Prakt. Met. Sonderband 38 (2006) 277 
ng of the uncertainty regarding the number of analyzed subgrains and may vary for different steels 
ccur and depending on their subgrain-size. For each region three images were acquired using 
structure different tilt-angles to unveil the subgrain-size. 
xample a a 
the FBS initial state after creep 
| be seen subgrain size 
dislocation density 
iil 
Fig. 4: TEM metal foil images taken at the TAF steel before and after creep testing at 
650 °C and 100 MPa. 
pm The subgrain size of the FB8 material increased from 291 + 102 nm for the as-received 
sample to 758 + 326 nm after creep rupture; the TAF material showed an increase from 
289 +47 nm to 494 + 242 nm. When analysing the dislocation density, one should be 
ils in the aware that the dislocation density can be strongly influenced by the TEM specimen 
extraction preparation and the imaging conditions. However, if the same specimen preparation 
tion from technique and similar imaging conditions are used for the as-received and creep-tested 
g a more specimen, a comparison between the obtained values is useful in order to get some insight 
ts can be on the influence of the mechanical deformation on the microstructure. For the specimen 
ents that investigated in the present work, the dislocation density within the subgrains was 
liffraction estimated from micrographs acquired with a magnification of 100k corresponding to an 
rain size area of 0.43 ym2. In total, an area of 8.6 um? was analyzed. Again, as for the subgrain-size 
seen the determination, the magnification as well as the total area should be adjusted to the 
jithin the material under study. In the case of the investigated FB8 material the dislocation density 
least 9 within the subgrains reduces from 7,3+1,2 10m? of the as-received material to 
rain-size 2,2 + 1,0 10" m™. For the more creep resistant TAF material, the dislocation density within 
n area of the subgrains decreased from 3.4 + 0.8 10" m? of the as-received material to 0.8 + 0,6 
statistical 10" m=.