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

tel-00751814, version 1 - 14 Nov 2012
Chapter 3. Oxide Dispersion Strengthened Steels
800°C). If to consider measured mean concentrations in the matrix (Table 3.8), they exceed
the equilibrium values reported above. In contrast, Ti has a reasonable solubility in iron (>3.0
at.% at 800°C [14]). However a significant smaller amount of Ti is detected in the matrix of
ODS model alloy. It should be mentioned that in multicomponent system, the Ti solubility
limit given above, may change. Supersaturation of the matrix in Y and O is not surprising
since capacity of ball milling to extend the solubility limits have been demonstrated [15].
Finally it should be mentioned that, the supersaturation of the matrix is more pronounced
for O (0.48 ± 0.06 at.%) comparing to Y (0.08 ± 0.03 at.%) and Ti (0.12 ± 0.03 at.%). First
principles calculations by Fu et al [1, 16], show that O form strong bonds with vacancies.
Therefore, the higher solubility of O in bcc iron is possible due to its strong interaction with
vacancies introduced during the MA process.
As a comment, the results from the 12 th analyse in Table 3.8, show smaller Y and O
concentrations. The values are more comparable to solubility limits, whereas again a
significant smaller amount of Ti is measured. A good agreement with the nominal
composition of the FeCrW pre-alloy powder without addition of TiH2 and Y2O3 is found. So,
it can be assumed that, in that specific experiment, no TiH2 nor Y2O3 dissolution occur.
I.2. Discussion
The same ODS model alloy was also analysed by neutron and X-ray diffraction (XRD) at
different stages of mechanical alloying and subsequent annealing treatments [5, 6].
Concerning the first stage, when FeCrW prealloyed powder is mechanically alloyed with
TiH2, no characterisation by APT is available. The results of X-ray as well as neutron
diffraction at this step show that peaks corresponding to TiH2 decrease with milling time and
disappear after 48 hours. From these results the dissolution of a major part of TiH2 in the Fe-
9CrW matrix [6] is supposed.
At second stage, FeCrW-Ti powder is mechanically alloyed with Y2O3. Both neutron
diffraction [5,6] and APT results are available. Neutron diffraction shows that the intensity of
the signal corresponding to the Y2O3 decreases with milling time. A single phase (alpha Fe) is
observed after 24 h of milling. The authors suggest two possible mechanisms for Y2O3
diffraction peaks diminution [5,6]: i) fragmentation with subsequent dissolution of Y2O3
powder into Fe-rich matrix resulting in the formation of a supersaturated solid solution and ii)
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