- Email: [email protected]
A novel approach to the characterization of thin oxide layers
Author’s Accepted Manuscript A novel approach to the characterization of thin oxide layers B. Rutkowski, A.S. Galanis, A. Gil, A. CzyrskaFilemonowicz www.elsevier.com
PII: DOI: Reference:
S0167-577X(16)30270-1 http://dx.doi.org/10.1016/j.matlet.2016.02.104 MLBLUE20393
To appear in: Materials Letters Received date: 19 December 2015 Revised date: 9 February 2016 Accepted date: 21 February 2016 Cite this article as: B. Rutkowski, A.S. Galanis, A. Gil and A. CzyrskaFilemonowicz, A novel approach to the characterization of thin oxide layers, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.02.104 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A novel approach to the characterization of thin oxide layers B. Rutkowskia*, A. S. Galanisb, A. Gilc, A. Czyrska-Filemonowicza a
International Centre of Electron Microscopy for Materials Science and Faculty of Metals Engineering and Industrial Computer Science, AGH University of Science and Technology, Al. A. Mickiewicza 30, PL-30-059 Kraków, Poland, b
NanoMEGAS SPRL, Blvd Edmond Machtens 79, B-1080 Brussels, Belgium
c
Faculty of Materials Science and Ceramics, AGH University of Science and Technology, Al. A. Mickiewicza 30, PL-30-059 Kraków, Poland. *
corresponding author: [email protected]
Abstract A novel approach to the characterization of thin coatings or surface layers by means of the ASTAR hardware attached to the transmission electron microscope (TEM) and the dedicated software, used for orientation and phase map analyses, was applied to examine the Sanicro 25 steel after 500 h of oxidation in water vapor at 700 C. Phase composition and grain orientation maps as well as texture analysis of thin (i.e. with a thickness of below 1 μm) oxide layers were performed using a new TEM technique similar to electron backscatter diffraction in the scanning electron microscope. The spatial resolution of this technique was sufficient for the imaging of nm-sized grains and a full characterization of the thin oxide scale. The acquired results were verified using data obtained by means of conventional selected area electron diffraction (TEMSAED), energy-dispersive X-ray spectroscopy and texture analyses using X-ray diffraction techniques. It should be noted that high-quality TEM samples are required for such investigations.
Keywords: microstructure, TEM, ASTAR, oxide layer, oxidation, Sanicro 25, Cr2O3 scale 1. Introduction Progress in the design and manufacture of corrosion- and heat-resistant alloys entails novel, state-of-the-art research techniques for the investigation of the physico1
chemical properties of materials. In many modern metallic materials, surface degradation is so small that the thickness of the oxide layer is lower than 1 μm, even after long-term exposure at high temperatures. In these cases, conventional methods such as light microscopy or even scanning electron microscopy have so far proven to be inadequate for investigating the microstructure of corrosion products. Transmission electron microscopy (TEM) and the focused ion beam (FIB) technique developed for the preparation of ultra-thin TEM samples have thus become the methods used increasingly often as the primary research tools in corrosion studies. An example of a new generation of corrosion- and heat-resistant metallic materials is the Sanicro 25 austenitic steel [1], which was introduced to the market by SANDVIK. The target applications of this steel include super-heaters in fossil fuel power plants operating in ultra-supercritical (USC) steam conditions [2]. A material used in super-heaters needs to exhibit long-term creep resistance and very good corrosion resistance under exposure to exhaust gases and water vapor. Results of the research on the corrosion of the Sanicro25 steel in exhaust gas [3] and water vapor [4, 5] media have shown that the resulting oxide layer is not only very thin, but also that it has a complex, multi-layered structure. In multi-component, multi-layered systems, it is not easy to predict the scale composition and microstructure that forms during oxidation. Consequently, it is often necessary to characterize oxide surface layers using advanced analytical techniques, especially cross-section TEM. In the presented study, a new technique for the analysis of coatings or surface layers (e.g. scales), utilizing ASTAR [6, 7] and based on precession electron diffraction [8], was used for the generation of accurate grain orientation, texture and phase maps of a scale with nanometric resolution. The micro- and nanostructure of a thin scale formed on the Sanicro 25 steel after oxidation at 700°C were studied in order to better understand the changes in its morphology and chemical element partitioning that ultimately affect the steel’s properties at high temperatures. The study presents a new approach to this problem, which features the use of a novel electron microscopy technique. 2. Materials and methods 2
The Sanicro 25 (SANDVIK) was delivered as a solution-annealed (1220 °C/5 min/ water-cooled) tube. The chemical composition of the Sanicro 25 is shown in Table 1 (manufacturer data).
