280 Andrew J. Breen et al. Transmission Kikuchi diffraction (TKD) in the
scanning electron microscope (SEM) (Keller & Geiss, 2012; Trimby, 2012) offers a fast and convenient means of col- lecting complementary crystallographic information on atom probe specimens. Also known as transmission electron back-scattered diffraction (t-EBSD), it offers significant improvements in lateral spatial resolution compared with conventional EBSD. The technique works by passing the primary electron beamthrough the sample and analyzing the transmitted diffracted electrons on a standard EBSD detec- tor. The lateral spatial resolution depends on a combination of experimental parameters, including sample thickness, step size, and beam spot size, but is typically <10nm (Trimby, 2012) and small enough to observe changes in crystal- lographic texture in nanocrystalline materials. The angular resolution is similar to that of conventional EBSD and dependent on the parameters used for pattern indexing, as well as the pixel resolution used for pattern binning, and is typically ~0.5° using standard Hough-transform-based indexing (Zaefferer, 2011; Trimby et al., 2014). For the results shown in this manuscript, it was measured to be ~0.3° after solution refinement techniques were used in the commercial software. The technique is conveniently suited to atom probe
specimens for a number of reasons. Optimal sample thick- ness for TKD in the SEM primarily depends on beamenergy and the material being analyzed, but is typically in the range of 100 nm, similar to the thickness of the end of an atom probe tip. It is also common to use a dual beamSEM/focused ion beam (FIB) for atom probe sample preparation and many of these instruments have an EBSD detector already installed, so TKD can often be done without any additional transporting of the sample. The time it takes to do the additional TKD experiments depends on the step size, size of the specimen and pixel resolution used for pattern binning, but multiple orientation maps of high quality are achievable within a typical single session on the microscope. It is important to point out that TKD is currently a 2D
technique, the orientation maps produced are representative of the bottom ~10–20nm of the specimen, relative to the detector, and depending on the material (Babinsky et al., 2014a). In one respect, this is a great advantage and is what enables such clear texture to be observed at the nanometer scale. On the other hand, the information provided does not fully define the grain morphology in 3D, nor the complete crystallographic nature of individual interfaces, such as the boundary planes. However, when combined with APT, it becomes possible for very accurate and complete crystal- lographic analysis of individual grains in 3D. Several authors have recently reported combining APT
experiments with complementary crystallographic informa- tion from other techniques. TKD has been used to facilitate site-specific FIB-based sample preparation, of individual interfaces, in atom probe samples (Babinsky et al., 2014a; Rice et al., 2016). Recently, nanobeam diffraction in the transmission electron microscope, was used for com- plementary crystallographic information on a cold-drawn
pearlite atom probe tip (Herbig et al., 2015). Another study, involving the in situ determination of the misorienation angle of a grain boundary using complementary field ion microscopy (FIM), was also reported by (Takahashi et al., 2014). Here, we extend these studies by directly comparing misorientation measurements between TKD and state-of- the-art APT crystallographic measurements on the same specimen, in order to gain insight into the angular resolution currently achievable by APT. In addition, crystallographic measurements made on straight flight path atom probes are compared to those on reflectron fitted instruments. Areflectron, while dramatically improving mass resolution of the technique, has also been suspected of degrading the spatial resolution and reconstruction accuracy, due to com- plications in reconstructing the curved ion trajectories. The following study will investigate this theory. TKD offers a fast and convenient way to guide crystallographic measurements made directly on APT reconstructions.
MATERIALS ANDMETHODS
To initiate the study, a sample that was easy to prepare, had a high chance of running successfully in the atom probe, was likely to have clear crystallographic information from both APT and TKD, and numerous grains within the first several nanometer of an atom probe tip, was desired. A nano- crystalline Al–0.5Ag alloy, with Si impurities, that was severely plastically deformed through high pressure torsion (HPT) satisfied these requirements and was chosen for the experiments on the straight flight-path instrument. In contrast, technically pure Mo was selected as a more
challenging specimen for comparison purposes and was used for the reflectron-fitted atom probe experiments. A different specimen preparation method was required (because the grain size was much larger) and less crystallographic signal was present in the APT data. It allowed some insight into the robustness of the study being performed. For additional information on the technically pure Mo, including compo- sition and processing conditions the reader is directed to (Babinsky et al., 2014b; Primig et al., 2015).
Nanocrystalline Al–0.5Ag Sample Preparation and Data Acquisition
The needle-shaped specimen of the nanocrystalline Al–0.5Ag alloy, with Si impurities, was prepared using the standard two-stage electropolishing technique with universal electrolytes (Miller et al., 1996). The grain size was small enough, so that there was a high chance to capture multiple grain boundaries within a specimen prepared this way. The sample was then loaded into a Zeiss® Ultra® Plus SEM equipped with anOxford Instruments® Aztec® EBSD system (version 3.0) and Nordlys® Nano detector forTKD mapping. Before acquisition, the chamber and sample were plasma cleaned using an Evactron® plasma cleaner for ~1min using room air. The sample was then tilted by 20° to obtain a horizontal orientation. The step size of the maps was chosen
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