Transmission Kikuchi diffraction in a scanning electron microscope: A review

Materials Science and Engineering R 110 (2016) 1–12

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Transmission Kikuchi diffraction in a scanning electron microscope: A review Glenn C. Sneddon a,b, Patrick W. Trimby b, Julie M. Cairney a,b,* a b

School of Aerospace, Mechanical, Mechatronic Engineering, The University of Sydney, NSW 2006, Australia Australian Centre for Microscopy and Microanalysis, The University of Sydney, NSW 2006, Australia

A R T I C L E I N F O

A B S T R A C T

Article history: Received 16 May 2016 Accepted 31 October 2016 Available online

Transmission Kikuchi diffraction (TKD), also known as transmission electron backscatter diffraction (tEBSD), has received significant interest for the characterisation of nanocrystalline materials and nanostructures. In this paper, we will review the development of TKD, including forescatter detector imaging and ongoing parameter optimisation, as well as some of the current applications of the technique. A comparison to other microanalysis techniques is also included, highlighting their relative strengths and weaknesses and their complementarity with TKD. Finally, potential applications of the technique and possible future developments are discussed. ß 2016 Published by Elsevier B.V.

Keywords: TKD t-EBSD Nanocrystalline Nanomaterials Corrosion Highly deformed materials

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Data acquisition and analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Forescatter detector dark-field imaging. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Parameter selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Hardware development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanocrystalline and ultra-fine grained materials . . . . . . . . . . . . . . . . . . . . . . 3.1. Highly deformed materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Geological samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Nanostructures and functional materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Comparison and complementarity with other characterisation techniques . . . . . . . Transmission Kikuchi diffraction and electron backscatter diffraction. . . . . . 4.1. Transmission Kikuchi diffraction and transmission electron microscopy . . . 4.2. Transmission Kikuchi diffraction and energy X-ray dispersive spectroscopy. 4.3. Transmission Kikuchi diffraction and atom probe tomography . . . . . . . . . . . 4.4. Future opportunities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1. Introduction * Corresponding author at: School of Aerospace, Mechanical, Mechatronic Engineering, The University of Sydney, NSW 2006, Australia. E-mail address: [email protected] (J.M. Cairney). http://dx.doi.org/10.1016/j.mser.2016.10.001 0927-796X/ß 2016 Published by Elsevier B.V.

In recent years, there has been significant interest in nanocrystalline materials and nanostructures, due to their superior or unique physical properties [1–4]. In order to understand their

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behaviour and further tailor their properties, characterising these materials on a nanoscale is essential. Orientation mapping – determining the crystal orientation at each point within the material – is an important part of characterising a material, giving useful information such as grain size distribution, grain misorientation, grain boundary character and crystallographic texture. Electron backscatter diffraction (EBSD) is an orientation mapping technique that has typically been used to characterise materials with grain sizes in the order of microns [5,6]. However, the spatial resolution of EBSD is limited to approximately 20 nm for dense materials and up to 50 nm for lighter materials such as aluminium [7,5,8]. Additionally, EBSD samples are typically tilted towards the detector by 708 and the resolution down the tilted surface is approximately three times worse. These limitations mean that EBSD cannot be used for routine analysis of truly nanostructured materials and therefore orientation mapping techniques with a higher spatial resolution are required. Recently, transmission Kikuchi diffraction (TKD) – also known as transmission EBSD (t-EBSD) or transmission electron forward scatter diffraction (t-EFSD) – was introduced by Keller and Geiss [9], with a significant improvement in spatial resolution. The key difference between TKD and EBSD is that the sample is electron transparent and mounted horizontally or backtilted away from the EBSD detector, level with or above the top plane of the EBSD detector. This geometry results in the diffraction pattern originating from the bottom surface of the sample and a smaller diffraction source volume. The result is an improved spatial resolution, with resolution of down to 2 nm being measured in nickel [10]. Transmission electron microscope (TEM) based orientation mapping techniques are being developed and significant progress has been made in this area in recent years. These methods can provide extremely high spatial resolution [11–16]. Advantages and disadvantages of these techniques will be discussed in a later section. TKD has received significant interest in recent years, already being applied to a wide range of fields, including nanocrystalline and ultra-fine grain materials [10,17–28], corrosion studies [29– 36], geological samples [37–42], nanostructures [43,9,44,45] and functional materials [46–49]. However, despite the significant uptake of TKD, there is still much that remains to be understood regarding the technique, such as how the various experimental parameters affect the resolution. Progress in this area will enable the technique to be more effectively applied. 2. Data acquisition and analysis

Fig. 1. In-chamber video camera image of a typical experimental configuration for TKD analysis in an SEM.

