Application of Transmitted Kikuchi Diffraction in Studying Nano-oxide

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Application of Transmitted Kikuchi Diffraction in Studying Nano Oxide and Ultrafine Metallic Grains Majid Abbasi, Dong-Ik Kim, Hwanuk Guim, Morteza Hosseini, Habib Danesh-Manesh, and Mehrdad Abbasi ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b04296 • Publication Date (Web): 20 Oct 2015 Downloaded from http://pubs.acs.org on October 22, 2015

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Application of Transmitted Kikuchi Diffraction in Studying Nano Oxide and Ultrafine Metallic Grains Majid Abbasi1, Dong-Ik Kim1*, Hwan-Uk Guim2, Morteza Hosseini3, Habib Danesh-Manesh3, Mehrdad Abbasi4

1- High Temperature Energy Materials Research Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea 2- Korea Basic Science Institute, Daejeon 34133, Republic of Korea 3- Department of Materials Science and Engineering, Shiraz University, Shiraz, Iran 4- Department of Mining and Metallurgy, Amirkabir University of Technology, Tehran, Iran. *

Corresponding Author:

Dong-Ik Kim [email protected] [email protected] Phone: (+82) 2-958-5432 Fax: (+82) 2-958-5449

Abstract:

Transmitted Kikuchi Diffraction (TKD) is an emerging SEM-based technique that enables investigation of highly refined grain structures. It offers higher spatial resolution by utilizing conventional Electron Backscattered Diffraction (EBSD) equipment on electron transparent samples. A successful attempt has been made to reveal nano oxide grain structures as well as ultrafine severely deformed metallic grains. Effect of electron beam current was studied. Higher beam currents enhance pattern contrast and intensity. Lower detector exposure times could be employed to accelerate the acquisition time and minimize drift and carbon contamination. However, higher beam currents increase the electron interaction volume and compromise the spatial resolution. Lastly, TKD results were compared to orientation mapping results in TEM

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(ASTARTM). Results indicate that combination of TKD and EDS is a capable tool to characterize nano oxide grains such as Al2O3 and Cr2O3 with similar crystal structures. Keywords: Transmitted Kikuchi Diffraction (TKD), Transmitted EBSD (t-EBSD), Orientation Imaging in TEM, Nano Grains, Ultrafine Grains, Accumulative Roll Bonding, Oxidation

Grain refinement can significantly alter mechanical, chemical, electrical, and magnetic properties of metallic or non-metallic systems. As the average grain size decreases, grain boundary area increases dramatically which can lead to property enhancement (diffusion, strength, toughness, i.e.) or degradation (electrical, magnetic, i.e.). Engineering the immense amount of grain boundaries as a result of grain refinement in nano- and ultrafine-grained structures facilitates material modification for different applications. Grain boundary engineering (GBE) is an ongoing research topic with various applications.1-19 However, GBE requires large-scale quantitative understanding of grain boundaries (their crystallographic type, fraction, distribution, properties etc.) to perform meaningful property-microstructure-parameter correlations.

Electron Backscattered Diffraction (EBSD) is a powerful tool that enables grain boundary characterization and quantification. However, it inherits spatial resolution limitations in the order of 50-100 nm in most practical cases. The relatively low spatial resolution is due to sample’s high angle of tilt which spreads the beam across the surface and increases the electron interaction volume. This makes it difficult to study nano and ultrafine grains in severely deformed metals, thin films, nano-oxide layers, nano-particles, and nano-wires. Additionally, high lattice distortion in severe deformation processes imposes a challenge in successfully obtaining index-able patterns in fine grains.

In recent years, a novel technique based on Kikuchi diffraction in scanning electron microscope (SEM) has been introduced. Characterizing nano-particles and nano-wires has been the motivation to develop this technique.20, 21, 22 It has been named Transmitted Kikuchi Diffraction (TKD), Transmitted Electron Backscattered Diffraction (t-EBSD), and Transmitted Electron


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Forward Scattered Diffraction (t-EFSD). Although the nomenclature varies among researchers, the technique has been called TKD in the current publication for simplicity. TKD is gaining a growing attention since its introduction in 2010.20-43 It offers higher spatial resolution and lower interaction volume than conventional EBSD to study nano-scale crystalline materials. These are due to electron beam vertical penetration in contrast with horizontal spreading of beam in EBSD as a result of high tilt angles. Resolutions as high as 5-10 nm were reported.21, 22, 25

