Atom Probe Tomographic Characterization of Nanoscale Cu-Rich Precipitates 343


Figure 4. APT results from analysis of 17-4 PH SS tempered at 420°C for 4 h after solution-treatment. Atom maps (32×32×37 nm3)of (a) Cu, (b) Ni, (c) Al, (d) Mn and (e) Si. f: Atom maps of Ni, Mn and Cu together with 10 at.% Cu isoconcentration surfaces. g: Enlarged part (12×6×3 nm3) and nearest neighbor analysis of (h) Ni and (i) Mn.


(Fig. 3a), but the curves for Ni and Mn atoms (Figs. 3f, 3g) more or less overlap. This indicates that Cu clustering occurs, whereas Ni and Mn atoms are randomly distributed without any distinct clustering or segregation for this tempering state.


Tempering at 450°C


Figure 4 shows the atom maps and the corresponding NND analysis of the sample tempered at 450°C. Some regions in the atom maps are obviously enriched in Cu and Ni (Figs. 4a–4f) and slightly enriched in Mn (Figs. 4c, 4g), which is consistent with NND analysis (Figs. 4h, 4i), whereas both Si and Al (Figs. 4d, 4e) remain randomly distributed. Isoconcentration surfaces of 10 at%Cu, outlining Cu-enriched regions (precipitates) and their corresponding proximity histograms are shown in Figure 5. The CRPs are equiaxed and contain a large amount of Ni and Mn. Moreover, the content of Cu in the core of the precipitates is ~70 at% and those of Ni and Mn are ~10 and 4 at%, respectively. On the other hand, the content of Fe in the core of the precipitates is about 15 at%, which is much lower than in the matrix. In addition, Al and Si are still randomly distributed in the core of the precipitates and Cr is rejected fromthe core of the Cu-rich particles into matrix. (The distribution of Cr in the samples tempered at 510 and 570°C is similar to that at 450°C, as shown in Figs. 7, 9.)


Figure 5. Proxigram concentration profiles of Fe, Cu, Cr, Ni, Al, Si and Mn based on 10 at.% Cu isoconcentration surfaces in Fig. 4 (spherical orange surfaces).


Tempering at 510°C


Figure 6 shows atom maps of the sample tempered at 510°C. The atom distributions indicate that the CRPs contain


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