Typically, an accelerating voltage of 30 kV (the maximum available in many commercial SEMs) is used to maximise penetration and obtain a suitable diffraction pattern. For materials with a low atomic number and for ultra thin samples, lower accelerating voltages of down to 15 kV have been used to increase the intensity of the diffracted signal [10,17,52]. During longer analyses at high resolution (step sizes on the order of 5 nm or less), beam/sample drift and contamination can be significant issues [17,50]. These issues can be minimised by allowing the sample to settle in the chamber for several hours prior to data acquisition, minimising sample drift and improving the vacuum in the chamber. In chamber plasma cleaning can also significantly reduce contamination. For these small step sizes, drift correction algorithms are critical for reliable analyses and should be used during acquisition. At lower resolutions, data acquisition can be started immediately and drift correction algorithms are often unnecessary. If a specific surface of the sample is of interest, this surface should be face-down in the sample holder as the dominant diffraction pattern comes from the bottom few tens of nanometres of the sample. This was best shown by Rice et al. [52], looking at the resulting diffraction patterns from a bilayer crystalline Au/ amorphous Si3N4 sample. The results (Fig. 2) show significantly clearer diffraction patterns when the gold surface is facing down.

2.1. Experimental configuration 2.2. Forescatter detector dark-field imaging The experimental configuration for TKD is in many ways similar to that of conventional EBSD; the key differences being that an electron transparent sample (mounted in a micro clamp or a dedicated holder) is required and that the sample is not tilted towards the detector. Instead the sample can either be positioned horizontally or backtilted away from the EBSD detector, with both options having their own advantages and disadvantages [17,50,51]. A short working distance is used, with the sample located level with or just above the top plane of the EBSD detector, in order to utilise most of the detector. Standard EBSD processing software can be used to process the diffraction patterns, with slightly modified algorithms to optimise the indexing for TKD. Following this, a noise reduction procedure is generally applied to extrapolate to appropriate unindexed points and remove misindexed points. All TKD maps presented here are after noise reduction, unless otherwise stated. A typical experimental configuration for TKD is shown in Fig. 1.

The same experimental configuration can also be used to collect dark field (DF) images in an SEM, using a forescatter detector system consisting of either backscatter electron diodes located around the EBSD detector phosphor screen [17], or by treating specific regions of interest on the phosphor screen as positional electron detectors [54–56]. The two lower diodes (or a region near the base of the phosphor screen) can provide a good DF image of the sample, showing thickness, density and channelling contrast. The thickness and density contrast in these images can assist in finding thin regions suitable for TKD. Conversely, if the signal on one of the diodes is inverted (or with appropriate weighting of regions of interest), the thickness and density contrast can be removed, enhancing the orientation contrast of the image, providing useful microstructural information prior to TKD analysis. Examples of each type of DF image are provided in Fig. 3, taken from a nanocrystalline copper sample [17].

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Fig. 2. Diffraction patterns from a bilayer crystalline Au/amorphous Si3N4 sample, with the crystalline Au facing up and down. Reprinted from [53], Copyright 2015 with permission from Cambridge Press, reproduced in part from [52], copyright of Wiley and Sons.

Fig. 3. TKD results from a nanocrystalline Cu sample. (a) DF image collected using forescatter detectors. (b) DF image with an inverted diode signal, enhancing orientation contrast. (c) TKD pattern quality map. Reprinted from [17], Copyright 2014, with permission from Elsevier.

2.3. Parameter selection There have been several studies looking at parameter optimisation for TKD, largely focusing on the effect of working distance, detector distance, specimen tilt and specimen thickness, with mixed findings [51,44,10]. There is general agreement that short working distances and detector distances (the detector being as close to the sample laterally as is practical and safe) improve the signal for TKD [44,51,17], although the optimal working distance will depend on the chamber and detector geometry. There is still a lack of consensus regarding the optimum tilt geometry, with suggested specimen tilts ranging from a 08 to 408 [17,51], with 08 and 208 being the most common. Higher tilt angles result in the pattern centre being more central to the diffraction pattern, reducing gnomonic distortion and improving the band detection reliability, resulting in higher indexing rates [51]. This effect is partially mitigated by modifications to the band detection software to account for this change in geometry. Tilting the sample can also reduce the impact of shadowing in the diffraction patterns that can be caused by protrusions or folds in the sample in regions