Automated data acquisition over large areas could be achieved by TKD utilizing already available EBSD hardware and software. Additionally, TKD could be combined with other techniques such as EDS in SEM for phase characterization as performed by Brodusch et al.25 Overall, it is a promising technique to study nano and ultrafine grains considering the availability of localized sample preparation methods such as focused ion beam (FIB) to prepare electron transparent samples already used in transmission electron microscopy (TEM) and atom probe microscopy (APM).20-43

TKD is fundamentally similar to EBSD except the fact that transmitted electrons instead of reflected electrons generate Kikuchi diffraction patterns. TKD patterns (intersecting bands) are projected on a commercial EBSD detector (phosphor screen) located on the side or below the electron-transparent sample. The patterns carrying crystallographic information (crystal structure and orientation) of near-exit surface are indexed using commercial EBSD systems in order to construct large-scale orientation and grain boundary maps.21-26

Parameters such as sample tilt angle, thickness, atomic mass, and electron beam accelerating voltage are known to affect the pattern quality.21-26 Other geometric and microscope parameters such as working distance, detector distance, and beam current may also affect the pattern quality. However, the effect of primary sample parameters such as tilt angle, thickness, and atomic mass was investigated in details.21-26


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No standard tilt angle has been defined in TKD unlike conventional EBSD where a standard -70º tilt angle is applied (between sample surface normal and electron beam). Tilt angles from 10º to 60º have been reported.20-43 However, 10º to 20º have been mostly used to prevent shadowing on the detector while patterns with sufficient intensity were produced. Higher tilt angles result in a beam broadening which increases the interaction volume and reduces the spatial resolution.

Similarly, no optimum thickness has been found. Electron transparent samples with a thickness of 5 nm (HfO2) to 3000 nm (Al) have provided results.21, 22, 23, 25, 35 The inelastic electron meanfree-path has been mentioned as a significant factor in generating TKD patterns versus thickness.25 Rice et al.28, 35 showed through simulations that significant broadening of the energy distribution occurs with increasing thickness leading to lower contrasts in Kikuchi patterns. This will lead to difficulty in indexing which ultimately lowers the resolution. They have also suggested that mass-thickness could be considered to roughly estimate the maximum thickness that would provide index-able patterns.35

In most TKD cases, electron transparent slices prepared via methods such as FIB with the typical thickness of 100 nm provided satisfactory patterns.20-43 It should be noted that chemical sample preparation methods such as electropolishing may result in improved diffraction signals since ion-milling techniques produce an amorphous layer at the sample surface.

Simulations have shown that the yield of forward-scattered electron is high compared to the backscattered electrons in EBSD.28, 35 This implies that even lower energy beams could be used to perform TKD.25 Higher energy beams (25-30 kV) are preferred to increase the fraction of transmitted electrons, generate patterns with sufficient intensity for indexing, reduce beam broadening, and improve spatial resolution. Lower accelerating voltages may be useful in scanning very thin or low density material samples.21, 23, 25, 35

Other parameters such as working distance, detector distance, and beam current seem to be microscope-dependant. However, short working distances (WD) of 3-7 mm have been reported effective in capturing index-able patterns.21, 23, 24, 28, 35 Detector distance (DD) defined as the distance between pattern center on the detector and the intersection point of incident beam on the 4 ACS Paragon Plus Environment

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sample has been studied in details by Brodusch et al.24 Shorter DD will provide larger field of view with narrower bands. Longer DD captures smaller pattern areas with wider bands and higher angular resolution. Wider bands could enhance the indexing accuracy depending on algorithm used by EBSD software.24 In most TKD applications, DD of nearly 10 mm has been used.

Although beam current seems an important parameter in general electron microscopy, there are no detailed investigations regarding beam current (probe size) effect on TKD results. Weak pattern definitions were reported for beam currents lower than 300 pA.26 An attempt has been made to address the effect of beam current and its significance in the present research. The results will be discussed in upcoming sections.

A growing number of areas have utilized TKD to characterize nano- and ultrafine-grained materials in recent years. Areas such as metal thin films,28, 37 nano-grained oxides,29, 38, 39, 40 atom probe and correlative tomography,30, 33 nano-crystalline steels and alloys,27, 31, 41 nano-grained graphene-MgB2 superconductors,32,

43

nano-particles and fibers,20,

24

and nano-crystalline

LiFePO4-based electrodes44 could be named. This is an indication of the interest to quantify nano- and ultrafine-grained structures with a capable tool such as TKD.

