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

tel-00751814, version 1 - 14 Nov 2012
Chapter 2. Materials, experimental and simulation techniques
contrast, for a particle with a lower field of evaporation, the image of the nanoparticle is
compressed and ions coming from the surrounding matrix fall into the particle (Figure 2.14
(d)). The local magnification can consequently modify the morphology, dimensions of
precipitates and introduce bias in the measured chemical composition. These effects have
been observed in the studied ODS and NDS materials and are discussed below.
(a)
(b)
Figure 2.14. Schematic representation of local magnification effects in case of
evaporation of (a) high-field precipitate; (b) low-field precipitate; Simulation of impact
images for binary AB alloy containing a spherical β particle (eight interatomic distances). β
phase is pure in B and its environment is pure in A. These two simulated images are cross
sections of reconstructed precipitates with (c) Eprec > Ematrix and (d) Eprec < Ematrix: B atoms
are in black and A are in gray. The images are reproduced from [36,37].
An indication of the presence of the local magnification effect during an analysis is the
detection of a local change in the atomic density (i.e. number of atoms per unit of a volume)
in the nanoparticles compared to matrix. Nanofeatures in ODS steels generally exhibit a lower
field of evaporation than the surrounding matrix (� p