MS-4-P-2044 Structural evolution of T91-ODS steel during high temperature deformation
Scanning and transmission electron microscopy (TEM) was applied to study the structure of oxide dispersion strengthened (ODS) ferritic–martensitic steel T91 (9 wt.% Cr, 1 wt.% Mo) before and after mechanical deformation at elevated temperatures.
The microstructure of investigated samples consists of two kinds of ferrite grains, with nearly no defects, and with a high defect density due to formerly martensite. In the as received samples, the grain boundaries are decorated by relatively large, non-regular shaped M23C6 carbides where M is metal. Additionally, there are finely distributed Y2O3 round ODS nanoparticles.
Figure 1a shows high angle annular dark-field (HAADF) scanning TEM (STEM) image of as received sample. The small precipitates with a dark contrast are Y2O3 particles showing energy-dispersive X-ray (EDX) linescan through the precipitate as illustrated in Figure 1b. The high resolution TEM (HRTEM) images (here not shown) yield the Y2O3 particle with bcc structure and a lattice parameter of 1.06 nm. Note that Y2O3 particles are arranged close to dislocations D inside the ferrite grain as is shown in enlarged region in Figure 1a.
In Figure 2a, conventional bright-field TEM image of as received sample is presented. Analysis of corresponding diffraction pattern in Figure 2b shows the coherence relationship between the ODS particles and the matrix: <100> Y2O3 || <110> Fe.
HRTEM analysis of M23C6 in as received samples (here not shown) reveals that carbide has fcc structure with a lattice parameter of ~ 1.07 nm. The relation between the carbide and ferrite is: <112> M23C6 || <110> Fe. The detail EDX analysis of the carbide regions shows the existence of Cr21Mo2C6. Many {111} stacking faults in the large carbide inclusions are observed.
The EDX study combined with the diffraction analysis of other inclusions in the investigated steel shows the presents of MX phases with fcc (rock salt) structure and lattice parameter of 0.44 nm, where M is metal and X is C,O or N.
The first investigations of the samples deformed at elevated temperatures from 500 to 700 °C show, that mean particle diameter of ODS particles in these samples increases and their size distribution broadens. The number of the grains without defects extremely decreases. Additionally, some extremely large non-regular shaped M23C6 carbides have been found that is caused by Ostwald ripening during the deformation at higher temperatures.
The present work was composed at the Institute for Applied Materials at the Karlsruhe Institute of Technology with support of the Helmholtz Association (Germany).
Fig. 1: a) HAADF STEM image of as received sample. In insert enlarged region by D marked dislocations. b) EDX linescan taken along dash line of a). |
Fig. 2: a) Bright-field TEM image of as received sample close to [001] Fe-zone axis with b) corresponding diffraction pattern. |