Although comparable high resolution orientation mapping techniques based on Transmission Electron Microscope (TEM) and Scanning Transmission Electron Microscope (STEM) have been introduced, their lack of automation in performing large-scale measurements has played a role in the growth of TKD. These techniques utilize various modes such as spot and Kikuchi diffractions to obtain crystallographic orientations in TEM/STEM.45-61 A newly developed automated package called ASTARTM has become available that utilizes precession diffraction mode combined with template-matching to extract crystallographic orientations in TEM.55,

57

This technique was used on a nano-oxide sample in the present research. ASTAR and TKD results on ultrafine-grained62 and nano-grained63 samples are compared and discussed in the upcoming sections.


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Results and Discussion:

Conventional EBSD is unable to reveal the grain structure of severely deformed ultrafine metallic and nano-oxide grains. Higher resolutions are required to obtain grain boundary structures. As an example, secondary electron image of ultrafine-grained Cu (bright) and Ti (dark) layers is shown in Fig. 1a. Conventional EBSD orientation map of the selected area in Fig. 1a as well as Kikuchi patterns of two grains with their grain boundary area are presented in Fig. 1b. Most grain boundaries and some elongated Cu grains are un-indexed (black areas in Fig. 1b). Ti layer does not provide any index-able patterns.

Lower resolution of conventional EBSD and severely deformed ultrafine-grained microstructure contribute to unsuccessful attempts in fully revealing the grain structure. Kikuchi patterns from grain boundaries are not well-defined due to overlapping patterns and high strains as a result of severe plastic deformation (compare GB pattern in Fig. 1b with G1 and G2). Higher spatial resolution techniques with smaller electron interaction volumes are required to properly reveal the grain structure and boundaries.

Figure 1

TEM investigations indicate that Cu and Ti layers are composed of fine grains with high density of dislocations. This explains why conventional EBSD was unsuccessful in revealing the grain structure. The location of FIB slice removed for TEM analysis is shown in Fig. 2a. Bright field TEM image of FIB slice is presented in Fig. 2b indicating Cu and Ti layers as well as Selected Area Diffraction (SAD) and TKD investigation areas. Ultrafine and nano grains are evident at high magnifications in Cu and Ti layers (see TEM1 and TEM2 diffraction rings and high magnification bright field images in Fig. 2b).

Qualitative grain size and grain shape information obtained in TEM is based on contrast variations in bright field images. They cannot be an accurate representative of actual grain size and grain boundaries since orientation information is required. Additionally, larger areas need to


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be investigated in order to achieve meaningful statistical grain size and grain boundary measurements.

Figure 2

Large scale orientation mapping via TKD shows promising results in revealing grain structure and grain boundary information. Forward-scattered electron image of Cu layer in Fig. 2b using ArgusTM detector is illustrated in Fig. 3a. TKD inverse pole figure maps of Cu and Ti layers in Fig. 2b are illustrated in Fig. 3b and 3c, respectively. These figures correspond to TKD1 and TKD2 areas in Fig. 2b.

Elongated ultrafine grains in Cu layer are evident (Fig. 3b). Finer Cu grains near the Cu/Ti interface were observed (Fig. 3b). The average grain size was measured as 380 nm with 5º grain boundary misorientation criteria. Accelerating voltage and beam current of 30 kV and 47 nA were employed.

Grains as small as 50 nm in Cu layer were detected. Un-indexed areas in Cu (along the dark horizontal band in Fig. 3b) are due to higher thickness of FIB slice as a result of ion-milling. This band corresponds to the bright horizontal band in Fig. 3a indicating thicker sections of the slice. Thick areas appear bright in Fig 3a similar to dark field TEM images since both are generated by diffracted electrons rather than the direct transmitted beam. The thick area in Fig. 3a exhibits weak pattern quality and indexing (Fig. 3b). Additionally, diffraction spots appear near the bottom edge of the phosphor screen when scanning thin areas of the foil. No diffraction features are expected where the foil is too thin (thicknesses close to electron mean-free path).

Figure 3

Beam conditions employed to scan Cu layer are only able to identify some Ti grains at the right side of the Cu/Ti interface (Fig. 3b). This is due to smaller grains and lower density of Ti which means electron interaction volume must be reduced. Therefore, beam current was decreased to 20 nA in order to reveal Ti nano grains (Fig. 3c). 7 ACS Paragon Plus Environment

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Grains as small as 30 nm in Ti were characterized. Unlike Cu layer, Ti layer exhibits finer grains with high fraction of subgrains (Fig. 3c). There are some un-indexed areas along Ti grain boundaries. Multiple patterns in grain boundary area or overlapping nano grains could cause indexing ambiguities as mentioned in previous EBSD studies.64-66 However, grain boundaries appear wide (diffused) in TKD maps of both Cu and Ti. Despite earlier conclusions that TKD information mostly originates from the near-exit surface, wide grain boundaries particularly for Ti imply that diffraction is occurring throughout thickness. Lower density of Ti compared to Cu may have contributed to the appearance of wider Ti grain boundaries. Further investigations are required to determine vertical resolution as a function of atomic number.