adjacent to the analysis area or, in the case of focussed ion beam (FIB) prepared samples, by the TEM grids they are attached to [57]. On the other hand, higher tilt angles also increase the effective thickness of the sample, increasing the amount of beam broadening, worsening the spatial resolution [51]. Additionally, using an untilted specimen eliminates the need for tilt correction and dynamic focusing [17] and is also the optimum geometry for simultaneous energy dispersive X-ray spectroscopy (EDS) acquisition. In both cases, if simultaneous EDS analysis is required, there is an additional constraint on the working distance in order to ensure the path to the detector is not blocked by the pole piece. Specimen thickness is another important consideration for TKD, with a significant impact on both pattern quality and spatial resolution. Indexable diffraction patterns have been obtained from specimens up to 3 mm for aluminium [58] and 400 nm for copper [53]. On the other end of the thickness scale, indexable patterns have been obtained from ultra-thin films of HfO2 of down to 5 nm thickness [59] and Fe–Co nanoparticles of down to 10 nm diameter [9]. Dynamical diffraction theory implies a lower limit of specimen thickness of a quarter of the extinction distance [60,61]. For these

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ultra-thin specimens, lower accelerating voltages were used to increase the diffracted signal. However, higher quality patterns are likely to be obtained from regions with a thickness in the range of 75–200 nm for aluminium [10] and 30–100 nm for copper, provided that the grain size is large enough that there is not pattern overlap from adjacent grains. For materials with a higher atomic number, the optimum thickness range will be lower. The impact of specimen thickness on pattern quality can be seen visually in Fig. 4 [51], with no grain overlap. The point labelled (a) is very close to the edge of the hole, in a very thin region of the sample and there is a relatively large amount of noise in the diffraction pattern. As the sample thickness increases, the diffraction pattern becomes clearer and less noisy at points (b) and (c) before the thickness increases further and the diffraction pattern blurs and band contrast reversal, due to dynamical diffraction effects [62], is seen at point (f). All of these individual diffraction patterns were able to be indexed but visually, it is clear that indexing rates are likely to be much higher and with better angular precision in regions with thickness similar to that of points (b) and (c). The specimen thickness also has a significant impact on the spatial resolution due to beam broadening [52,53,58,51,30,63]. Beam broadening increases with increasing specimen thickness, being roughly proportional to t3/2, where t is the specimen thickness [64]. Beam broadening is also dependent on the material and accelerating voltage, increasing with atomic number and lower electron energies. This is only a rough proportionality for the spatial resolution of TKD, as it does not consider the requirements for being detected by the off-axis EBSD detector. Specimen tilt and electron channelling are likely to have further effects on resolution. Current investigations have largely focussed on high thickness samples and Monte Carlo simulations [52,53,58,63] and experimental verification of these theoretical and simulation results is particularly important given some of the discrepancies between simulation and experiments for resolution measurements in EBSD [65]. Absolute spatial resolutions of down to 5–10 nm in nickel have been recorded, with deconvolution of overlapping patterns giving effective spatial resolutions of down to 2 nm [10], although the thickness of these samples were not recorded. The effect of various parameters on the depth resolution of the technique has largely been neglected by the literature thus far. The depth resolution is considered to be the distance normal to the

sample surface between two points: the bottom surface of the sample and the highest location of the final diffraction relevant event that noticeably contributes to the Kikuchi pattern. Monte Carlo simulations have been used to estimate the depth resolution of the technique under certain conditions [63], but the models used are not well suited to determining the depth of the final diffraction relevant event. Studies are currently underway to experimentally determine the depth resolution of TKD in a range of materials and how it is impacted by different acquisition parameters [66]. The majority of the experimental parameter optimisation thus far has focused on the impact on pattern quality and there is still significant scope to investigate the impacts of these parameters on spatial resolution, both depth and lateral. 2.4. Hardware development Due to the experimental configuration required for TKD, a dedicated sample holder is required to allow transmission and prevent shadowing on the EBSD detector. Commercial holders are currently available [67–69], however many groups have designed their own, predominantly consisting of a clamp to hold the sample such that one edge is free-standing, preventing shadowing (as shown in Fig. 1). Some TKD holders have also been designed to accept multiple samples to reduce exchange time. Other design considerations for TKD holders include avoiding stage collisions at the low detector distances desired and minimising interference from the scattering of transmitted electrons incident on the stage below. This is often achieved by having an additional back-tilt on the holders so that the stage can be tilted out of the way of the transmitted beam. The material and the profile of the holder can also affect the accuracy of any simultaneous EDS measurements, particularly when the sample consists of the same elements as the holder. There have also been developments regarding detectors for TKD, including using an on-axis detector rather than an off-axis detector [70–72] and an angularly sensitive detector [73]. An onaxis detector directly below the sample reduces the gnomonic distortion of the diffraction pattern and results in a stronger signal, allowing for shorter acquisition times or a reduced probe current. However, it also includes the transmitted beam and the diffraction spots around it, which may cause issues with indexing relatively indistinct patterns or when high angular resolution is desired.