Table 1. Chemical composition of the Sanicro 25 steel according to the certificate supplied with the material.
Element
C
Si
Mn
P
S
Cr
Ni
wt%
0.064
0.18
0.51
0.016
0.0005
22.35
25.36
Element
W
Co
Cu
Al
Nb
B
N
wt%
3.37
1.44
2.98
0.023
0.49
0.0035
0.23
Samples in the form of 20x15x2 mm blocks were cut from the tube and oxidized for 500 h in water vapor at 700 °C. The microstructure of the oxidized samples was investigated by means of scanning and transmission electron microscopy (SEM and TEM, respectively). A highresolution scanning electron microscope (Merlin Gemini II manufactured by ZEISS) was used to examine the morphology of the surface of the oxidized sample. For the TEM observations of the thin oxide layers, cross-sectioned samples in the form of small lamellae were prepared via FIB (NEON CrossBeam 40EsB by ZEISS) followed by FIB post-processing using NanoMill 1040 (Fischione). In order to protect the oxide layer against heavy Ga ions during FIB preparation, a Pt layer with a thickness of several µm was deposited on the specimens’ surface. A probe Cs-corrected Titan3 G2 60-300 with the ChemiSTEM™ system (FEI) was used for microstructural- and chemical composition analyses of the oxidized 3
samples. Scanning transmission electron microscopy (STEM) imaging using high-angle annular dark-field (HAADF) contrast and energy dispersive X-ray spectroscopic (EDS) chemical element mapping were used to characterize the layer’s micro- and nanostructure. Phases were identified by means of selected area electron diffraction (SAED) with the assistance of the JEMS software package [9]. The Tecnai G2 20 Twin LaB6 TEM operating at 200 kV and retrofitted with a precession electron diffraction (PED) unit (DigiSTAR P1000 system, NanoMEGAS) controlled by the ASTAR software was used for orientation and phase map analysis at the nanometer level. The ASTAR method in TEM is similar to the conventional EBSD technique applied in scanning electron microscopy. In ASTAR, Brag spot electron diffraction patterns are collected, in contrast to the Kikuchi patterns that are recorded in the EBSD method. However, orientation and phase maps provided by TEM-ASTAR are characterized by an improved spatial resolution (up to 1-2 nm) compared to the typical EBSD method, in which a TEM with a field emission gun (FEG) is used [10]. In the ASTAR approach, the sample is scanned by the electron beam (as in the STEM technique); however, the beam is not convergent, but parallel. At the same time, a large number of sequential electron diffraction spot patterns are acquired from the phosphorous screen of the microscope as sequential frames through an ultra-fast Stingray detector (CCD) attached to the binocular site of the TEM. The beam movement and precession is controlled by a dedicated external device called “Digistar”, which is connected to the beam and image deflection coil control boards present in each TEM. The ASTAR software controls the CCD detector and synchronizes it with the Digistar device. During scanning, thousands of electron diffraction patterns that correspond to each pixel of the scanned area are recorded and stored on a dedicated computer. In order to proceed with the identification of the nanocrystal orientation / phase, each of the experimental patterns is compared to theoretical patterns (templates) generated by the computer based on the crystallographic data uploaded by the user as CIF files for each of the known phases present in the examined sample. The ASTAR software automatically generates spot templates corresponding to every possible orientation of the crystal system. Every experimental pattern is automatically assigned to one template of a certain orientation / phase through a cross-correlation routine. Thus, 4
every experimental pattern is indexed and corresponds to a certain orientation / phase, leading to the final orientation and phase maps. For every measured pixel, ASTAR provides correlation and reliability index scores and maps related to the statistical significance of the orientation assignment of each pattern and for each corresponding point of the map. A correlation index score or map is assigned to the best match of each experimental pattern with the template. The reliability index, which is analogous to the SEM-EBSD confidence index, is defined in such a way that it has a minimum value when more than one solution is proposed and a high value when only one solution is clearly identified. Grayscale index correlation and reliability maps indicating wellmatched patterns and realistic results is visualized as white to light grey pixels; black to dark grey regions on the map demonstrate poorly matched patterns and ambiguous regions (see: Section 3 and Figs 2b and c). In most cases, it is recommended that the orientation/phase maps are presented in combination with index and reliability maps, increasing the level of confidence. Several data sets were collected by means of ASTAR in order to optimize the data acquisition conditions. Different sample areas were investigated using different camera length, step size, and detector exposure time. The presented results were obtained for a data set acquired in the TEM mode using a convergent beam from a 