22.08.2015   Comprehensive nuclear materials

CEC characterization

Evidence from various independent studies using techniques such as FEGSTEM or 3DAP has con­firmed the small solute clusters to be multi-alloyed with minor constituents of the steel. An example of the composition of a cluster formed in an irradiated low Ni A533B weld metal is shown in Figure 7. The results of the 3DAP study clearly show that the CECs are alloyed

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Table 6 Techniques for the characterization of irradiation-induced solute clusters

Подпись:Existing Comment

techniques

Shape and size of solute clusters

 

3DAP, SANS, TEM/FEGSTEM

 

Analysis of the irradiation-induced scattering data from SANS provides a feature size distribution (for clusters 1-10 nm diameter) averaged over a volume approximately 50 mm3. 3DAP (LEAP) provides a direct estimate of the size of clusters formed in a region ~60 x 60 x 100 nm3

3DAP yields a direct measure of the composition of each cluster observed. SANS measurements of irradiation-induced scattering in a strong magnetic field provide indirect evidence of cluster composition. Techniques are starting to be developed for measuring the level of Fe in Cu-enriched precipitates using advanced TEM techniques. At present, uncertainties on composition of small (~2 nm diameter) are large FEGSTEM and 3DAP can directly measure the number density of visible clusters in the analyzed volume. In practice, Nd estimates from all techniques are subject to considerable uncertainty 3DAP provides a direct measure of the level of Cu in the matrix that is not in clusters. In the FEGSTEM techniques have been developed for measuring the level of matrix Cu from areas where clusters are not observed51

 

3DAP, SANS, FEGSTEM/EDX

 

Composition of solute clusters

 

Number density (Nd) and volume fraction of solute clusters

Level of matrix Cu

 

3DAP, SANS, FEGSTEM/ EFTEM 3DAP,

FEGSTEM/EDX

 

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Silicon

Nickel

Copper

Figure 7 Example of single irradiation-induced cluster approximately 3nm in diameter.

with Mn, Ni, and Si; there is also evidence of an association with P near the cluster-matrix interface (not shown). Similar cluster compositions have been found by other workers using other atom probes and FEGSTEM (which is not sensitive to Fe in clusters), and there is good agreement between the techniques.

The atom probe data indicate that the clusters are dilute in that the majority element is Fe.57-59 In contrast, analysis of SANS data is often undertaken assuming that irradiation-induced clusters are non­magnetic (implicitly Fe-free). The assumption of low levels of Fe in the Cu clusters was supported by thermodynamic calculations that predicted low levels of Fe in such precipitates.47 Furthermore, PAS-OEMS on as-irradiated (290 °C) Fe-0.9Cu or Fe-0.9Cu-1Mn indicate that the clusters in this con­dition are not magnetic and therefore very unlikely to contain Fe,60 although it should be noted that clusters in model alloys may be different from those found in RPV steels. Carter demonstrated that the scattering observed in SANS experiments is not inconsistent with the presence of magnetic clusters.57 More recently, Morley et al61 attempted to characterize the extent of trajectory aberrations in the atom probe that might give rise to an incorrect estimate of the true Fe content in small clusters in thermally aged (330-405 °C) RPV steels. He concluded that the clusters do contain Fe, but the levels are lower than those measured. Furthermore, he found that the con­centration of Fe in the precipitate phase is a function of aging temperature with less Fe at higher aging temperatures. Consensus has not yet been reached on the precise Fe content in irradiation-induced clus­ters in ferritic steels and is the subject of ongoing research.

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