Core-Shell Structure of Intermediate Precipitates 363


Table 2. The Routine for Calculating the Composition of Small Precipitates Based on the Matrix Deduction From the Precipitate+Matrix Atoms.


Matrix Matrix+Particle


Elements Counts % Counts % Cr


N Fe


Nb W


Co V B C


Mn Ta Ni Si


Correction Based on Removing Co


CoPM/ CoM


2,431,673 12.30 60,411 26.05 0.0032 3,697 0.02 52,839 22.78


15,860,080 80.26 53,622 23.12 75 0 51,175 22.07 34,836 0.18 2,231 0.96 1,038,645 5.26 3,324 1.43 672 0 244 0 163 0


1,672 0.72 1,387 0.60 2,448 1.06


83,254 0.42 690 0.30 0 0


212 0.09


195,529 0.99 1,343 0.58 113,105 0.57 549 0.24


Expected Matrix


7,782 12


50,757 0


111


3,324 2 1 1


266 0


626 362


PM–EM (particle)


52,629 52,827 2,865


51,175 2,120 0


1,670 1,386 2,447 424 212 717 187


Particle (at%)


Correction Based on Removing Fe


FePM/ FeM


31.20 0.003 31.32 1.70


30.34 1.26 0


0.99 0.82 1.45 0.25 0.13 0.43 0.11


Expected Matrix


8,221 12


53,622 0


117


3,511 2 1 1


281 0


661 382


PM–EM (particle)


52,190 52,827 0


51,175 2,114 −187 1,670 1,386 2,447 409 212 682 167


Particle (at%)


31.61 32.00 0


31.00 1.28


−0.11 1.01 0.84 1.48 0.25 0.13 0.41 0.10


The table shows both the “Co correction” and “Fe correction” routines. The expected matrix in the precipitate+matrix atoms can be calculated based on the CoPM/CoM or FePM/FeMratio for “Co correction” and “Fe correction” routines, respectively. The minus value for the Co content in the “Fe correction” is due to the fact that Z-phase can dissolve some little Fe. PM, precipitate+matrix atoms; EM, expected matrix in the matrix+precipitate atoms.


Table 3. The Chemical Composition (in at%) of a Z-Phase Precipitate Obtained From the Z–Nb Trial Steel Aged for 3,000 h at 650°C. Fe


Ni Z-phase 1.96 0.09 Co 0.03 35.69


Cr W Nb 0.86


precipitate plus matrix composition (Table 2). At this point, using the built-in peak deconvolution tool in IVASTM allows us to solve peak overlap issues. By considering the fact that there is almost no Co in Z-phase (see Table 3), that Co is not enriched in the matrix–precipitate interface, and that Co has a uniformdistribution in the matrix (see Figs. 3a, 3b), one can subtract the Co content, and proportionally other alloying elements in the matrix, from the “precipitate plus matrix” composition. The remaining atoms belong to the precipitate, and hence the composition of the preci- pitate is obtained. A similar technique was first used by Fischmeister et al., 1988 to study the composition of small precipitates in high-speed steels using atom probe field ion microscopy. In case of very small precipitates, the number of Co


atoms is low and consequently large errors could be intro- duced to the composition. Thus, one can deduce the matrix composition based on the “Fe correction”, meaning that the Fe and proportionally other elements are removed from the “precipitate plus matrix” composition. By considering the fact that Z-phase can dissolve 1–3 at% Fe (see Table 3), a small systematic error in the composition was introduced. The minus value for the Co content in the “Fe correction” method is due to the small solubility of Fe in Z-phase. However, as can be seen in Table 2, there is a good agreement between the compositions obtained by “Co correction” and “Fe correction”.


Ta V 30.13 0.13 0.49 1.61 28.85


C B N Mn 0


0.11 In Figure 4, the composition of a precipitate obtained


from the “Fe correction” routine is presented for the trial steel aged for 24 h at 650°C. Note that the composition varies slightly between the iso-concentration surface value of 50 and 30. Iso 50 shows the composition of the core of the precipitate, whereas iso 30 contains both core and shell of the precipitate. The graph shows that the Cr concentration in the shell of the precipitate is higher compared with the core of the precipitates.


Figure 4. The composition of a precipitate in the trial steel aged for 24 h at 650°C obtained using different iso-concentration values and the “Fe correction” technique. The non-visible error bars (1 SD) are smaller than the data point symbols.


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