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

tel-00751814, version 1 - 14 Nov 2012
Chapter 3. Oxide Dispersion Strengthened Steels
Figure 3.4. SEM image of APT sample preparation by Focused Ion Beam (annular
milling). Two contrasts are clearly observed indicating that two phases are observed.
b) Evaporation artifacts
In the 3DAP reconstructions of ODS model alloy, a shift between Ti and Y atom
positions is observed, as shown in Figure 3.1 (c). It should be mentioned that, the major part
of Y and almost all Ti are detected as molecular ions. They are bonded with O in the form of
TiO, TiO2 and YO. In addition, CrO, CrO2 and FeO molecular ions are also observed. Thus,
in order to study the Y/Ti shift in more details, concentration profiles showing only YO, TiOx,
FeO and CrOx molecular ions are shown on Figure 3.5 (a-c).
The YO and TiO+TiO2, FeO, CrO+CrO2 peaks correspond to the Y-Ti-O rich phase on
the concentration profile. The shifts of TiO, TiO2 molecular ions relatively to YO are clearly
seen in the z direction (direction of analysis). The same observation is done for CrO, CrO2
and FeO molecular ions.
The shift between these ions is estimated as follow. First, the distance between YO and
TiOx top peaks, d, are determined (as indicated in Figure 3.5 (a)). Then, the width of each
YO, TiO and TiO2 peaks (namely LYO and LTiOx) at half maximum height is estimated and the
difference between these values (LTiOx - LYO) is calculated. All these characteristics (d, LYO,
LTiOx and LTiOx -LYO) are summarized in Table 3.2. d varies from 0.2 to 2.1 nm and (LTiOx -LYO)
varies from -0.9 to 0.7 nm. As it can be seen from Figure 3.5 (a), in one case the TiOx peak is
absent in comparison to corresponding YO peak (peak 4 on the figure).
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