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

tel-00751814, version 1 - 14 Nov 2012
Chapter 1.Bibliography
An example is the work of Okuda et al. [94] who studied the change of behaviour of a
Fe-13Cr alloy during mechanical alloying (with 3wt.% of Y2O3) and subsequent annealing
treatments. Another example can be the study of Alinger et al. [56] on Fe-14Cr ODS steels.
He confirmed that Y2O3 dissolution in Fe-14Cr alloy matrix is the dominating process during
MA, resulting in a highly supersaturated solid solutions.
Studies by Larson [49], Miller [51] and Yamashita [95] are also in this direction.
However, it should be mentioned that in their works, APT investigations of ODS material are
performed only in the final state, passing over investigation of material immediately after MA
step. Their conclusions are mainly based on size differences of nano clusters in final state and
the initial size of Y2O3 powder (~2 nm for nanoclusters in as-received state and 17-31 nm for
initial Y2O3 powder [51]) introduced during MA process. Another argument is that, the
chemical composition of nanoclusters estimated by APT, is more consistent with TiO than
Y2O3 stoichiometry.
However, some other groups of researchers show that the formation mechanism of these
nano-oxides is not limited to dissolution/precipitation mechanisms as reported above. Among
them, recent work by Brocq [96,97] on Fe-14Cr-2W-0.25Ti ODS showed that the role of
mechanical alloying appears to be more complex. It could be summarised as follow:
i) Dissolution of primary oxide and nucleation of first nanoclusters can occur
simultaneously during ball-milling;
ii) Nucleation and growth of nanoclusters continue after subsequent annealing.
The nano-oxides formed this way can be controlled through variation of ball-milling
parameters (intensity, temperature and atmosphere) and annealing parameters (duration and
temperature). However it should be mentioned that specific conditions of ball milling
(reactive ball milling: YFe3 and Fe2O3 are used as reactants) are used for this study.
Nevertheless, the possibility that nanoclusters in conventional ODS can originate from the
same mechanism are not excluded.
An alternative formation mechanism of nano-oxides is suggested by Hsiung et al
[43,92,93]. HRTEM investigations of Fe-16Cr-4Al-2W-0.3Ti ODS (designated as K3) in its
final state, revealed some Y2Al2O9 crystallite (in the core) surrounded by an amorphous shell.
Based on this observation, the author suggests that the formation mechanism of nanoparticles
may include three stages (Figure 1.13.):
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