364 Masoud Rashidi et al.


Figure 5. The composition of the core and the shells of the precipitates in the trial steel aged for (a) 24, (b) 1,005, (c) 3,000 h at 650°C obtained using the “Fe correction” technique on different iso-concentration surfaces. “Iso x–y” means a shell between the iso-concentration surfaces of Nb+Cr+N >x% and Nb+Cr+N >y% was analyzed. The non-visible error bars (1 SD) are smaller than the data point symbols.


By subtracting the obtained composition of different iso- surfaces, one can measure the composition of different layers of the precipitate. Figure 5 shows the composition of the precipitates layer by layer in the specimens aged for 24, 1,005, and 3,000 h at 650°C. In the specimen aged for 24 h, the Cr concentration in the core of theMXprecipitate is around 20 at%, and in the shell of the precipitate reaches 30 at% (Fig. 5a). The in-diffusion of Cr continues during thermal ageing. Figure 5b shows the composition of a precipitate after 1,005 h ageing at 650°C. The shell of this precipitate has reached the Z-phase composition, whereas the core of the precipitate requires more Cr for a full transformation to Z-phase. After 3,000 h ageing (Fig. 5c), the MX to Z-phase transformation is completed and the core and the shell of the precipitate have the same composition, showing that Cr diffusion into the MX is now completed and the precipitate has reached its equilibrium composition. The Cr diffusion into the existing MX precipitate has


been reported earlier (Danielsen & Hald, 2009; Cipolla et al., 2010) for V and Ta-based Z-phase precipitates using TEM techniques. The results from the current study show that the Nb-based Z-phase follows the same pathway in its formation. It is worth mentioning that the Z-phase is known to


have a composition of Cr1+xM1 −xN (Ettmayer, 1971). The results from APT also show that some of the Nb is replaced by Cr and from the Cr/Nb ratio x was determined to be 0.08. The N content is believed to be at 33 at%, whereas our APT results show that the N concentration is around 29 at% (and the N+C concentration around 30 at%). This difference in the N content can be presumably attributed to the lower detection efficiency for multiple hits in APT, which can lead to the loss of some of the N ions. As shown in Table 3, Z-phase can dissolve limited


amounts of other alloying elements such as Fe, and W. Even though the amount of C is very small in the trial steel, we can


see that some C can dissolve in Z-phase, see Table 3. Trace amounts of V and Ta existed in the trial steel as impurities. The concentration of these elements in the matrix was below the detection limit, so apparently all Ta and V are incorpo- rated in Z-phase. It is worth mentioning that the Ta and V concentrations in the core and the shell of the precipitates were almost constant.


CONCLUSION


This study provides a routine to analyze the chemical com- position of an entire precipitate rather than the core of the precipitate using high iso-concentration values. This tech- nique involves the following three steps; the precipitate is selected by using low iso-concentration values in a way that most ions belonging to the precipitate are collected. The gathered data gives the composition of the precipitate plus a contribution from matrix, due to the local magnification effect. The built-in peak deconvolution tool in IVASTM solves possible peak overlaps in the mass spectra. The matrix contribution can be deduced based on an element, which only exists in the matrix (in this case Co or Fe). The remaining ions belong to the precipitate. It was also shown that by comparing ions within different


iso-concentration surfaces, the composition of the core and the shell of a precipitate could be measured accurately. This technique was employed to analyze precipitates


with a high evaporation field in a Z-phase strengthened 12% Cr steel. It was shown that Z-phase forms by diffusion of Cr into the MX precipitates. The equilibrium composition of Z-phase corresponds to Cr1+xNb1 −xN with x =0.08.


ACKNOWLEDGMENTS


Swedish Energy Agency (contract number 31139-1), Swed- ish research programKME(contract numbers: 510 and 710),


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