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

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tel-00751814, version 1 - 14 Nov 2012<br />

Chapter 4. Nitri<strong>de</strong> Dispersion Strengthened Steels<br />

A threshold of 32 at.% (Cr+N) is used for cluster search algorithm. The corresponding<br />

filtered atom map is represented on Figure 4.35. Particles having different habit p<strong>la</strong>nes can<br />

be clearly distinguished on 3D reconstruction. In the first <strong>par</strong>t of the analysis, re<strong>la</strong>tively thin<br />

p<strong>la</strong>te-like <strong>par</strong>ticles are observed. The angle between the normal of these p<strong>la</strong>tes and the<br />

direction of analysis is about 30°. For further discussion, they are <strong>de</strong>noted as (1). In the<br />

second <strong>par</strong>t of analysis, ap<strong>par</strong>ently thicker p<strong>la</strong>te-like <strong>par</strong>ticles are visible. Their normal is<br />

perpendicu<strong>la</strong>r to the analysis axis. They are <strong>de</strong>noted as (2).<br />

Figure 4.35. Iso<strong>la</strong>ted Cr-N rich phases observed in nitri<strong>de</strong>d NDS steel after annealing<br />

at 700°C during 1h (only Cr, N species are shown). Two orientations of <strong>par</strong>ticles can be<br />

distinguished <strong>de</strong>noted as (1) (beginning of the analyse) and as (2) (second half of the<br />

analyse). Direction of composition profiles shown on Figure 4.36 and Figure 4.38 are<br />

indicated by dashed b<strong>la</strong>ck arrows. Analysed volume is 190× 50× 50 nm 3 .<br />

A composition profile drawn perpendicu<strong>la</strong>rly to the <strong>par</strong>ticles (1) (as indicated by dashed<br />

b<strong>la</strong>ck arrow on Figure 4.35) is shown in Figure 4.36. Enrichment of Cr and N as well as<br />

<strong>de</strong>pletion of Fe corresponds to Cr-N rich <strong>par</strong>ticles. The contents of Cr and N insi<strong>de</strong> these<br />

<strong>par</strong>ticles reaches 40 at.% and 26 at.% respectively. This approximately corresponds to the<br />

Cr2N stoichiometry of hexagonal V2N-type Cr nitri<strong>de</strong> [33]. Here again, a slight <strong>par</strong>t of the<br />

iron measured here certainly comes from the matrix [32].<br />

As for <strong>par</strong>ticles <strong>de</strong>noted as (2), they are also p<strong>la</strong>te-like, but appear thicker than the<br />

popu<strong>la</strong>tion (1). The thickness of these p<strong>la</strong>tes varies from 5 to 20 nm, width from 10 to 42 nm<br />

and their length from 17 to more than 44 nm. The length of the bigger <strong>par</strong>ticle (that is clearly<br />

seen in the end of analyze) cannot be evaluated from APT, since its size exceeds the analyzed<br />

volume (more than 44 nm). These <strong>par</strong>ticles are thick enough to have no or limited bias in<br />

core composition.<br />

20 nm<br />

190×50×50 nm 3<br />

(2)<br />

(1)<br />

172

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