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

tel-00751814, version 1 - 14 Nov 2012
� Fe content
Chapter 4. Nitride Dispersion Strengthened Steels
The concentration of Fe into the particles has the opposite behaviour than the
concentration of Ti and N (Figure 4.23 (a)). The highest level is measured in as-milled
samples (~45 at.%), the lowest in powders annealed at 850 and 1000°C (~5at.%), and an
intermediate constant value (~30-35%) is measured in samples heat treated at 600, 700 and
hot extruded state.
It is worth mentioning that, the fraction of Fe in Ti-N rich particles can be non-zero. As
an example, it is suggested, that in the case of nanosized precipitates, thermodynamic barrier
for nucleation may be lowered by increasing concentration of matrix atoms in the precipitates
[19, 20]. However, this effect is less pronounced for particles reaching larger sizes (advanced
growth stage and coarsening). This could explain that be biggest particles (observed in
samples annealed at 850 and 1000°C) contain only few percents of iron. The presence of few
percents of iron in nitrides is already reported elsewhere [17]. However, in all analyzed
states, the measured Fe concentration is higher than the maximum value compatible with the
interpretation of SANS data.
Local magnification effects [12,13] could be at the origin of this slight discrepancy (~30-
35at.% of Fe according to APT and less than 20 at.% according to SANS). Modelling of field
evaporation of high-field nanoparticles in MA NDS matrix (section ΙΙ.4. (d) of the Chapter
2), shows that when radius of nanoparticle exceed 1 nm, the composition in the core is not
affected by trajectory aberrations. However, as already said in the case of Cr atoms, even if
the model reproduce qualitatively the experimental artefacts, it could underestimate their
intensity and spatial extension because all the input parameters are not known (e.g. field
evaporation of N and Ti inside a nitride) and because it is possible that some mechanisms are
not considered (e.g. poor conductivity of nitrides). In addition, when measuring the
composition, by introducing a small box inside particle, it is not possible to clearly define the
limit between the area containing Fe actually present in the particle, and Fe introduced by
overlap. In order to estimate the actual level of Fe, concentration profiles were drawn through
Ti-N rich particles with Rg > 2.5 nm in all samples (except as-milled one, where nanoparticle
with Rg=2.2 nm was studied). On the profiles, the level of Fe observed in the core of Ti-N
rich particles is in the range 10 to 20 at % in case of particles annealed at 600 and 700°C as
well as in hot extruded states. Smaller content of Fe (~5 at.%) is detected in particles after
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