2.25 μm x 1.62 μm area, with a scanning step size of 9.0 nm, a condenser aperture of 50 μm, spot size 10, and camera length of 135 mm. The probe was measured to be around 10 nm and data were collected using the 8 bit Stingray detector with a rate of 14 frames per second, collecting 45000 diffraction patterns with a size of 144x144 pixels. The obtained data were processed by the ASTAR integrated software to index the diffraction patterns and generate the orientation and phase maps. Crystallographic data of compounds present in the sample (Cr2O3, MnO and austenite) were uploaded to the ASTAR software as CIF files to generate templates for every crystal orientation. Distribution corrections due to the Stingray detector position, diffraction pattern center localization and camera length refinement were performed to optimize the indexing and generation of the final maps. All presented phase and orientation maps were created by combining the maps with the correlation index and reliability index maps as described above, which increased the confidence level of results. Further analyses of the generated 5
maps, including grain disorientation, pole figures, and texture analysis, were performed using the integrated software package. 3. Results and discussion Fig. 1a shows the surface morphology of the oxide scale grown on the Sanicro 25 steel after 500 h oxidation in water vapor at 700 °C. Cr2O3 plates in a form similar to a hexagon were observed. Fig. 1b shows a cross-section through the scale, as visualized via STEM-HAADF imaging. The fine-grained structure of the scale is visible. Due to the high amount of chromium (22.5 wt%) in the steel, a continuous, ca. 1 µm thick scale consisting mainly of Cr2O3 (identified by electron diffraction, Fig. 1d) had formed on the surface of the steel. The element distribution map of Cr (Fig. 1c), recorded using STEM-EDS, shows that the scale is enriched with Cr.
Figure 1. Oxide scale after 500 h of oxidation in water vapor at 700 °C: a) surface morphology (SEM image), b) STEM-HAADF image of scale cross-section, c) STEM-EDS element map of chromium (yellow), d) diffraction pattern (SAED) of the oxide scale identified as Cr2O3, with a superimposed pattern calculated using JEMS.
Due to the fact that the oxide scale formed on the Sanicro 25 steel was only ca. 1 µm thick, advanced TEM methods had to be utilized to examine the phase composition and grain orientation of the scale. Fig. 2a shows the STEM-HAADF image of the scale, whereas results acquired and processed by the ASTAR software are presented in Figs 2 b-h.
6
Figure 2. Oxide scale: a) STEM-HAADF image, b) correlation index map, c) reliability map, d) phase composition map, e-g) orientation maps along x,y, z directions, respectively, h) color-coded orientation triangle corresponding to the colors visible in figures e-g.
All presented ASTAR maps were created taking into account the statistical values that validate the maps’ quality, as described in the Materials and methods section. Thus, the combination of the preliminary orientation/phase maps with the correlation index (Fig. 2b) and reliability maps (Fig. 2c) was performed for the presented maps. The fine-grained structure of the scale can be seen clearly. The phase composition map (Fig. 2d) confirms the presence of Cr2O3 (marked in red) in the entire scale. In addition, some grains of MnO (marked in green) were detected. The austenite matrix of the steel is marked in violet. Regions presenting as black areas on the map are unassigned, either due to the absence of the sample (holes) or due to grain boundaries. The slight variation in the independent grains’ color is due to slight variations in orientation and/or index correlation and reliability values of every pixel, as described above. Darker pixels inside independent grains are likely due to variation in grain thickness or the overlapping of grains. As shown in Fig. 2, the spatial resolution was sufficient to visualize even nanometer-sized grains. More specifically, grain boundaries can be clearly identified by means of the ASTAR software, and were detected to be 7
around 15 nm in size (data not shown). It should be noted that these results were obtained using a microscope with an LaB6 gun, which features a resolution that is limited compared to that of an FEG-TEM. In any case, acquisition conditions (e.g. spot size, C2 aperture size, raster scan, scanned area, scanning step, camera length) have to be carefully optimized to ensure the successful generation of orientation/phase maps with increased resolution. Figs 2e-g show a scale grain orientation in three different projections; the color-coded orientation triangle for the Cr2O3 phase is presented in Fig. 2h. In addition to generating phase- and orientation maps, grain texture analyses of the scale were also performed using the described technique. As seen in Fig. 3a, five maxima are present. These maxima originate from elongated, plate-like Cr2O3 crystals shown in Fig. 1a. Since the pole figure is created using all diffraction patterns of Cr2O3 obtained via measurements, a higher amount of diffraction patterns with a specific orientation is present due to the size and elongation of the Cr2O3 plates.
