Core-Shell Structure of Intermediate Precipitates 361


Table 1. Chemical Composition of the Investigated Trial Steel in Both Atomic and Weight Percent (Fe in balance). Steel


Z–Nb wt% at%


0.005 0.02


C Mn 0.50


0.50 Si


0.30 0.60


Cr


11.64 12.64


strengthened steels are described elsewhere (Danielsen & Hald, 2009; Rashidi et al., 2014; Liu et al., 2016a, 2016b). Table 1 presents the chemical composition of the studied trial steel in both weight and atomic percent. APT specimens were prepared using the standard two-


step electropolishing procedure (Miller & Forbes, 2014). The needle shape specimens with a final radius <50nm were analyzed in an Imago local electrode atom probe (LEAP) 3000X HR instrument (CAMECA, Madison, WI, USA). During theAPT measurement, the specimens were held in the temperature range of 55–60 K. The instrument was operated in the pulsed laser mode with a pulse energy of 0.3nJ (Liu & Andrén, 2011) and a pulse frequency of 200 kHz. The acquired data were analyzed using the IVASTM software.


RESULTS AND DISCUSSION


Several specimens from the trial steel were successfully ana- lyzed using APT after ageing time of 24, 1,005, and 3,000 h at 650°C. Figure 1 shows the APT data reconstruction of Z–Nb steel after 24 h of ageing at 650°C. At this stage, small


1.47 1.47


Ni W Co 2.82


0.87 B


5.4 5.2


0.004 0.02


N


0.036 0.15


Nb


0.26 0.16


blade-like precipitates are formed in the steel. They are highlighted using iso-concentration surfaces (Miller, 2000) of Cr+Nb+N >25 at%. In Figure 1, several precipitates are seen, some isolated, some interconnected. The interconnected precipitates at the bottom of the APT reconstruction seem to lie on a plane, presumably a lath boundary; and the ones at the top appear to lie on a dislocation. Although all are rich in Cr, Nb, and N, there are some fluctuations in the composition. This is probably due to the ongoing dissolution of unstable (Cr, Nb)2Nand the transformation ofMX to Z-phase. As this transformation occurs byCr diffusion into the precipitate, it is of great interest to investigate any concentration gradients in individual precipitates. These precipitates have a higher evaporation field


compared with the matrix; this means that they locally gen- erate a higher field by assuming a smaller radius that increases the local magnification. Thus some ions stemming from the precipitates overlap with ions from the matrix at the detector, and a decrease in the density of hits appears (Gault et al., 2012), see Figure 2. Threepossiblewaystoanalyze thesesmall precipitates


using IVASTM software are compared: one-dimensional (1D) concentration profiles, proxigrams, and iso-concentration sur- faces. The two latter methods require that the precipitate stu- died be well separated from other precipitates and boundaries. A 1D concentration profile across a precipitate and a


proxigram (see Figs. 3a, 3b) provide important information on the distribution of the elements. For example, it is shown that the precipitate is depleted in Fe and contains Cr, N, and Nb. However, one must be very careful interpreting the results because of peak overlaps between different ions, for


example, 56Fe2+ and 14N2+, 28Si2+ and 14N+, and 54Fe2+ and 54Cr2+. In addition to the peak overlaps, the obtained data are disturbed by the local magnification effect that gives a contribution from the matrix to the analysis. Out of the three aforementioned techniques, it is only


Figure 1. Atom probe tomography data reconstruction of Z–Nb trial steel aged for 24 h at 650°C. Pink dots represent Cr ions. The brown iso-concentration surfaces define volumes, within which the concentration of Cr+Nb+N >25 at%. A presumed lath boundary is highlighted by an arrow.


for the iso-concentration technique that there is a possibility to extract small volumes of ions and treat overlaps using the built-in peak deconvolution function and background sub- traction in IVASTM. This is not feasible for 1D concentration profile and proxigrams. To solve the issue with peak over- laps, one can extract the ions belonging to the precipitate using iso-concentration surfaces and use the built-in peak deconvolution tool in IVASTM. Normally, the iso- concentration value is selected in a way that the least con- tribution from matrix is involved. However, by doing this a lot of ions belonging to the precipitate are not measured, and only the composition of the core of the particle is considered as the composition of the whole particle (see also Fig. 2).


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