron microscopy (TEM). Specimens have been prepared by focused ion-beam (FIB) milling at an area previously characterized by EBSD. They cover the ferritic base material, but mainly the precipitation zone and the Fe–Cr spinel layer. Energy filtered selected area diffraction (SAD) in the conventional (C)TEM and high-angle annular dark field (HAADF) imaging in the scanning (S)TEM mode were employed in the characterization of the specimens.
Automated electron backscatter diffraction (EBSD) is generally unable to distinguish between multiple cubic phases in a specimen without additional information, such as that obtained by simultaneous energy dispersive X-ray spectroscopy (EDS). Small particles of phases with relatively similar compositions push the limits of phase identification using simultaneous EBSD and EDS, and a mismatch exists between the spatial resolutions of these two techniques due to them having different electron interaction volumes. In a recent paper, the present authors explored using backscatter detectors mounted on top of the EBSD detector to obtain atomic number (Z) contrast images that could be used for phase segmentation in cases where the results from the EBSD and EDS signals remain ambiguous. In the present work, we show that similar information can be obtained from the raw EBSD patterns themselves at higher spatial resolution than was obtained from the backscatter detectors, with the additional advantage of having no spatial mismatch between the data collection grids.