Figure 3. a) texture analysis for the (0001) plane, b) sketch of the Cr2O3 unit cell, as generated by the JEMS software.
The TEM sample preparation process is crucial for such investigations; the FIB technique was therefore used. The lamellae must be very thin and free of artifacts. Aside from offering the possibility of detailed surface layer characterization, the lamellae 8
prepared via FIB allow the proper chemical composition analyses to be performed due to the constant sample thickness over the entire investigated area, which is impossible in the case of the conventional cross-section method. Due to the fact that every point in the scanned area should only yield a single pattern of one phase, which the software can fit and index, FIB lamella should be sufficiently thin to avoid the grain overlapping; the latter would disrupt the phase / orientation identification process and would cause problems with further processing (only the patterns with the best fit would be accepted). In certain cases, the preparation of lamellae with a thickness of about 10-20 nm is required, which is a considerably demanding task. 4. Conclusions The presented novel microscopy technique that utilizes TEM and ASTAR provides interesting and important information concerning the phase composition and grain orientation of thin coatings or surface layers (e.g. oxide scales). It offers an unprecedented possibility of phase identification and texture analyses of surface layers at the micro- and nanometer levels. However, high-quality TEM samples are required for such investigations. The samples prepared by means of FIB must be sufficiently thin, artifact-free and of equal thickness to avoid grain overlapping, which would disrupt the analyses. The combination of an efficient sample preparation with TEM for orientation and phase map generation is crucial in order to shed light on the detailed local properties of materials of high interest at the nanometer level. Acknowledgments The presented research received funding from the European Union Seventh Framework Programme under Grant Agreement 312483 - ESTEEM2 (Integrated Infrastructure Initiative–I3). The authors would like to acknowledge the contribution of Dr. A. Aguero (INTA) for providing the oxidized samples within KMM-VIN cooperation. Authors would also like to thank Prof. W. Ratuszek and MSc. A. Gruszczyński (AGH-UST) as well as Prof. E. Rauch and Dr. M. Veron (Phelma) for their valuable help.
9
References [1] SANDVIK, (2015). http://www.smt.sandvik.com. [2] R. Viswanathan, J.F. Henry, J. Tanzosh, G. Stanko, J. Shingledecker, B. Vitalis, R. Purgert, U.S. Program on materials technology for ultra-supercritical coal power plants, J. Mater. Eng. Perform. 22 (2013) 2904–2915. [3] B. Rutkowski, A. Gil, A. Czyrska-Filemonowicz, Microstructure and chemical composition of the oxide scale formed on the Scanicro 25 steel tubes after fireside corrosion, Corros. Sci. 102 (2016) 373. [4] L. Intiso, L.G. Johansson, S. Canovic, S. Bellini, J.E. Svensson, M. Halvarsson, Oxidation behaviour of Sanicro 25 (42Fe22Cr25NiWCuNbN) in O2/H2O mixture at 600 ˚C, Oxid. Met. 77 (2012) 209–235. [5] J. Zurek, S.-M. Yang, D.-Y. Lin, T. Hüttel, L. Singheiser, W.J. Quadakkers, Microstructural stability and oxidation behavior of Sanicro 25 during long-term steam exposure in the temperature range 600-750 °C, Mater. Corros. 66 (2015) 315–327. [6]
E. F. Rauch, M. Veron, J. Portillo, D. Bultreys, Y. Maniette, S. Nicolopoulos, Automatic crystal orientation and phase mapping in TEM by precession diffraction, Microsc. Anal. 22 (2008) S5.