Fig. 4. Example of pattern quality variation with specimen thickness for an 8Cr tempered martensite specimen at 25 keV. Reprinted from [51], Copyright 2013, with permission from Springer.

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These difficulties are likely to be more apparent for thin samples and samples from material with a low atomic number. The angularly sensitive detector designed by Jacobsen et al. [73] may mitigate this, using a digital micromirror device to separate the direct and diffracted beams to enhance the achievable dynamic contrast in the diffraction pattern and produce a bright field image simultaneously. However, a blank spot will still remain where the directly transmitted beam would be. Due to geometric considerations, a worsening of the depth resolution is also expected with an on-axis detector. The shallower net scattering angle required for detector incidence for an on-axis detector results in shorter path lengths through a sample at a given depth. This increases the likelihood of an electron reaching the detector without additional scattering events, resulting in a thicker source region for diffraction relevant scattering events. This will also result in a slight improvement in lateral resolution, due to beam broadening increasing with the depth travelled into the sample. However, the different electron trajectories are required for incidence on an on-axis detector, resulting in different locations of the final diffraction relevant scattering event and this is expected to have a greater effect on lateral resolution. 3. Applications 3.1. Nanocrystalline and ultra-fine grained materials By far, the highest uptake of TKD so far has been for the analysis of nanocrystalline and ultra-fine grain materials, including steels and ferrous alloys [74–76,28,26,77,78,24,79,20,17,80], aluminium [81,82,22,83,21,17,50], nickel [84,23,17], titanium [18,19,17], copper [17] and gold [25,85]. The TKD analysis of a duplex stainless steel, including phase maps and inverse pole figures (IPF)

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of the ferrite and austenite phases, is shown in Fig. 5 [79]. The distribution of the two phases can be seen readily and although the average austenite grain size is approximately 300 nm, many grains on the sub 100 nm scale are clearly resolved. For comparison, a TKD analysis of an ultra-fine grained titanium alloy, processed by dynamic plastic deformation, is provided in Fig. 6. The ability to resolve the smaller grains and the nano twins illustrates the suitability of TKD for high resolution orientation mapping of materials with a lower atomic number, which is a limitation of EBSD. 3.2. Highly deformed materials The need for high spatial resolution is often overlooked for highly deformed materials with a larger grain size. Despite the larger grain size, EBSD analysis of these materials can often be problematic. The high dislocation density and lattice deformation in these samples result in blurred and missing patterns [17], leading to poor indexing rates. The smaller interaction volume of TKD mitigates this, enabling the analysis of materials such as severely plastically deformed titanium [27,18,19], aluminium [21] and highly deformed steel alloys [86]. It should also be noted that many of the ultra-fine grained and nanocrystalline materials mentioned in the previous section were also formed using severe plastic deformation techniques. Fig. 6, showing the TKD analysis of dynamically deformed titanium, demonstrates the capacity of TKD to resolve highly deformed material as well as nanocrystalline materials. The large extent of intragranular misorientation, shown by the colour gradient within a grain, indicative of lattice deformation, is cleanly indexed whereas EBSD analysis of the same region would be much more difficult.

Fig. 5. Phase map (a) of a duplex stainless steel and IPF-z plots for the ferrite (b) and austenite (c) phases. Reprinted from [79], Copyright 2015, with permission from Elsevier.

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Fig. 6. An orientation map of an ultra-fine grained titanium alloy, the black and white arrows indicating twin boundaries and twin boundary segments. Reprinted from [18], Copyright 2013, with permission from Elsevier.

