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

tel-00751814, version 1 - 14 Nov 2012
Chapter 1.Bibliography
HIP is more interesting resulting in nearly isotropic characteristics of the material.
III.3. Microstructure and influence of elaboration parameters
ODS steels is designed with body-centered cubic (bcc) matrix (which is less sensitive to
swelling issues in comparison to face-centered cubic austenitic type matrix), reinforced by
dispersion of nano-oxides. Presently, different ODS steels are being developed for fission and
fusion applications in Japan –F4, K1, K4, K3 [22,43,58], in Europe - Fe-18Cr-1W-0.3Ti
[54,59], Eurofer [60] and in United States 12YWT and 14YWT [27,51,61]. These ODS
contain different amounts of Cr (9-20 wt.%). Major part of them contains Ti and W as well as
other alloying elements (Mo, Ni, Mn, V, C, Ta, Al).
The current understanding of the microstructures of current ODS steels, the effect of
composition and key processing parameters are discussed below.
a) Ferritic or ferritic/martenticitic (F/M) matrix
Due to the different chromium content, ODS steels may have different microstructures:
→ODS steels that contain between 9 and 12 wt.% of Cr, (see Fe-Cr phase diagram in
Figure 1.8) have martensitic structure. These steels are austenitic above 800°C. Martensitic
transformation occurs during the quench from the austenitic temperature down to room
temperature. After such treatment, the microstructure consists of laths with a very high
density of dislocations and the material is extremely hard (and brittle) [62]. A tempering heat
treatment must be given to provide a partial recovery of the dislocation network. Martensitic
9-12 %Cr ODS can be used for application in a temperature range below 800°C in order to
avoid high temperature phase transformation into austenite [13].
→ODS steels containing more than 12 wt.% of Cr, present a fully ferritic matrix up to the
melting temperature (see Fe-Cr phase diagram in Figure 1.8). This type of materials is
envisaged for application in extended range of temperatures up to 1100°C in comparison to
martensitic ODS steels [59].
In order to decrease the degree of anisotropy in the direction of extrusion (if hot extrusion
is used) for ferritic MA-ODS steel, recrystallization process may be used [64]. However, this
is not so easily achievable. Up to now, all worldwide efforts are directed to overcome
“bamboo-like grain structure and a strong deformation texture” [23] observed after hot
extrusion in ferritic ODS steels.
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