[7]
E. F. Rauch, J. Portillo, S. Nicolopoulos, D. Bultreys, S. Rouvimov, P. Moeck, Automated nanocrystal orientation and phase mapping in the transmission electron microscope on the basis of precession electron diffraction, Zeits. Kristallogr. 225 (2010) 103.
[8]
R. Vincent and P. Midgley, Double conical beam-rocking system for measurement of integrated electron diffraction intensities, Ultramicroscopy 53 (1994) 271.
[9] P. Stadelmann, JEMS Java Electron Microscopy Software, http://cime.epfl.ch/, 2004. [10] A. Darbal, K. J. Ganesh, X. Liu, S.-B. Lee, J. Ledonne, T. Sun, B. Yao, A. P. Warren, G. S. Rohrer, A. D. Rollett, P. J. Ferreira, K. R. Coffey, and K. Barmak, Grain Boundary Character Distribution of Nanocrystalline Cu Thin Films Using Stereological Analysis of Transmission Electron Microscope Orientation Maps, Microscopy and Microanalysis, 19 (2013) 111-119.
Highlights
New technique for characterisation of surface layers (e.g. oxide scales)
10
Possibility of collecting complex crystallographic data (e.g. grain orientation maps, texture) in TEM
Results of ASTAR technique successfully verified by conventional TEM, EDS and XRD analyses
11
PII: DOI: Reference:
S0167-577X(16)30270-1 http://dx.doi.org/10.1016/j.matlet.2016.02.104 MLBLUE20393
To appear in: Materials Letters Received date: 19 December 2015 Revised date: 9 February 2016 Accepted date: 21 February 2016 Cite this article as: B. Rutkowski, A.S. Galanis, A. Gil and A. CzyrskaFilemonowicz, A novel approach to the characterization of thin oxide layers, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.02.104 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A novel approach to the characterization of thin oxide layers B. Rutkowskia*, A. S. Galanisb, A. Gilc, A. Czyrska-Filemonowicza a
International Centre of Electron Microscopy for Materials Science and Faculty of Metals Engineering and Industrial Computer Science, AGH University of Science and Technology, Al. A. Mickiewicza 30, PL-30-059 Kraków, Poland, b
NanoMEGAS SPRL, Blvd Edmond Machtens 79, B-1080 Brussels, Belgium
c
Faculty of Materials Science and Ceramics, AGH University of Science and Technology, Al. A. Mickiewicza 30, PL-30-059 Kraków, Poland. *
corresponding author: [email protected]
Abstract A novel approach to the characterization of thin coatings or surface layers by means of the ASTAR hardware attached to the transmission electron microscope (TEM) and the dedicated software, used for orientation and phase map analyses, was applied to examine the Sanicro 25 steel after 500 h of oxidation in water vapor at 700 C. Phase composition and grain orientation maps as well as texture analysis of thin (i.e. with a thickness of below 1 μm) oxide layers were performed using a new TEM technique similar to electron backscatter diffraction in the scanning electron microscope. The spatial resolution of this technique was sufficient for the imaging of nm-sized grains and a full characterization of the thin oxide scale. The acquired results were verified using data obtained by means of conventional selected area electron diffraction (TEMSAED), energy-dispersive X-ray spectroscopy and texture analyses using X-ray diffraction techniques. It should be noted that high-quality TEM samples are required for such investigations.