3.3. Corrosion Many of the TKD studies of oxides and corrosion have used simultaneous TKD and EDS at the interface between a metal and a surface oxide, providing both orientation and chemical information from this region, both of which can have a significant impact on the oxidation behaviour. Many corrosion studies to date have focussed on stainless steels, looking at intergranular corrosion [29], nanograined oxide scale formation [31] and stress corrosion cracking [34,30], although there have also been investigations into corrosion in other materials such as zirconium alloys [32,33,35] and nickel superalloys [36]. Fig. 7 shows a TKD analysis of a zirconium alloy being developed for use in nuclear power plants, looking at the metaloxide interface and the oxide itself. The phase map shows the blocky suboxide ZrO grains, columnar monoclinic ZrO2 and several small tetragonal ZrO2 grains. The orientation map also shows that most of the suboxide ZrO grains grow epitaxially, sharing an orientation with the metal grain from which they form. While the indexing is still relatively poor in some regions, it should be noted that the average grain size of the detected tetragonal ZrO2 grains was 20 nm (although there could be more below the resolution limit) and that TKD was able to highlight the suboxide microstructure in a way that was not possible with conventional TEM due to the overlapping patterns at the grain boundaries [32]. Due to the non-conductive nature of most of the corrosion specimens, charging and beam drift can be a significant issue, especially when mapping using a small step size. Specimen preparation is also a challenge, with site specific, non-conductive specimens being required, often necessitating the use of focussed ion beam (FIB) milling. 3.4. Geological samples There have been comparatively few studies using TKD on geological samples thus far, and they range from identification of different asbestos fibres [40] to the structure of meteorites [38,87], amongst others [41,88,89,42,37]. The TKD analysis of the meteorite Vigarano, shown in Fig. 8, revealed crystallographic and mineralogical changes on a small scale that were not visible in EBSD analysis of the bulk sample [38]. Analysis of the texture of regions within the meteorite also indicated certain orientation relationships between different minerals, providing insights into how they were formed and the conditions they may have experienced afterwards. Due to the nature of the geological materials, the most pressing problems normally encountered for these samples is beam damage

Fig. 7. Band contrast map (a), phase map (b) and orientation map (c) of a zirconium alloy. The ellipses in (c) show epitaxial growth or the ZrO suboxide on the zirconium substrate. Reprinted from [32], Copyright 2015, with permission from Elsevier.

and charging effects. The electron dose must be carefully considered in order to obtain high quality data throughout an entire map. 3.5. Nanostructures and functional materials TKD has been used to analyse a wide range of functional materials, including ferroelectrics [47], semiconductors [48], superconductors [46] and lithium-based electrodes [49]. There are also a number of examples in the literature showing the potential for the analysis of nanoparticles and nanorods using TKD [43,90,9,44,45,91], though currently there are relatively few involving the analysis of a specific material towards an end goal. The most notable thus far involved the characterisation of tungsten nanotendrils, formed under conditions simulating their growth in a fusion reactor [92]. An IPF-z map of a single nanotendril is shown in Fig. 9, with grains down to 16 nm wide being seen. Given the small step size required to accurately map the orientation of many of the nanostructures, drift can be a significant issue for the analysis of these nanostructures. In some cases, particularly with nanoparticles, the support film may also cause

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4. Comparison and complementarity with other characterisation techniques 4.1. Transmission Kikuchi diffraction and electron backscatter diffraction

Fig. 8. Band contrast image and phase map of a refractory metal nugget within a spinel in a calcium-aluminium rich inclusion of the Vigarano meteorite. Reprinted with permission from [38], 46th Lunar and Planetary Science Conference.