Observation of elongated grains in Cu as well as nano grains and subgrains in Ti was made possible via TKD. These features are absent in TEM results (compare Fig. 2 and Fig. 3). Additionally, orientation maps obtained in TKD present grain size and grain shape more accurately than bright/dark field TEM images. However, electron beam current seems to be an important factor when different grain sizes are investigated.

An increased quantity of transmitted and forward-scattered electrons is expected by increasing the beam current. While higher beam current increases the probe size and interaction volume, it enhances signal intensity which leads to brighter Kikuchi patterns. This implies that lower detector exposure times could be employed resulting in shorter scans. Shorter scans can minimize beam drift, mechanical (stage) drift, and carbon contamination.

Lowering acquisition time is particularly important when scanning large areas is intended. An example is provided in Fig. 4. Shorter times are achieved by increasing the beam current (Fig. 4a to 4c). Applying beam currents of 10 nA, 25 nA, and 40 nA results in approximately 40 min, 30 min and 15 min scanning times. Drift has been minimized at the highest beam current (compare the arrow length in marked grains in Fig. 4a to 4c). Improved grain size and shape accuracy is achieved as drift is minimized by reducing the acquisition time. It must be noted that beam broadening due to high probe currents may cause the grain boundaries to appear wider. Therefore, a compromise between probe current and spatial resolution seems necessary. 8 ACS Paragon Plus Environment

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Increasing beam current is beneficial in reducing the number of un-indexed points away from grain boundaries. The large un-indexed region in Fig. 4a corresponds to a bright area in the TEM bright field image (TKD3 in Fig. 2b). Number of transmitted electrons is higher for bright grains. This implies that the area has low thickness or low scattering angles. Increasing beam current leads to improved band contrast. Consequently, Kikuchi patterns with higher number of detected bands are generated (compare [-131] band for points 1, 2, and 3 with the simulated pattern in Fig. 4d).

It should be noted that the improved band contrast with higher beam currents could be mainly related to signal-to-noise ratio. Higher currents above the threshold of the phosphorescent screen (which is limiting the contrast at low currents) intensify weak diffraction signals to reach the camera. Consequently, the bands with low signal (lower than the background) could then be successfully detected because the signal was increased over the phosphorescent screen light emission threshold.

Figure 4

Although increasing beam current reduces scan time and drift, it compromises the spatial resolution. This may be a concern when investigation of nano grains with sizes comparable to beam probe size is intended. In these cases, lower beam currents in smaller scan areas could be employed to avoid drifting while resolution is maintained.

TKD is able to reveal misorientation gradients in elongated and severely deformed grains. An elongated Cu grain in Fig. 2b and Fig. 4c is demonstrated in Fig. 5 as an example. Inverse pole figure map along with transmitted Kikuchi patterns of selected points are shown in Fig. 5a. Slight pattern rotation is evident from points 1 to 9 indicating gradual orientation change (Fig. 5a). Corresponding orientation deviation (angle of rotation) map has also been presented in Fig. 5b where misorientations are compared to point 1 and presented as a scaled color-coded map.


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There is no clear evidence of low or high angle grain boundaries from points 1 to 8 within the grain presented in Fig. 5. This is an interesting observation indicating the severe elongation as a result of ARB while no static or dynamic recrystallization has occurred. Measured point-toorigin misorientation, point-to-point misorientation, and image quality values along the line connecting points 1 to 8 are plotted in Fig. 5c. Significant variations in point-to-point misorientation and image quality between points 4 and 5 are related to potential low angle grain boundaries. However, no well-defined grain boundary appears in orientation maps (Fig. 5a and 6b).

Total misorientation of nearly 12º from points 1 to 8 in the absence of clear grain boundaries implies that the lattice has undergone a large distortion. The distortion is visible in color-coded orientation maps (Fig. 5a and 5b). However, contribution of possible thickness and surface curvature variations in the FIB slice to the lattice distortion is unknown.

Figure 5

TKD reveals the grain structure of internal nano-oxides in INCONEL 740 nickel-based superalloy which was previously unachieved.63 Bright or dark field TEM images are unable to provide such information which is crucial in understanding oxidation mechanisms. Bright field TEM image of internal Al2O3 and Cr2O3 as well as SAD pattern of the area marked by dashed circle are illustrated in Fig. 6a and 6b, respectively. Although diffraction spots and contrast variations in TEM images indicate the presence of multiple grains, they are unable to quantitatively reveal grains and grain boundaries.