Keywords: microstructure, TEM, ASTAR, oxide layer, oxidation, Sanicro 25, Cr2O3 scale 1. Introduction Progress in the design and manufacture of corrosion- and heat-resistant alloys entails novel, state-of-the-art research techniques for the investigation of the physico1
chemical properties of materials. In many modern metallic materials, surface degradation is so small that the thickness of the oxide layer is lower than 1 μm, even after long-term exposure at high temperatures. In these cases, conventional methods such as light microscopy or even scanning electron microscopy have so far proven to be inadequate for investigating the microstructure of corrosion products. Transmission electron microscopy (TEM) and the focused ion beam (FIB) technique developed for the preparation of ultra-thin TEM samples have thus become the methods used increasingly often as the primary research tools in corrosion studies. An example of a new generation of corrosion- and heat-resistant metallic materials is the Sanicro 25 austenitic steel [1], which was introduced to the market by SANDVIK. The target applications of this steel include super-heaters in fossil fuel power plants operating in ultra-supercritical (USC) steam conditions [2]. A material used in super-heaters needs to exhibit long-term creep resistance and very good corrosion resistance under exposure to exhaust gases and water vapor. Results of the research on the corrosion of the Sanicro25 steel in exhaust gas [3] and water vapor [4, 5] media have shown that the resulting oxide layer is not only very thin, but also that it has a complex, multi-layered structure. In multi-component, multi-layered systems, it is not easy to predict the scale composition and microstructure that forms during oxidation. Consequently, it is often necessary to characterize oxide surface layers using advanced analytical techniques, especially cross-section TEM. In the presented study, a new technique for the analysis of coatings or surface layers (e.g. scales), utilizing ASTAR [6, 7] and based on precession electron diffraction [8], was used for the generation of accurate grain orientation, texture and phase maps of a scale with nanometric resolution. The micro- and nanostructure of a thin scale formed on the Sanicro 25 steel after oxidation at 700°C were studied in order to better understand the changes in its morphology and chemical element partitioning that ultimately affect the steel’s properties at high temperatures. The study presents a new approach to this problem, which features the use of a novel electron microscopy technique. 2. Materials and methods 2
The Sanicro 25 (SANDVIK) was delivered as a solution-annealed (1220 °C/5 min/ water-cooled) tube. The chemical composition of the Sanicro 25 is shown in Table 1 (manufacturer data).
Table 1. Chemical composition of the Sanicro 25 steel according to the certificate supplied with the material.
Element
C
Si
Mn
P
S
Cr
Ni
wt%
0.064
0.18
0.51
0.016
0.0005
22.35
25.36
Element
W
Co
Cu
Al
Nb
B
N
wt%
3.37
1.44
2.98
0.023
0.49
0.0035
0.23
Samples in the form of 20x15x2 mm blocks were cut from the tube and oxidized for 500 h in water vapor at 700 °C. The microstructure of the oxidized samples was investigated by means of scanning and transmission electron microscopy (SEM and TEM, respectively). A highresolution scanning electron microscope (Merlin Gemini II manufactured by ZEISS) was used to examine the morphology of the surface of the oxidized sample. For the TEM observations of the thin oxide layers, cross-sectioned samples in the form of small lamellae were prepared via FIB (NEON CrossBeam 40EsB by ZEISS) followed by FIB post-processing using NanoMill 1040 (Fischione). In order to protect the oxide layer against heavy Ga ions during FIB preparation, a Pt layer with a thickness of several µm was deposited on the specimens’ surface. A probe Cs-corrected Titan3 G2 60-300 with the ChemiSTEM™ system (FEI) was used for microstructural- and chemical composition analyses of the oxidized 3
samples. Scanning transmission electron microscopy (STEM) imaging using high-angle annular dark-field (HAADF) contrast and energy dispersive X-ray spectroscopic (EDS) chemical element mapping were used to characterize the layer’s micro- and nanostructure. Phases were identified by means of selected area electron diffraction (SAED) with the assistance of the JEMS software package [9]. The Tecnai G2 20 Twin LaB6 TEM operating at 200 kV and retrofitted with a precession electron diffraction (PED) unit (DigiSTAR P1000 system, NanoMEGAS) controlled by the ASTAR software was used for orientation and phase map analysis at the nanometer level. The ASTAR method in TEM is similar to the conventional EBSD technique applied in scanning electron microscopy. In ASTAR, Brag spot electron diffraction patterns are collected, in contrast to the Kikuchi patterns that are recorded in the EBSD method. However, orientation and phase maps provided by TEM-ASTAR are characterized by an improved spatial resolution (up to 1-2 nm) compared to the typical EBSD method, in which a TEM with a field emission gun (FEG) is used [10]. In the ASTAR approach, the sample is scanned by the electron beam (as in the STEM technique); however, the beam is not convergent, but parallel. At the same time, a large number of sequential electron diffraction spot patterns are acquired from the phosphorous screen of the microscope as sequential