Etude par Sonde Atomique Tomographique de la formation de nano ...

tel-00751814, version 1 - 14 Nov 2012
Chapter 3. Oxide Dispersion Strengthened Steels
concentration of element, i, that is Fe, Cr, Ti, Y and O in the nanoparticle then can be deduced
from relation:
where Ni is the number of chemical specie, i, and
C
N
i
� (3.2)
i real
artificial
N cluster � N cluster
real
N cluster the number of atoms that form the
nanoparticle (Y, Ti and O). The results of calculations for as-milled, annealed and hot
extruded nanoparticles are reported in Figure 3.16 (b-d).
As it was previously mentioned the artificially introduced matrix atoms, in particular Fe
and Cr, result in an increase of the relative atomic density inside the nanoparticles. In
accordance with experimental results, the relative atomic density increases by a factor 2 in
nanoparticles in as-milled state and factors 2 and 2.5 in annealed at 850°C during 1h and hot
extruded states. So, taking into account the increase of atomic density, several conclusions
can be done:
a) the radius of overlapped zone, Roverlap, from 0.2 to 0.8 nm give the experimentally
observed increase of relative atomic density.
b) around 36-42 at.% of Fe as well as 8.7-10.5 at.% of Cr can be introduced in
nanoparticles in as-milled state as a result of the local magnification effect. It should be
mentioned that for as-milled nanoparticles, calculations are performed in accordance to
assumption that there is no Cr-rich shell around Y-Ti-O-rich nanoparticle (since no clear
evidence was observed in experimental data). From these calculations, the content of Fe and
Cr that can be artificially introduced into nanoparticles is slightly smaller in comparison to the
experimentally measured one (70 and 19 at.% respectively for Fe and Cr).
c) about 10-33 at.% of Fe and 30-46 at.% of Cr can be introduced in nanoparticles
after annealing and hot extrusion (resulting in an increase of the relative atomic density from
1.8 to 3.2 times). It should be mentioned that the content of Cr estimated from these
calculations are higher than the experimentally observed ones (26.6 and 26.3 at.%
respectively in annealed and hot extruded samples). It should be keep in mind that in reality,
the concentration of Cr rich shell may be lower than 100 at.%. As a consequence smaller
content of Cr atoms would be introduced into Y-Ti-O-rich nanoparticles, decreasing the
difference with experimental results.
Therefore, following these calculations the measured level of Fe in the nanoparticles
can not be only explained by trajectory overlaps. Some underplaying level of Fe may be
present originally in these nanoparticles.
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