Nano oxide grains are evident in the image quality map produced by TKD (Fig. 6c). This area corresponds to the dashed-rectangle in Fig. 6a. However, simultaneous EDS is required in order to distinguish different phases such as Al2O3 and Cr2O3. These phases have similar crystal structures which make phase detection solely on Kikuchi patterns challenging. Although Kikuchi patterns could be used to characterize phases in conventional EBSD,67 characterization results solely based on pattern analysis were not satisfactory in the current study. This is due to slight


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differences in lattice parameters between phases, pattern center lying outside of the phosphor screen, and severe Kikuchi band widening from top to bottom of the screen.

Performing simultaneous EDS analysis along with TKD facilitates characterization of the oxides despite their similar trigonal crystal structure. TKD phase map demonstrating Al2O3, Cr2O3, and Ni is presented in Fig. 6d. Segregated oxide regions composed of only Cr2O3 or Al2O3 imply that selective internal oxidation has occurred at high angle nickel grain boundaries.

Orientation (inverse pole figure) map of Cr2O3 nano grains as well as coarse Al2O3 and Ni crystals is illustrated in Fig. 6e. The finest detected Cr2O3 grain was approximately 30 nm in length (see Cr2O3 region in Fig. 6c and 6e). The average Cr2O3 grain size was measured as 82 nm. Grains as large as 150 nm in length were identified (Fig. 6c and 6e). These TKD findings confirm that internal chromium oxidation has occurred in INCONEL 740 superalloy.63

Figure 6

There is no strong orientation relationship between Al2O3 and Cr2O3 crystals along the interface in Fig. 6. This may be due to fast kinetics of oxidation. Magnified interface between Al2O3 and Cr2O3 crystals in Fig. 6e is illustrated in Fig. 7. Some Cr2O3 crystals such as 3, 6, and 8 maintain low misorientations with 1 (Al2O3) especially along the basal plane (see lattice orientations represented by red hexagons in Fig. 7 and corresponding Kikuchi patterns). Other crystals appear to have random orientations (compare 2, 4, 5, and 7 with 1 in Fig. 7). Orientation dependence and interface crystallinity between Al2O3 and Cr2O3 have been reported for various applications.68-70 However, no strong orientation dependence has been observed in the current study.

Formation of nano Cr2O3 crystals along Al2O3 interface and their low misorientation imply that sequential internal oxidation has occurred. Chromium has oxidized following the initial oxidation of Aluminum at high angle nickel boundaries as thermodynamic models predicted.63

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TEM orientation mapping of TKD area in Fig. 6 confirms the existence of nano-oxide grains. ASTAR reliability map, phase map, and orientation map in standard diffraction mode are presented in Fig. 8a, 8b, and 8c, respectively. Corresponding maps using nano-beam diffraction with 0.3º precession are illustrated as d, e, and f in Fig. 8.

Despite proven high resolution in TEM/STEM, some nano-grains particularly along Al2O3/Cr2O3 interface are not detectable (compare orientation maps in Fig. 8c and 8f with TKD results in Fig. 6c and 6e). This is due to overlapping grains. Unlike TKD, transmitted electrons in TEM originate from the entire sample thickness due to higher beam energy. This leads to higher vertical resolution in TKD while ASTAR provides higher lateral resolution. Additionally, applying precession in ASTAR may limit spatial resolution. Reducing sample thickness to avoid overlapping grains will improve the grain boundary definition in ASTAR.

Diffused grain boundaries similar to TKD results are evident in reliability and orientation maps (Fig. 8a, 8c, 8d, and 8f). This is due to the angle of grain boundary plane with respect to electron beam direction and the fact that electrons are diffracted at different depths. Applying lower beam currents (smaller aperture size) may improve vertical resolution and provide less-diffused boundaries as shown in the case of Ti nano-grains in Fig. 3c. Lower beam currents result in less beam broadening meaning smaller interaction volumes (both in horizontal and vertical directions). This factor may contribute significantly in enhancing horizontal and vertical resolution. Further experimentation and simulation are required to understand and quantify the beam current effect.

Improved orientation and phase mapping are achieved when precession with 0.3º tilt is applied to nano-beam which has lower current relative to standard diffraction mode (compare Fig. 8a, 8b, and 8c with 8d, 8e, and 8f). Grain boundaries are better-defined as a result of applying precession (compare Fig. 8a and 8d).