In practice, the major difference between EBSD and TKD is the improved spatial resolution. As mentioned earlier, the absolute spatial resolution of EBSD is limited to 20–50 nm, depending on the atomic mass, and worsens threefold along the tilted surface [7,5,8]. In contrast, for TKD, absolute spatial resolutions of 5–10 nm have been recorded for both aluminium and nickel [50,10]. The higher resolution is due to the smaller interaction volume for TKD, as shown by the Monte Carlo simulation in Fig. 10 [57]. While the spatial resolution of EBSD improves with increasing atomic number [95], the reverse is true for TKD (for a constant sample thickness). In both cases, this is due to the increased interaction between the beam and the material at higher atomic numbers. For EBSD, the increased interaction reduces the penetration depth into the sample before the beam is scattered out, reducing the interaction volume whereas for TKD the increased beam interaction results in more beam broadening, increasing the interaction volume. This results in a poorer EBSD spatial resolution for light materials, such as aluminium, than for materials with a higher atomic number. However, the ideal sample thickness for TKD is not independent of atomic number and that high resolutions require suitably thin samples. As mentioned in Section 2, the formation mechanisms and the appearance of the diffraction patterns for TKD are very similar to that for EBSD, with comparable angular resolutions. The similarities between the diffraction pattern obtained from EBSD and TKD is likely to enable techniques based on further analysis of the diffraction patterns, such as strain mapping based on crosscorrelation between high quality patterns [96–98], to be extended to TKD [34]. Another potential advantage of TKD arises when analysing nonconductive specimens. The smaller interaction volume results in less charging in the thin sample when compared with a bulk sample, increasing the likelihood of a successful analysis, or enabling a smaller step size if charging was the limiting factor. One disadvantage of TKD compared with EBSD is that an electron transparent sample is required, resulting in more intensive sample preparation for specimens from a bulk material. This also results in both a smaller region being available for analysis and additional stress release considerations, as there are two free surfaces rather than one. Both are useful techniques for orientation mapping, with EBSD being better suited to materials with a coarser grain size whereas TKD is more suited for finer grained materials, particularly

Fig. 9. IPF-z map of a tungsten nanotendril. Reprinted from [92], Copyright 2017, with permission from Elsevier.

charging issues for the sample. Functional materials may also experience similar issues, depending on the type of material. In addition to the applications already outlined, there are also several more unique applications of TKD, including looking at the behaviour of tin under compression at cryogenic temperatures [93] and in situ verification of vitreous ice in FIB-prepared cryobiological samples [94], showing a wider range of potential applications.

Fig. 10. Monte Carlo simulations of electron scattering for 40 nm Ni/2.5 nm Ta/ 40 nm Si3N4 at 25 keV for (a) EBSD configuration and (b) TKD configuration. Reprinted from [57], Copyright 2013, with permission from Cambridge Press.

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materials with a lower atomic number where EBSD suffers from poorer resolution. 4.2. Transmission Kikuchi diffraction and transmission electron microscopy Transmission electron microscopy has been widely used for studying nanocrystalline and nanoscale materials, with its high spatial resolution and a number of electron diffraction techniques enabling orientation measurements to be taken on the nanoscale [11–14,99,100,33,15,101,16]. The electron diffraction techniques that are capable of nanoscale orientation measurements include conical dark field scanning (CDFS), convergent beam electron diffraction (CBED) and precession electron diffraction (PED), each with their own strengths and weaknesses. A comprehensive overview of these techniques can be found in a review paper by Zaefferer [100]. Precession electron diffraction is the most comparable to TKD in that it can be automated to produce an orientation map of a thin region of a specimen. However, the pattern matching used in PED gives an angular resolution of 0.5–28 [100], which is poorer than that achievable using TKD (typically better than 0.58 [17]), and tends to be less robust in determining the orientation. Conical dark field scanning can also be used to produce orientation maps but the angular resolution is generally lower than that of PED. Kikuchibased techniques in a TEM have better angular resolution but suffer from problems with automation and have a far greater sensitivity to defects such as dislocations. A common advantage of these TEM diffraction techniques is that they can all achieve a higher resolution than TKD, due to the higher electron energies resulting in reduced beam broadening, with absolute spatial resolutions of under 5 nm being achievable [100]. A comparison of orientation maps of similar regions produced using TKD and PED is provided in Fig. 11 [101], with TKD appearing achieve clearer phase and orientation maps, although the EDS was also used to assist phase characterisation. However, different comparison studies have found that opposite result with spatial resolution of PED being superior to TKD, together with more reliable indexing [33]. The discrepancies between the studies can likely be attributed to the fact that, while PED should have a superior

