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

tel-00751814, version 1 - 14 Nov 2012
Chapter 1.Bibliography
V-and Cr-enriched shells are also observed around all investigated particles by TEM
(EDS/EELS) in Eurofer ODS [90].
Taking into account difficulties of quantification of chemical composition by TEM, these
results are in good agreement with results reported by E. Marquis [87].
However, the same MA 957 ODS has been studied by Miller et al. [91] in the as-received
state and after isothermal annealing treatments up to 24 h at 1300°C (by APT). This author
does not report any information about the presence of a core shell structure!
Hsiung et al [43,92,93] studied by HRTEM the interfacial structure of nanoparticles in
Fe-16Cr-4Al-2W-0.3Ti (designated as K3) mechanically alloyed with 0.3 wt.% of Y2O3.
After hot extrusion at 1100°C and additional annealing at 1050°C/1h, the sizes of the studied
particles vary from 1.7 to 30 nm. A structure with an Y4Al2O9 core associated with a shell has
been observed in both large and small nanoparticles, Figure 1.12. However, in contrast to the
previous reported core-shells structures (by Marquis [87], Castro [89], Klimenkov [90]), the
author shows that the shell is amorphous. The author suggests that amorphous shell results in
a partial crystallization of amorphous nanoparticles during consolidation, whereas
amorphization of these oxides particles appear during MA process. The core/shell structure
disappears after prolonged annealing treatment (900°C/168 h) for particle with diameter
larger than 20 nm, indicating that these core/shell structures are far from chemical
equilibrium.
The same structure of crystallize core and amorphous shell is found in hot extruded
(1107°C) Fe-20Cr-4.5Al-0.34Ti+0.5 wt.% of Y2O3 (designated as MA 956) ODS steel. In this
case significantly thicker shell is observed in comparison to K3.
By comparing the HRTEM observations for K3 and MA 956 materials with results of
Marquis [87], the author suggest that the content of these solute-enriched shells may differ
and depend on the initial composition of the ODS steel. He suggests that Y4Al2O9 and YAlO3
core forms in the case of Al-containing K3 and MA materials, Y2TiO5 and Y2Ti2O7 may form
in the case of Ti-containing ODS steel and Y2O3 may form in the case of Eurofer that do not
contain either Al nor Ti. It is supposed that shell forms as a result of solutes depletion from
the core during crystallization of the particles upon annealing. The formation mechanism
proposed by the author to rationalize the existence of these core-shell structures are discussed
in details in next section.
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