Beam precession improves template matching leading to enhanced orientation and phase mapping. Precession of the incident beam using deflection coils located above the sample and 12 ACS Paragon Plus Environment

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collecting coils below the sample allows beam rotation speeds of several hundred hertz with small tilt angles (precession angle).57 This provides a higher number of diffraction spots per point which enhances the accuracy of template matching and orientation determination.

Figure 8

Although applying precession improves phase characterization, ASTAR is still unable to fully differentiate phases such as Al2O3 and Cr2O3 particularly along their interface (Fig. 8e). This is because of close Al2O3 and Cr2O3 lattice parameters. For instance, lattice parameters of ideal Al2O3 and Cr2O3 trigonal crystals used in the current characterization are (a=b=4.91 Å, c=13.58 Å)Cr2O3 and (a=b=4.76 Å, c=12.99 Å)Al2O3. The difference in lattice parameters between Al2O3 and Cr2O3 is less than 5% which makes characterization solely based on diffraction patterns challenging.

Exact crystallographic information such as lattice parameters and atomic positions are required to generate diffraction pattern templates and extract orientations in ASTAR. Slight changes in ideal lattices caused by solid solution atoms and high density of defects can lead to deviations in template matching leading to misidentification. Incorporation of chemical signals into crystallographic data is necessary to distinguish phases with similar crystal structures and overcome slight deviations caused by non-ideal lattices.

Combination of TKD and EDS proves to be more capable in phase characterization since chemical information has been utilized. Even with future TKD developments by introducing a dedicated horizontal phosphor screen below the sample to maintain uniform band width for phase characterization,71 incorporation of EDS data seems necessary in order to achieve accurate phase detection.


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Conclusions:

Transmitted Kikuchi Diffraction (TKD) is a promising technique that offers high spatial resolution to investigate nano grains and grain boundaries. It is capable of revealing grain structure of nano oxides as well as severely deformed metallic ultrafine grains. High beam currents enhanced forward scattered signal intensity leading to lower detector exposure times. Reducing detector exposure time resulted in shorter scans. This minimized drifting and potential carbon contamination.

Despite earlier conclusions that TKD information mostly originates from the near-exit surface, appearance of wide grain boundaries particularly for lower density metals such as titanium imply that diffraction is occurring throughout thickness. Further investigations are required to determine vertical resolution as a function of atomic number.

Combination of TKD and EDS proved to be a powerful tool to characterize nano oxide phases such as Al2O3 and Cr2O3 with similar crystal structures. Characterization solely based on crystallographic data in TKD or TEM (ASTAR) does not provide satisfactory results since lattice parameters of these oxides exhibit slight differences. Although applying precession nano-beam with a small tilt angle improves orientation mapping and phase determination in ASTAR, characterization by combining TKD and EDS exhibited better results since chemical information were incorporated to crystallographic data.

Methods

Two samples with complex grain structures were selected for TKD studies. The first sample is a Cu/Ti bilayer produced via Accumulative-Roll-Bonding (ARB) with ultrafine grains.62 The second is a nano-chromia-alumina structure formed at a high angle nickel grain boundary as a result of grain boundary oxidation of INCONEL 740 superalloy.63 Revealing grain structure of these samples is crucial for understanding their formation mechanisms and properties. However, conventional techniques such as EBSD and TEM are inadequate in providing satisfactory results. 14 ACS Paragon Plus Environment

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EBSD data were acquired via Hitachi S-4300SE scanning electron microscope equipped with Bruker Quantax EBSD/EDS package. Accelerating voltage, beam current, step size, working distance, and detector distance were 20 kV, 4.5 nA, 25 nm, 15 mm, and 15 mm, respectively. Pattern resolution was set at 200×150 for both EBSD and TKD modes.

Electron transparent slices were prepared from selected areas using FEI Helios NanoLab 600 FIB. Average slice thickness was approximately 100 nm. This thickness should limit the number of overlaying nano grains in TKD analysis. However, areas may still exist containing overlaying nano-grains (especially in the nano-oxide sample). Further TEM analysis was carried out using FEI Titan HR-TEM/STEM.

TKD analysis was performed on Zeiss Merlin FE-SEM equipped with Bruker Quantax EBSD/EDS package at Korea Basic Science Institute, Daejeon, Republic of Korea. Accelerating voltage of 29-30 kV was selected. Beam current of 10-47 nA was applied to different scans. Probe current was measured using Faraday cup in Hitachi S-4300SE and a built-in high tension unit in Zeiss Merlin. Step sizes of 5 nm, 3 nm, and 1.5 nm were used to scan the Cu layer, Ti layer, and nano-oxide sample, respectively. TKD samples were tilted 10-15º away from the beam using Bruker TKD holder. Working distance was 4-5 mm. Details of EBSD and TKD configurations are schematically shown in Figure 9.