frames through an ultra-fast Stingray detector (CCD) attached to the binocular site of the TEM. The beam movement and precession is controlled by a dedicated external device called “Digistar”, which is connected to the beam and image deflection coil control boards present in each TEM. The ASTAR software controls the CCD detector and synchronizes it with the Digistar device. During scanning, thousands of electron diffraction patterns that correspond to each pixel of the scanned area are recorded and stored on a dedicated computer. In order to proceed with the identification of the nanocrystal orientation / phase, each of the experimental patterns is compared to theoretical patterns (templates) generated by the computer based on the crystallographic data uploaded by the user as CIF files for each of the known phases present in the examined sample. The ASTAR software automatically generates spot templates corresponding to every possible orientation of the crystal system. Every experimental pattern is automatically assigned to one template of a certain orientation / phase through a cross-correlation routine. Thus, 4
every experimental pattern is indexed and corresponds to a certain orientation / phase, leading to the final orientation and phase maps. For every measured pixel, ASTAR provides correlation and reliability index scores and maps related to the statistical significance of the orientation assignment of each pattern and for each corresponding point of the map. A correlation index score or map is assigned to the best match of each experimental pattern with the template. The reliability index, which is analogous to the SEM-EBSD confidence index, is defined in such a way that it has a minimum value when more than one solution is proposed and a high value when only one solution is clearly identified. Grayscale index correlation and reliability maps indicating wellmatched patterns and realistic results is visualized as white to light grey pixels; black to dark grey regions on the map demonstrate poorly matched patterns and ambiguous regions (see: Section 3 and Figs 2b and c). In most cases, it is recommended that the orientation/phase maps are presented in combination with index and reliability maps, increasing the level of confidence. Several data sets were collected by means of ASTAR in order to optimize the data acquisition conditions. Different sample areas were investigated using different camera length, step size, and detector exposure time. The presented results were obtained for a data set acquired in the TEM mode using a convergent beam from a 2.25 μm x 1.62 μm area, with a scanning step size of 9.0 nm, a condenser aperture of 50 μm, spot size 10, and camera length of 135 mm. The probe was measured to be around 10 nm and data were collected using the 8 bit Stingray detector with a rate of 14 frames per second, collecting 45000 diffraction patterns with a size of 144x144 pixels. The obtained data were processed by the ASTAR integrated software to index the diffraction patterns and generate the orientation and phase maps. Crystallographic data of compounds present in the sample (Cr2O3, MnO and austenite) were uploaded to the ASTAR software as CIF files to generate templates for every crystal orientation. Distribution corrections due to the Stingray detector position, diffraction pattern center localization and camera length refinement were performed to optimize the indexing and generation of the final maps. All presented phase and orientation maps were created by combining the maps with the correlation index and reliability index maps as described above, which increased the confidence level of results. Further analyses of the generated 5
maps, including grain disorientation, pole figures, and texture analysis, were performed using the integrated software package. 3. Results and discussion Fig. 1a shows the surface morphology of the oxide scale grown on the Sanicro 25 steel after 500 h oxidation in water vapor at 700 °C. Cr2O3 plates in a form similar to a hexagon were observed. Fig. 1b shows a cross-section through the scale, as visualized via STEM-HAADF imaging. The fine-grained structure of the scale is visible. Due to the high amount of chromium (22.5 wt%) in the steel, a continuous, ca. 1 µm thick scale consisting mainly of Cr2O3 (identified by electron diffraction, Fig. 1d) had formed on the surface of the steel. The element distribution map of Cr (Fig. 1c), recorded using STEM-EDS, shows that the scale is enriched with Cr.
Figure 1. Oxide scale after 500 h of oxidation in water vapor at 700 °C: a) surface morphology (SEM image), b) STEM-HAADF image of scale cross-section, c) STEM-EDS element map of chromium (yellow), d) diffraction pattern (SAED) of the oxide scale identified as Cr2O3, with a superimposed pattern calculated using JEMS.
Due to the fact that the oxide scale formed on the Sanicro 25 steel was only ca. 1 µm thick, advanced TEM methods had to be utilized to examine the phase composition and grain orientation of the scale. Fig. 2a shows the STEM-HAADF image of the scale, whereas results acquired and processed by the ASTAR software are presented in Figs 2 b-h.
6
Figure 2. Oxide scale: a) STEM-HAADF image, b) correlation index map, c) reliability map, d) phase composition map, e-g) orientation maps along x,y, z directions, respectively, h) color-coded orientation triangle corresponding to the colors visible in figures e-g.
