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

tel-00751814, version 1 - 14 Nov 2012
Chapter 3. Oxide Dispersion Strengthened Steels
hot extruded state) incorporation, whereas just slight enrichment in Y is observed (~2 times
higher for both annealed and hot extruded state). As it will be shown below, this can be linked
with the fact that Y is the slowest diffuser. In addition, some part of Y (36% owing to APT) is
located in Y-O rich nanoclusters just after mechanical alloying step.
c) Formation mechanism of nanoparticles in ODS steel
Different formation mechanisms of nanoparticles have been detailed in Chapter 1
(Section ΙΙΙ.3 (c)). In present case, nanometric nanoparticles are observed in the ODS steel
just after mechanical alloying step.
Considering the materials that have been studied in this work, a formation mechanism for
oxide nanoclusters could be suggested. Mechanical alloying results in: a) dissolution of Y2O3
initial oxide inducing supersaturation of Y and O in the matrix (64 and 72 % of Y, O
respectively) and b) formation of Y-O-enriched aggregates. These Y-O enriched phases could
contain some amount of Ti (up to 1.6±0.5 at.%). However, the origin of YO-rich aggregates is
still not clear. Two possibility can be considered: (i) they are produced by the direct
transformation (under ball milling conditions) of Y2O3 oxide towards Y-O-rich phases or (ii)
complete dissolution of Y2O3 occurs first and then Y-O-rich phase nucleates. Since Y2O3 is
very stable oxide (heat of formation of - 1907 kJ.mol -1 [9]), milling time and intensity of
industrial attritor may not be sufficient fully dissolve it in Fe-Cr matrix. From this point, the
hypothesis that MA results in only a partial dissolution of original Y2O3 powder seems to be
probable. Tendency of Y2O3 to amorphization during ball milling process has been widely
demonstrated [17–19].
Subsequent annealing or hot extrusion, results in significant depletion of matrix in Y, O
and Ti, whereas nanoparticles become more enriched. Y-O nanoparticles in as-milled state,
act as nuclei for the formation of the nano-oxide particles. Y, O and Ti are incorporated to
these nuclei, resulting in their enrichment. After annealing or hot extrusion, most of Ti and Y
are located into nanoparticles. The schematic representation of nanoparticles formation is
summarized in Figure 3.17.
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