Modern FIB-Based Specimen Preparation for APT 197


Figure 5. Various TEM/APT specimen holders, available either commercially or manufactured by various research groups. a: The E.A. Fishione Instruments 2050 on-axis rotation tomography holder which accommodates standard wire-formatted specimens (image courtesy E.A. Fishione), (b) the Hummingbird Scientific single-tilt tomography holders (half-grid holder shown here) (Gorman et al., 2008) (image courtesy Hummingbird), half-grid holder by Felfer et al. (reprinted from (Felfer & Cairney, 2011a; Felfer et al., 2012) with permission), (d) half-grid holder by Herbig et al. (reprinted from (Herbig et al., 2015) with permission).


a finished specimen. In this section, advances in specimen fixtures and preparation methods, enabling growth of TEM-


based and electron backscatter diffraction (EBSD)-based correlative analysis, are highlighted.


TEM


TEM has been used to characterize and aid end-pointing of APT specimens (Henjered & Norden, 1983; Seto et al., 1999; Arslan et al., 2008; Thompson et al., 2009; Cojocaru-Mirédin et al., 2011; Haley et al., 2011; Larson et al., 2011; Hartshorne et al., 2014; Xiong & Weyland, 2014; Grenier et al., 2015; Herbig et al., 2015; Lefebvre et al., 2015; Diercks et al., 2016), but the process of moving specimens between instruments is laborious, increases opportunity for handling damage of specimens, and invites electron-beam contamination, oxidation, and other damage that may interfere with APT analysis. FIBs equipped with 30kV scanning-TEM (STEM) capability have the advantage of avoiding multi-instrument issues (Felfer& Cairney, 2011a), but may not provide sufficient imaging capability for many situations. The most popular solution has been to develop specimen holders which facilitate transfer between FIB, TEM, and APT without the need to re-fixture the primary specimen holder (Gorman et al., 2008; Cojocaru-Mirédin et al., 2011; Felfer & Cairney, 2011a; Felfer et al., 2012; Xiong & Weyland, 2014; Herbig et al., 2015).


A number of these holders are shown in Figure 5. These fixtures, coupled with protocols that either prevent deposition or remove contamination deposits on specimens, make the proposition of performing TEM-based analysis and successfully collecting APT data on the same specimen feasible. Electron tomography (ET) has been successfully


performed on specimens subsequently analyzed with APT (Arslan et al., 2008; Xiong & Weyland, 2014; Grenier et al.,


2015). In these examples, precipitates and other features are identified by ET and used to aid calibration of APT recon- structions. Analyses from both techniques can be directly overlaid in 3D for comparison, and although each technique uses different material properties to discover features (electron-scattering density versus local composition), there is generally good agreement in overall particle shape from both methods. This illustrates the high resolution and spatial accuracy of both techniques. Although the direct visualization of features with both techniques is very powerful, these types of comparisons are important for aiding the understanding of the field-evaporation processes in APT, better determining optimal reconstruction para- meters for both techniques, and providing important information for evaluating the quality of the ET and APT reconstructions (Arslan et al., 2008). Another example of correlative TEM and APT


measurements on the same specimen region, was recently reported by Herbig et al. (2015, 2014) to study GB segregation in a nanocrystalline pearlitic steel. The half-grid holder shown in Figure 5d was used to transport lift-out specimens between FIB, TEM, and APT, and the process used to create appropriate half-grid specimen carriers is described in (Herbig et al., 2015). The holder is critical because exact control of the sample orientation in all instruments is necessary formerging analyses from multiple instruments, and this is especially critical for enabling optimum TEM measurements. Although this study did not utilize energy-dispersive X-ray spectroscopy or electron energy-loss spectroscopy (EELS), these measurements are possible as well, with a properly equipped TEM(Thompson et al., 2009; Larson, 2011b). The combination of STEM and nanobeam diffraction


(NBD) has been used to characterize the location of columnar grains (Fig. 6d), to accurately measure the relative grain orientations (Fig. 6e), and to discover the locations of secondary phase precipitates for subsequent APT analysis. The APT analysis was used to provide the local chemistry, especially detailing carbon segregation to the GBs (Fig. 6f). The sample was purposely shaped so that the top ~400nm had a maximum thickness of 60nm to provide quality TEM imaging at 200kV for the entire region. As shown in Figure 6, NBD was used to successfully characterize grain orientation down to ~1nm resolution, even for the top 10nm of a specimen where the grain volume is exceedingly small. Compared with a TEM foil, the confined sample volume of an APT specimen makes it easier to locate the same region after tilting. In addition, the sample thickness can be easily calculated from the width of the conical APT tips (Herbig et al., 2015, 2014).


Transmission Electron Backscatter Diffraction (tEBSD)


tEBSD (also known as transmission Kikuchi diffraction) is a recently developed technique that allows the crystallography of


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