lateral resolution, TEM-based techniques tend to probe through the entire thickness of the sample. This means that overlapping features through the thickness of the sample can still cause problems during analysis whereas this is less of an issue with TKD due to the dominant diffraction signal originating largely from the bottom surface. One significant advantage of TKD over most TEM diffraction techniques is that it has a much higher accessibility and availability and requires a lower level of expertise. All that is needed for TKD is an SEM with an existing EBSD system and a relatively inexpensive sample holder, whereas a comparable TEM orientation mapping technique requires a TEM and a dedicated PED attachment system. If the slightly higher resolution obtainable with PED is not required, TKD provides a more accessible, simpler alternative. Regarding the complementarity with conventional and high resolution TEM, TKD can also be used to quickly map a large region of the sample, obtaining the distribution and orientation of different phases, prior to more detailed TEM investigation into the regions of interest. This rapid mapping can provide statistics, such as phase fractions and grain size distributions, over a large area and help identify promising regions for later high resolution TEM analysis, including locating grains with a zone axis normal to the sample surface. 4.3. Transmission Kikuchi diffraction and energy X-ray dispersive spectroscopy The ability to acquire EDS data simultaneously with orientation data from TKD can provide useful insight, particularly when looking at chemical segregation at grain boundaries [17] and for corrosion studies [29]. An example of simultaneously acquired EDS and TKD for corrosion studies is provided in Fig. 12. Dedicated software can be used to measure the variations in specimen thickness [102], enabling further analysis of parameter dependence and locating ideal regions for further analysis. EDS maps can also be obtained simultaneously with orientation information when combined with other orientation mapping techniques. However, many of the advantages and disadvantages mentioned in the previous two sections still apply here, such as the resolution of EDS acquired with EBSD having a poorer resolution

Fig. 11. PED reliability, phase and orientation (left) and TKD (right) image quality, phase and orientation maps of the same region of nano-oxides in an INCONEL 740 nickelbased superalloy. Reprinted with permission from [101], Copyright 2015, American Chemical Society.

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Fig. 12. (a) TEM bright field image of intragranular corrosion region of interest with (b–d) TKD phase map, IPF-z orientation map and EDS elemental maps. Reprinted from [29], Copyright 2014, with permission from Elsevier.

than EDS acquired simultaneously with TKD. It is important to note that, while the dominant signal originates from the base of the sample for TKD, the EDS signal originates from the entire thickness. In most cases this will not cause a significant difference and there will be a strong correlation between the techniques. However, it will affect the correlation between EDS and TKD maps for features such as extremely fine precipitates or highly inclined grain boundaries. A precipitate which is not at the base of the sample would be readily observed using EDS, but may not be visible using TKD (or visible only as a decrease in pattern quality) if it is above the diffraction source volume for TKD. This can be seen in Fig. 13, with several zinc precipitates appearing in the EDS map whereas in the TKD map, only one can be indexed and many of the smaller ones cannot be seen at all. There are several complications involved with acquiring EDS simultaneously with TKD data. One is that background fluorescence can come from the sample holder used to mount the thin specimens. This can be mitigated by appropriate holder design, as mentioned in Section 2.4, but some background fluorescence is likely to remain. The chamber geometry also imposes further constraints on the working distance and specimen tilt to ensure the path from the specimen to the EDS detector is unblocked. The thin specimen itself can also be an issue, resulting in low count rates

compared to bulk specimens as there is less material to actually interact with the beam and because most EDS systems in SEMs are optimised for analysing bulk samples [17]. 4.4. Transmission Kikuchi diffraction and atom probe tomography Atom probe tomography (APT) is a growing field, capable of achieving three-dimensional atomic resolution with chemical information from a needle shaped specimen [103]. However, a limitation of APT is the complex sample preparation required, particularly when producing a site-specific specimen [103]. One option that has been used for the preparation of samples containing a grain boundary, involves annular milling with a focussed ion beam (FIB) system and tracking the location of the grain boundary with TEM, requiring repeated exchanges between TEM and FIB/SEM being [104]. However, the repeated exchanges are time-consuming and increase the risk of damage to the sample. Recently, Babinsky et al. [105–108] have successfully used TKD in a FIB/SEM to sequentially mill and image an atom probe specimen within a single microscope at the same tilt angle, as shown in Fig. 14, negating the need for exchange between multiple microscopes. While the tilt angle and sample thickness may not be optimal for high resolution and indexing rates for TKD, the grain

Fig. 13. (a) TKD map (phase and band contrast) of an Mg-Zn alloy and (b) Zn EDS map of the same region.

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Fig. 14. Schematic of FIB/TKD set-up at (a) 08 tilt and (b) 528 tilt for FIB milling. This gives an effective backtilt of 388 for TKD. Reprinted from [106], Copyright 2015, with permission from Elsevier.

