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

tel-00751814, version 1 - 14 Nov 2012
Chapter 4. Nitride Dispersion Strengthened Steels
Individual atom maps, showing GB as well as nanoparticles (dashed grey circles) in
powder annealed at 600° during 1h are depicted on Figure 4.1 (a). From visual inspection, Cr
enrichment is clearly seen and traces of W, Ti and N(Si) can be distinguished.
(a)
(b)
Figure 4.1. Grain boundary observed in MA NDS powder annealed at 600°C during
1h. (a) 3D atom maps are shown for Cr, Ti, W and N(Si) species; Ti-enrichment
corresponding to nanoparticles can be seen in Ti map (dashed circles). (b) Variation of
Cboundary/Cmatrix measured across the grain boundary plane as indicated by arrow in (a). (Cr
and Fe are on the left profile and N(Si), Ti, and W species on the right one). Cboundary
corresponds to the concentration measured in the sampling volume 6×8×1 nm 3 . Cmatrix is
equal to the concentration of the element measured in the matrix.
Composition profile drawn across this grain boundary (as indicated in Figure 4.1 (a))
confirms Cr enrichment (Figure 4.1 (b)) as well as enrichment in Ti, N(Si) and W. However,
it should be mentioned that concentration profile exhibit asymmetric behaviour with respect
to Cr, which segregates up to 10 nm in the grain 1. The maximum peak of W is slightly
shifted in comparison to Ti, N(Si) and Cr peaks (Figure 4.1 (b)). Normally, the highest solute
concentrations correspond to GB`s core, however, elastically strained region near GB also
can result in significant segregation outside the core [5, 6].
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