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

tel-00751814, version 1 - 14 Nov 2012
Chapter 4. Nitride Dispersion Strengthened Steels
metallurgical state. In the present case, since there was no systematic study of the nature and
misorientation of GB, the effect of annealing temperature on Cr segregation intensity can not
be deduced.
In addition, MA NDS powders are subjected to mechanical alloying, which is known as
a highly non-equilibrium process. During this process, non-equilibrium point defects are
introduced into material. This super-saturation of point defects (vacancies) can lead to non
equilibrium intergranular segregation. Indeed, two basic segregation mechanisms can be
distinguished: (a) equilibrium and (b) non-equilibrium [4, 10]. The basis of equilibrium
segregation relies on the existence of crystalline disordered region, boundary or interface,
where misfitting impurity atoms can find low-energy sites [11]. Non equilibrium segregation
is due to the excess of vacancies which can migrate toward GB where they annihilate.
Vacancy fluxes can induce solute fluxes by solute dragging. If the time of annealing is
enough, so that diffusion process is allowed to reach full equilibrium, equilibrium
redistribution of solutes appears. In the present case, such phenomena can occur during
annealing of as-milled powder. Since as-milled powder is subjected to high temperature
annealing within short time (1 hour), this time might not be sufficient to reach equilibrium in
the system. The observed segregation behaviour in MA NDS steel may be combination of
both equilibrium as well as non-equilibrium aspects.
To clarify these points a specific experimental study is needed (but not in the program of
this PhD work) to: (i) evaluate segregation intensity depending on GBs nature; ii) study
segregation kinetics as a function of longer ageing times as well as varying temperatures.
I.3. Nanoparticles characterization
a) As-milled condition
Small nanoparticles are observed in the as-milled powder (Figure 4.5 (a, b)). The
shape of these nanoparticles is roughly spherical. However, slight elongation in the plane
perpendicular to the evaporation direction can be noted. Such a shape can result from local
magnification effects [12, 13] occurring during field evaporation of a spherical particle with
an evaporation field higher than the matrix one (high field particle). To clarify this point,
concentration profiles were plotted through particles. Such profile, oriented parallel to the
specimen axis, is shown on Figure 4.6 (b). An enrichment in Ti, N and O can be clearly seen
(Figure 4.6 (a)). In the Ti, N and O enriched area, the atomic density is 5 times lower than in
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