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

tel-00751814, version 1 - 14 Nov 2012
General conclusions and perspectives
powder. The origin of the latter one is not clear at the moment, as suggested above. 2) During
subsequent annealing and consolidation, Y-O-rich clusters act as nuclei for formation of Y-Ti-
O-rich nanoparticles.
On that part, perspectives works are numerous and further questions should be answered:
� Origin and the nature of these core-shell structures of nanoparticles are not
clear at the moment. Moreover, influence of ageing times on the stability of these core-
shell structures need to be studied. For instance, presence of shell has been observed after
prolonged annealing during 4, 24 and 48 h at 750º in Fe-12Cr ODS alloy [12].
� An important question remains concerning the real compositions of oxide
particles. As an example, the Fe content measured by APT varies in a wide range [5, 10,
13] and should be deeply studied. In order to answer this question, a specific study (PhD,
CPR ODISEE) started in the Groupe de Physique des Matériaux (GPM), University of
Rouen. In this study, combination of APT and TEM characterisations, coupled with
modelling of field evaporation during APT experiments should give some information on
the accuracy of chemical composition obtained from the analyses of nanooxides particles
embedded in a ferritic matrix.
As for MA NDS steel, studies of the microstructure evolution have been carried out
directly after MA as well as after annealing treatments up 1000°C. The APT technique
revealed presence of nanoparticles in all states. They are mainly enriched in Ti and N whereas
their stoichiometry differs from the equilibrium stoichiometry TiN. This can results from the
“non-equilibrium” MA process. A small amount of O (1-5 at.%) is also measured, indicating
that these phases can be designated as oxy-nitrides [14]. Moreover, a second family of
particles has been observed: Ti-Al-O-rich. Al and O are considered as impurities coming from
the elaboration process. This indicates that the control of the TiN particles size distribution is
also controlled by the purity of the alloy and process itself (introduction of impurities).
The precipitation kinetics of Ti-N-rich particles can be described as a three stages
process, according to the experimental observations: i) nanoparticles are formed during the
MA process and therefore can contain considerable amounts of matrix Fe and Cr atoms. ii)
subsequent annealing results in a partial rejection of Fe and Cr atoms from the nanoparticles
leading to the formation of a Cr-rich shell (annealing at 600, 700°C/1h and hot extruded
state); iii) coarsening of nanoparticles starts when all N and Ti is removed (down to solubility
limits) from the matrix.
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