Orientation imaging in TEM/STEM was performed on the nano-oxide sample to compare with TKD results. JEOL JEM-2100F TEM/STEM equipped with ASTARTM orientation mapping unit was utilized. Maximum accelerating voltage of 200 kV was used. Aperture size for standard diffraction mode and nano-beam diffraction mode were 100 µm and 10 µm, respectively. Camera distance was set at 30 cm. ASTAR diffraction templates for Cr2O3 and Al2O3 were generated using trigonal lattice parameters of (a0=b0=4.91 Å, c0=13.58 Å)Cr2O3 and (a0=b0=4.76 Å, c0=12.99 Å)Al2O3. EBSD, TKD and ASTAR data were converted to an exchangeable text format and post-processed with TSL OIM 7.1 software. Clean-up procedure was carefully applied to reduce the noise. Simulated Kikuchi pattern presented in Fig. 4d was generated by Bruker EBSD software utilizing Euler angles obtained from marked points in Fig. 4a through 4c. 15 ACS Paragon Plus Environment

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Figure 9

It should be noted that coordinate system correction may be needed when TKD and EBSD orientations are compared. TKD information mostly originate from the near-exit surface (-ND) of the electron-transparent sample while EBSD information is obtained from the top surface (ND). This means that the orientations measured by TKD need to be inverted if the EBSD acquisition software lacks the correction. Additionally, thickness and surface curvature variations in thin foils should be considered when local misorientations measured by TKD and TEM-based techniques are compared.

Acknowledgements:

The authors gratefully acknowledge the financial support provided by KIST (grant number: 2E25322). H. Guim acknowledges the New & Renewable Energy R&D program (20113020030020) under the Ministry of Knowledge Economy, Republic of Korea. Assistance from Mrs. Min-Kyung Cho, Mrs. Hyun-Hye Young, and Mr. Cheol-Hwee Shim at Advanced Analysis Center (KIST) and Mr. Chang-Yeon Kim (KBSI) in sample preparation and characterization is greatly appreciated. The authors would like to thank Dr. Daniel Goran at Bruker for constructive TKD discussions.

References:

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Figure Captions:


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Figure 1: Secondary electron image of severely deformed Cu/Ti bilayers (a) and orientation map of the selected area (b). Kikuchi patterns from two neighboring grains (G1 and G2) as well as their grain boundary area (GB) in (b). Step size was 25 nm.

Figure 2: Area selected for FIB sampling (a) which includes Ti and Cu layers. TEM bright field image of FIB slice (b). Areas selected for TEM and TKD investigations are marked with dashedboxes. SAD rings as well as high magnification bright filed images of Cu (TEM1) and Ti (TEM2) layers are presented. Figure 3: TKD results of FIB slice in Fig. 2. a) Forward-scattered detector (ArgusTM) image revealing approximate grain structure. b) TKD inverse pole figure map of Cu layer (TKD1) in Fig. 2b. c) TKD inverse pole figure map of Ti layer (TKD2) in Fig. 2b. Step size of 5 nm and 3 nm were used for (b) and (c), respectively.

Figure 4: Effect of beam current on the scanning time and quality. TKD3 area in Fig. 2 has been scanned with beam currents of 10 nA (a), 25 nA (b), and 40 nA (c). Arrows with different length are indications of drift at low beam currents and long scans. Kikuchi patterns captured at points 1, 2, and 3 exhibit improved pattern contrast and number of bands. Simulated Kikuchi pattern for these points is shown for comparison (d). Step size of 5 nm was used for all scans.

Figure 5: An elongated Cu grain in Fig. 2b and Fig. 4c. Inverse pole figure map along with transmitted Kikuchi patterns of selected points (a). Corresponding orientation deviation (angle of rotation) map (b). Measured point-to-origin misorientation, point-to-point misorientation, and image quality values along the line connecting points 1 to 8 (c). Step size was 5 nm.

Figure 6: Combination of TKD and EDS in characterizing nano-oxides with similar crystal structure. Bright field TEM image of internal Al2O3 and Cr2O3 (a). Selected area diffraction pattern of the area marked by dashed circle indicating multiple grains (b). TKD image quality map corresponding to the dashed-rectangle in TEM bright field image (c). TKD phase map demonstrating Al2O3, Cr2O3, and Ni phases (d). EDS was utilized along with Kikuchi diffraction


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for phase characterization. TKD orientation (inverse pole figure) map combined with image quality (e). Step size of 1.5 nm was used for scanning.