Fig. 15. TKD maps (IPF overlaid on IQ) of an APT specimen between FIB milling steps to obtain a grain boundary near the specimen tip. Reprinted from [105], Copyright 2015, with permission from Elsevier.

boundary is still clearly visible in the orientation maps. Fig. 15 shows orientation maps obtained by subsequent TKD analyses between annular milling steps. Aside from assisting with site-specific specimen preparation, the high angular resolution of TKD on a nanoscale complements the high chemical sensitivity of APT well. The combination of these techniques has enabled investigations into the relationship between grain boundary chemistry and misorientation [109] and segregation of specific elements into dislocations and deformed regions [41]. This has helped show the importance of deformation processes and microstructures on trace element concentrations in zircon, which plays an important role in geological dating. Due to the surface sensitivity of TKD, it is also necessary to analyse the APT tip in several orientations using TKD prior to conducting APT to get a clearer picture of the overall threedimensional structure. 5. Future opportunities The future opportunities regarding TKD can be broadly categorised into two areas; further developing and understanding the technique and further applying it to both new and existing areas. There is still much to be understood regarding the effect of different experimental parameters – such as specimen thickness, tilt and atomic number – on the spatial resolution of TKD, both depth and lateral. A better understanding of the effect of these parameters would help optimise the technique and help us determine just what we can see when looking at different regions of a specimen. Hardware developments, such as on-axis detectors and direct electron detectors [110,111], have the potential to obtain a stronger diffraction signal, allowing for reduced acquisition times or smaller currents and probe sizes. This would help mitigate drift and contamination issues and improve the lateral resolution limit, although there are potential downsides, such as a loss of depth resolution when using an on-axis detector.

There are already a large number of applications of TKD and, with the accessibility of the technique (only requiring an SEM with an EBSD system), this is only expected to increase. With the slower uptake in the fields of corrosion, functional materials and nanoparticles, there is still significant scope for expansion in these areas. TKD also shows significant promise in both aiding the preparation of FIB-milled specimens, such as atom probe tips, and as a routine pre-analysis technique, providing additional crystallographic information for atom probe specimens or narrowing down the region of interest for high resolution TEM studies. There is also the potential for nanoscale in situ deformation and heating experiments, with the large amount of available space in the SEM simplifying the design of the apparatus. For these experiments, an on-column plasma cleaner would likely be essential to minimise the contamination that would be caused by multiple scans. 6. Conclusion To conclude, TKD is a promising new technique for the analysis and routine characterisation of nanocrystalline materials and nanostructures, capable of effective resolutions of down to 2 nm. It is already in use in a number of fields, including materials engineering, corrosion, geology and functional materials, with scope for further expansion of the application areas, although there is still work remaining to better understand and optimise the technique. The comparable resolution to TEM orientation mapping techniques and the complementarity of TKD with EDS and APT presents exciting possibilities. Acknowledgements The authors acknowledge the facilities and the scientific and technical assistance of the Australian Microscopy and Microanalysis Research Facility at the Australian Centre for Microscopy and Microanalysis at the University of Sydney. The authors would also like to acknowledge Suqin Zhu for providing the Mg-Zn sample and related analysis. References [1] M.A. Meyers, A. Mishra, D.J. Benson, Prog. Mater. Sci. 51 (4) (2006) 427–556, http://dx.doi.org/10.1016/j.pmatsci.2005.08.003. [2] H. Gleiter, Acta Mater. 48 (1) (2000) 1–29, http://dx.doi.org/10.1016/S13596454(99)00285-2. [3] Y.H. Zhao, X.Z. Liao, S. Cheng, E. Ma, Y.T. Zhu, Adv. Mater. 18 (17) (2006) 2280– 2283, http://dx.doi.org/10.1002/adma.200600310. [4] P.V. Liddicoat, X.-Z. Liao, Y. Zhao, Y. Zhu, M.Y. Murashkin, E.J. Lavernia, R.Z. Valiev, S.P. Ringer, Nat. Commun. 1 (2010) 63, http://dx.doi.org/10.1038/ncomms1062. [5] D. Dingley, J. Microsc. 213 (3) (2004) 214–224, http://dx.doi.org/10.1111/j.00222720.2004.01321.x. [6] A.J. Schwartz, M. Kumar, B.L. Adams, D. Field, Electron Backscatter Diffraction in Materials Science, Springer Science & Business Media, 2010. [7] D. Chen, J.-C. Kuo, W.-T. Wu, Ultramicroscopy 111 (9–10) (2011) 1488–1494, http://dx.doi.org/10.1016/j.ultramic.2011.06.007.

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