Correlating Atom Probe Crystallographic Measurements 281
to be 10 nm, with a pattern resolution of 168×128 pixels (8 ×8 binned from full resolution). The acquisition speed was 67 points per second. The accelerating voltage was 30kV and the probe current was ~15 nA. TKD maps were taken over a range of projections (0°, 90°, 180°, and 270°) by manually rotating the sample in the holder and analyzed using the Oxford Instruments® HKL® software. The same sample was then loaded into a CAMECA®
LEAP® 4000X Si atom probe, which has a straight flight path design. Data were collected at 40 K, using voltage pulsing (20% pulse fraction, 2000Hz, 0.5% evaporation rate). Data were reconstructed using the commercially available IVAS® software. The mass spectrumand corresponding ranging can be found in the Supplementary Material (Figure S1). A reconstruction calibration protocol, described by Gault et al. (2009), was also performed to ensure spatial integrity of the tomogram, as well accuracy in the subsequent crystal- lographic measurements. To mitigate changes in the image compression factor (ICF) and field factor (kf), which are known to occur throughout the experiment (Gault et al., 2011), crystallographic calibration was performed within an approximately one million atom slice in z, where crystal- lography was approximately constant. Any subsequent crystallographic measurements were taken from this region. To determine the effect of crystallographic calibration on the accuracy of the misorientation measurements, a reconstruction that was not calibrated was also used for comparison. MATLAB® was then used to further analyze the crystallographic information in the detector hit maps and reconstruction from within the epos (extended pos) file that can be generated from IVAS®.
Supplementary Materials
Supplementary Materials can be found online. Please visit
journals.cambridge.org/jid_MAM.
Technically Pure Mo Sample Preparation and Data Acquisition The technically pure Mo had a much larger grain size, so samples were prepared using electropolishing, as well as additional sharpening via a correlative TKD and FIB annular milling procedure, to ensure a grain boundary was positioned close to the apex of the specimen (Babinsky et al., 2014a). An FEI® Versa® 3D DualBeam® (FIB/SEM) workstation equip- ped with an EDAX® Hikari® XP EBSD system was used for this part of the
study.The TKDmapping conditionswere very similar to those used on the previous sample. An accelerating voltage of 30 kV and probe current of 11nA were used. The diffraction pattern resolution was 160×120 pixels with 4×4 binning from full resolution, enabling an acquisition speed of 40 frames/s. A 10nm step size was used. TKD maps for a series of projections (0°, 90°, 180°, and 270°) were again taken. The EDAX® OIM® Analysis 7 software was then used for the analysis of the EBSD data files.
3000X HR atom probe, which has a reflectron design. Aspecimen temperature of 60 K, target evaporation rate of 1% and laser pulsing with a green laser (λ = 532nm), with 0.6nJ laser energy at 250 kHz pulse frequency, were used during experimental acquisition. The conditions used were to help with specimen success rate, but are not ideal for the observa- tion of crystallographic information. The data collected were subsequently reconstructed using IVAS® and calibrated simi-
The sample was then loaded into a CAMECA® LEAP®
larly. One notable difference being the use of a 40mm virtual flight path length, rather than the physical 382mm. This was found to be a better representation of the flight displacement along the z-direction from detector to specimen apex, and offered improved convergence of calibration metrics. It was difficult to get enough crystallographic information to deter- mine misorientation between the two grains contained within the reconstruction from a single approximately one million atom z-slice, so instead, two different regions, each containing enough information about one of the grains, were calibrated instead. The calibration parameters were then averaged across thewhole reconstruction before misorientation measurements were taken. A non-calibrated reconstruction was also used for comparison. The resultant epos file was again used for subsequent crystallographic analysis using MATLAB®.The mass spectrumand corresponding ranging can be found in the SupplementaryMaterial (Figure S2).
RESULTS
The results from TKD and atom probe crystallographic measurements are presented for each material and atom probe design. Mapping the grain orientation relative to the detector and measuring the associated misorientation between grains is highly automated for the TKD analysis when using the commercially available EBSD software. However, making misorientation measurements in atom probe reconstructions remains a very manual process, which has only been discussed in limited detail previously, so some attention is given here to the procedure undertaken.
Straight Flight Path Atom Probe Analyses Versus TKD Misorientation Measurements Figure 1a shows the calibrated tomographic reconstruction
(ICF = 1.63, kf = 4, ε = 0.57, L = 90mm) and associated crystallographic measurements of the nanocrystalline sample of the Al–0.5Ag alloy with Si impurities. The Si impurities, as well as density fluctuations in the reconstruction, highlight the captured grain
boundary.An approximately onemillion atom slice has been taken out of the centre of the reconstruction which, when viewed along the z-projection, clearly shows zone line and poles (Fig. 1b). By comparing this to a stereo- graphic projection of the crystal structure of Al, the Miller indices corresponding to each pole can be determined. A plane orientation extraction algorithm (POE), as
described previously (Araullo-Peters et al., 2015), has been used to very precisely determine the normal to the sets of
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