Figure 7: Magnified Al2O3/Cr2O3 interface in Fig. 6e. Label 1 represents Al2O3 crystal. Labels 2 to 8 represent Cr2O3 nano crystals at the interface with Al2O3 crystal. Lattice orientations of crystals are represented by red hexagons relative to sample coordinate system. Transmitted Kikuchi patterns corresponding to each point are illustrated.

Figure 8: TEM orientation mapping results (ASTAR) of the TKD area in Fig. 6. Reliability map (a), phase map (b), and orientation map combined with reliability (c) in standard diffraction mode. Corresponding reliability, phase, and orientation maps of the same area when nano-beam diffraction with 0.3º precession was applied are illustrated in d, e, and f, respectively. Step size was 2.4 nm.

Figure 9: Schematic of sample configuration in EBSD (a) and TKD (b). ND, WD, and DD represent Normal Direction to sample surface, Working Distance, and Detector Distance.


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Figure 1: Secondary electron image of severely deformed Cu/Ti bilayers (a) and orientation map of the selected area (b). Kikuchi patterns from two neighboring grains (G1 and G2) as well as their grain boundary area (GB) in (b). Step size was 25 nm. 104x88mm (300 x 300 DPI)

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Figure 2: Area selected for FIB sampling (a) which includes Ti and Cu layers. TEM bright field image of FIB slice (b). Areas selected for TEM and TKD investigations are marked with dashed-boxes. SAD rings as well as high magnification bright filed images of Cu (TEM1) and Ti (TEM2) layers are presented. 247x74mm (96 x 96 DPI)


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Figure 3: TKD results of FIB slice in Fig. 2. a) Forward-scattered detector (ArgusTM) image revealing approximate grain structure. b) TKD inverse pole figure map of Cu layer (TKD1) in Fig. 2b. c) TKD inverse pole figure map of Ti layer (TKD2) in Fig. 2b. Step size of 5 nm and 3 nm were used for (b) and (c), respectively. 97x38mm (300 x 300 DPI)


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Figure 4: Effect of beam current on the scanning time and quality. TKD3 area in Fig. 2 has been scanned with beam currents of 10 nA (a), 25 nA (b), and 40 nA (c). Arrows with different length are indications of drift at low beam currents and long scans. Kikuchi patterns captured at points 1, 2, and 3 exhibit improved pattern contrast and number of bands. Simulated Kikuchi pattern for these points is shown for comparison (d). Step size of 5 nm was used for all scans. 98x51mm (300 x 300 DPI)


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Figure 5: An elongated Cu grain in Fig. 2b and Fig. 4c. Inverse pole figure map along with transmitted Kikuchi patterns of selected points (a). Corresponding orientation deviation (angle of rotation) map (b). Measured point-to-origin misorientation, point-to-point misorientation, and image quality values along the line connecting points 1 to 8 (c). Step size was 5 nm. 122x117mm (300 x 300 DPI)


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Figure 6: Combination of TKD and EDS in characterizing nano-oxides with similar crystal structure. Bright field TEM image of internal Al2O3 and Cr2O3 (a). Selected area diffraction pattern of the area marked by dashed circle indicating multiple grains (b). TKD image quality map corresponding to the dashed-rectangle in TEM bright field image (c). TKD phase map demonstrating Al2O3, Cr2O3, and Ni phases (d). EDS was utilized along with Kikuchi diffraction for phase characterization. TKD orientation (inverse pole figure) map combined with image quality (e). Step size of 1.5 nm was used for scanning. 149x142mm (300 x 300 DPI)


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Figure 7: Magnified Al2O3/Cr2O3 interface in Fig. 6e. Label 1 represents Al2O3 crystal. Labels 2 to 8 represent Cr2O3 nano crystals at the interface with Al2O3 crystal. Lattice orientations of crystals are represented by red hexagons relative to sample coordinate system. Transmitted Kikuchi patterns corresponding to each point are illustrated. 102x60mm (300 x 300 DPI)


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Figure 8: TEM orientation mapping results (ASTAR) of the TKD area in Fig. 6. Reliability map (a), phase map (b), and orientation map combined with reliability (c) in standard diffraction mode. Corresponding reliability, phase, and orientation maps of the same area when nano-beam diffraction with 0.3º precession was applied are illustrated in d, e, and f, respectively. Step size was 2.4 nm. 147x93mm (300 x 300 DPI)


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Figure 9: Schematic of sample configuration in EBSD (a) and TKD (b). ND, WD, and DD represent Normal Direction to sample surface, Working Distance, and Detector Distance. 106x74mm (300 x 300 DPI)


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Application of Transmitted Kikuchi Diffraction in Studying Nano-oxide

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