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accuracy of several degrees and there is significant scope for improvement. It is also worth noting that pole and zone line structure is often less clear on reflectron instruments, even though lattice planes may still be conserved within the resultant reconstructions. Crystallographic signal intensity mapping approaches (Araullo-Peters et al., 2015) would be useful to enhance this information, particularly in situations where zone and pole line signal is too poor to index. The reason for the discrepancy is likely to be caused


predominantly from inaccuracies in the reconstruction cali- bration and measurement itself. A likely source of error is caused from the imperfectly reconstructed atom probe data and the requirement to fitthe Cs frame to this structure. It is worth noting that the image compression factor (ICF) and kf, which are used in the typical reconstruction protocol, change throughout the atom probe experiment (Gault et al., 2011) and may have added to measurement error. Some effort was made to mitigate this effect. For the nanocrystalline Al–0.5Ag dataset, itwas possible tomeasure the orientation of each grain from a single calibrated slice within the recon- struction, thereby avoiding any change in crystallography in depth. For the technically pure Mo, it was difficult to obtain sufficient crystallographic information of each grain within a single slice in depth. Consequently, an average ICF and kf from two slices, that contained sufficient crystallographic information about each grain, was calculated and applied to the entire reconstruction. The orientation of each grain was then measured at each slice. A dynamic reconstruction approach as proposed by (Gault et al., 2011), would have enabled the observed crystal structure in each grain to remain approximately constant in depth and hence would have probably improved the accuracy of the measurement further. However, this reconstruction calibration protocol takes much longer to implement for a small gain in accuracy. Perhaps a more accurate approach for future work


would be to measure the orientation directly from the pole and zone lines observed in the detector hit map, thereby alleviating any need to interrogate the tomographic recon- struction directly. Such an approach should achieve similar accuracy to that reported by Takahashi et al. (2014) (± 0.4°), with the added benefit that the APT experiments could continue without the time consuming process of a FIM experiment, which would require large changes to the vacuum within the analysis chamber. Further improvements could also be possible with improved reconstruction meth- ods and measurement protocols, such as improved indexing and interpretation of spacing between observed zone line and pole patterns, as well as averaging multiple measure- ments based on a different selection of poles.


Correlating the TKD and APT Orientation Maps


Figure 2 outlines the process used to filter out the boundary using a NN analysis, which worked well due to the strong segregation of impurity species in both examples. In cases where this type of segregation is not observed, density fluc- tuations within the reconstruction could be used instead to


highlight the boundaries using techniques such as the inter- face detection method proposed by (Liddicoat et al., 2010). Figures 3 and 5 are useful for directly comparing the


crystallographic information each technique provides. In both samples, the calculated color of the grains, based on the IPF, was very similar between the two techniques, indicating that crystallographic alignment was very close in the vertical direction. The 3D orientation maps from the atom probe reconstructions highlight the unique ability of APT to give additional information about grain morphology and com- pletely describe the boundary to 5 degrees of freedom, including the boundary plane orientation if desired. It should be noted that the 3D APT orientation maps only show the global orientation average of each grain, whereas the TKD orientation maps can show local changes in crystallography within the grains themselves due to strain or dislocations. However, all misorientation measurements in the presented study were based on average grain orientation. Although local changes in crystallography can be observed in APT, this is generally only in the pole and zone line regions and it is difficult to determine whether this change is due to inaccuracies in the reconstruction or true changes in local crystallography. Alignment between the TKD and APT results is an


important consideration. It is interesting to note that the longer TKD maps in Figure 3 demonstrate clearly that, depending on orientation, the grain structure can appear quite different as only 2Dcrystallographic information in the bottom 10–20nm of the sample, relative to the detector, is being displayed. This potentially adds a level of complexity to working out corresponding grains between the two techni- ques, and may even render some grains invisible to TKD if they are buried within the centre of the atom probe tip. However, this was not an issue for the examples shown as only a single boundary was captured in each APT recon- struction. Due to the angular field-of-view of APT being limited to ~30–40° because of the electrode and detector configuration, ions on the periphery of most tips are not detected and so what is reconstructed is a conical sub- volume of the original specimen. It is therefore difficult to precisely align the location of the APT reconstruction to the TKD map without multiple boundaries being captured in each. In cases where sufficient crystallographic information is present within the atom probe reconstructions to deter- mine grain orientation, this information can be used to help match up individual grains, otherwise density fluctuations and atomic segregation to the interfaces can be a useful means for grain alignment. TKD could also potentially guide reconstructions of these materials, if changes in grain mor- phology and boundary orientation are identified, however, this process would currently be challenging.


Efficiency and Future Outlooks


Although APT crystallography is important for reasons outlined previously, it is also time consuming, particularly for the non-expert with limited programming experience.


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