Comparing the Consistency of Atom Probe Tomography Measurements of Small-Scale Segregation 235 DISCUSSION
Comparing Cluster and Carbide Size with Increased Detection Efficiency
The implications of this study suggest that the efficiency gain due to the change in microchannel plate design in the LEAP 5000 generation is close to that expected by an increase from 37 to 52%, subject to caveats regarding parameter choice. As the smaller Cu clusters in the SG steel were not detected in the LEAP 3000, due to its reduced detection efficiency, the decrease in cluster size in the LEAP 5000 data is due to the increased sensitivity to these small features, making com- parison of the cluster size between the two detectors difficult. Similarly, the propensity of the maximum separation method to merge small clusters together in the LEAP 3000 analysis of the ODS steel results in a lower than expected increase in cluster size in the LEAP 5000—it is possible that improvements in cluster-finding algorithms may make such comparisons more accurate. In the case of the carbide-containing steel and
phosphorus-implanted silicon, the increase in atoms in the features increased roughly in line with the expected efficiency gain, although in both cases the choices of analysis parameters, especially the isoconcentration surface value used to define the edge of the carbide and the capping layer, respectively, make a big difference to this calculation. Hence, it is difficult to ascribe too much confidence to this calcula- tion—analysis parameter choice remains vital to accuracy. The third case, where dramatically more carbon was
observed inM2C carbides of the same size in the LEAP 3000, is slightly different. The increase in detector efficiency does appear to be resulting in a reduction in carbon content, as a result of increased loss of data from carbon multiple hit events, despite the change in hit-finding algorithm on the LEAP 5000. It will remain the case that for increased detector efficiency, there will be decreased ability to distinguish multiple ion events (Thuvander et al., 2013), and as a result the actual concentration of C may be underestimated further on higher efficiency instruments. The phosphorus-implanted silicon material, perhaps
even more so than the other materials, shows the importance of parameter selection during analysis of atom probe data. Varying the parameters of the isoconcentration profile used to define the start of the analyzed region can dramatically impact the number of phosphorus ions observed, resulting in an effective efficiency of the LEAP 5000 anywhere from 41 to 67%.
Changes in Stoichiometry Between LEAP Instruments While the change in detected atoms within these features either corresponds well to the detector change or can be explained, chemistry variations in the three steel samples are harder to account for. In all three cases, the composition was observed to differ between the LEAP 3000 and LEAP 5000. In the Cu clusters, more Fe was included from the matrix in
the LEAP 3000, whereas in the ODS steel, the reverse was observed, with more Fe content in the LEAP 5000 clusters. In the SG steel, the higher percentage of Fe observed in
the LEAP 3000 data is slight, but present. The ODS particles also saw a change in average Fe content between the two machines, although in this case the effect was reversed, with more Fe in the LEAP 5000 data. There are several possible reasons for this effect—it may be that the hit-finding algorithm is also playing a part here—the sample is mostly Fe, so if the rate of detected multiple ions increasing, the Fe concentration would be more effected by a general increase in multiple ions. It could also be that the increased number of
atoms detected in the LEAP 5000 requires a change in cluster finding parameters—as the Dmax and erosion parameters were kept the same for both instruments, it is possible that this has affected the behavior of Fe included at the edge of the clusters. The reason behind the reversed direction of the increased Fe content between the two materials is intriguing. If the nature of the matrix aberration was different—i.e. that one was a high field precipitate where the Fe inclusion was due to spreading of the higher field precipitate across the matrix, and the other was a low field precipitate where the Fe was included mainly as a magnification effect—itmay be that the change in detector efficiency results in a different effect on the two aberration types. While the improvements in detector efficiency and hit-
finding algorithms are to be welcomed in the LEAP 5000 instrument, it will remain the case that careful data analysis is often the most important factor in the accurate use of APT. It is important that the differences between measure- ments taken on both instruments is understood so that experiments using different generations of APT instruments can be compared.
CONCLUSION
Atom probe tomography data sets from four materials were compared using a LEAP 3000X HR with a 37% detection efficiency and a 532-nm green laser, and a LEAP 5000 XR with 52% detection efficiency and a 355-nm UV laser. Small segregations of clusters or impurity elements were compared to determine the effect of the change in instrument on the size and chemistry of these small-scale features. When comparing the ion count of the segregated elements in a carbide-containing steel and phosphorus-implanted silicon, the LEAP 5000 showed an increase in counts roughly equal to the expected change in detector efficiency, although it was also shown that the choice of isoconcentration surface used to perform this analysis can dramatically impact this calcu- lation. The size distribution of Cu clusters in an SG steel was harder to directly compare, due to the detection of additional small particles using the LEAP 5000 that were previously undetectable in the LEAP 3000, while in the case of YO particles in an ODS steel, the difference was smaller than expected, likely due to merging of small particles during the cluster analysis.
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