All presented ASTAR maps were created taking into account the statistical values that validate the maps’ quality, as described in the Materials and methods section. Thus, the combination of the preliminary orientation/phase maps with the correlation index (Fig. 2b) and reliability maps (Fig. 2c) was performed for the presented maps. The fine-grained structure of the scale can be seen clearly. The phase composition map (Fig. 2d) confirms the presence of Cr2O3 (marked in red) in the entire scale. In addition, some grains of MnO (marked in green) were detected. The austenite matrix of the steel is marked in violet. Regions presenting as black areas on the map are unassigned, either due to the absence of the sample (holes) or due to grain boundaries. The slight variation in the independent grains’ color is due to slight variations in orientation and/or index correlation and reliability values of every pixel, as described above. Darker pixels inside independent grains are likely due to variation in grain thickness or the overlapping of grains. As shown in Fig. 2, the spatial resolution was sufficient to visualize even nanometer-sized grains. More specifically, grain boundaries can be clearly identified by means of the ASTAR software, and were detected to be 7
around 15 nm in size (data not shown). It should be noted that these results were obtained using a microscope with an LaB6 gun, which features a resolution that is limited compared to that of an FEG-TEM. In any case, acquisition conditions (e.g. spot size, C2 aperture size, raster scan, scanned area, scanning step, camera length) have to be carefully optimized to ensure the successful generation of orientation/phase maps with increased resolution. Figs 2e-g show a scale grain orientation in three different projections; the color-coded orientation triangle for the Cr2O3 phase is presented in Fig. 2h. In addition to generating phase- and orientation maps, grain texture analyses of the scale were also performed using the described technique. As seen in Fig. 3a, five maxima are present. These maxima originate from elongated, plate-like Cr2O3 crystals shown in Fig. 1a. Since the pole figure is created using all diffraction patterns of Cr2O3 obtained via measurements, a higher amount of diffraction patterns with a specific orientation is present due to the size and elongation of the Cr2O3 plates.
Figure 3. a) texture analysis for the (0001) plane, b) sketch of the Cr2O3 unit cell, as generated by the JEMS software.
The TEM sample preparation process is crucial for such investigations; the FIB technique was therefore used. The lamellae must be very thin and free of artifacts. Aside from offering the possibility of detailed surface layer characterization, the lamellae 8
prepared via FIB allow the proper chemical composition analyses to be performed due to the constant sample thickness over the entire investigated area, which is impossible in the case of the conventional cross-section method. Due to the fact that every point in the scanned area should only yield a single pattern of one phase, which the software can fit and index, FIB lamella should be sufficiently thin to avoid the grain overlapping; the latter would disrupt the phase / orientation identification process and would cause problems with further processing (only the patterns with the best fit would be accepted). In certain cases, the preparation of lamellae with a thickness of about 10-20 nm is required, which is a considerably demanding task. 4. Conclusions The presented novel microscopy technique that utilizes TEM and ASTAR provides interesting and important information concerning the phase composition and grain orientation of thin coatings or surface layers (e.g. oxide scales). It offers an unprecedented possibility of phase identification and texture analyses of surface layers at the micro- and nanometer levels. However, high-quality TEM samples are required for such investigations. The samples prepared by means of FIB must be sufficiently thin, artifact-free and of equal thickness to avoid grain overlapping, which would disrupt the analyses. The combination of an efficient sample preparation with TEM for orientation and phase map generation is crucial in order to shed light on the detailed local properties of materials of high interest at the nanometer level. Acknowledgments The presented research received funding from the European Union Seventh Framework Programme under Grant Agreement 312483 - ESTEEM2 (Integrated Infrastructure Initiative–I3). The authors would like to acknowledge the contribution of Dr. A. Aguero (INTA) for providing the oxidized samples within KMM-VIN cooperation. Authors would also like to thank Prof. W. Ratuszek and MSc. A. Gruszczyński (AGH-UST) as well as Prof. E. Rauch and Dr. M. Veron (Phelma) for their valuable help.
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Highlights
New technique for characterisation of surface layers (e.g. oxide scales)
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Possibility of collecting complex crystallographic data (e.g. grain orientation maps, texture) in TEM
Results of ASTAR technique successfully verified by conventional TEM, EDS and XRD analyses
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