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

tel-00751814, version 1 - 14 Nov 2012
Chapter 4. Nitride Dispersion Strengthened Steels
length. These values are measured manually. The width for both families (orientation) of
particles varies from 3 to 13 nm, length from 5 to 28 nm and thickness from 1.6 to 5 nm. It is
rather difficult to compare these values with the SANS data [2] since, in this latter case, the
data are interpreted on the assumption that particles are spherical, which is not at all
consistent with APT observations.
Concerning chemical composition, the nitride core composition as a function of
thickness of the plates is given on Figure 4.33. Numbers (1) and (2) given next to values of
thickness, correspond to the two orientations of particles (as indicated in Figure 4.32). No
significant difference of chemical compositions as a function of the thickness or the
orientation of the particles is observed, thus they are presented together for both orientations.
Variations of measured chemical composition are observed. For instance, the Cr content
varies from 29 to 51 at.% and N from 17 to 44 at.%. The mean chemical composition for
both families of particles is: 46.4±0.80 Cr, 32.3±0.80 N, 20±0.6 Fe, 0.74±0.20 Ti and
0.13±0.05 Al at.%. The chromium over nitrogen ratio (Cr/N) is 1.4±0.1.
Content of Cr, N, Al, Ti and Fe (at,%)
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
Cr N Ti AL Fe
mean
5,19 (1)
5 (2)
4,62 (1)
4,4 (2)
4,11 (1)
4,1 (2)
3,99(1)
3,94 (1)
3,86(1)
3,78 (1)
3,65 (2)
3,65 (2)
3,62 (1)
3,6 (2)
3,6 (2)
3,56 (1)
3,53 (1)
3,5 (1)
3,43 (1)
3,35 (1)
Thickness (nm)
3,3 (2)
3,3 (2)
3,26(1)
3,24(1)
3,2 (1)
3,1 (2)
3,0 (1)
2,88 (1)
2,8 (2)
2,64(1)
2,51(1)
2,48(1)
2,43(1)
2,17 (1)
2,13 (1)
2,12(1)
2,07(1)
2,02(1)
1,87 (2)
1,71 (1)
1,67(1)
1,6 (2)
Figure 4.33. Composition of particles observed in nitrided NDS steel annealed at
600°C/1h as a function of their thickness. Composition is calculated for two families of
particles (denoted as (1) and (2)) as indicated in Figure 4.32. Number (1) and (2) next to
thickness indicate on family of particles. Only Cr, N, Ti, Al and Fe (at.%) are shown.
In APT, Cr-N rich particles have higher evaporation field than the ferritic matrix (similar
observation is done in case of Fe-5Cr alloy in [29]). This leads to a local magnification
effects [12, 13] and as a consequence, slight increase of particles dimensions in the lateral
direction may occur. As for their chemical composition, it was shown in Ph.D. work of
Jessner, that the nitrides can contain some iron [18]. However, trajectory overlaps can
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