Atomic-Scale Characterization of Nanostructured Metallic Materials by HAADF/Z-contrast STEM

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1 Materials Transactions, Vol. 44, No. 10 (2003) pp to 2041 Special Issue on Nano-Hetero Structures in Advanced Metallic Materials #2003 The Japan Institute of Metals OVERVIEW Atomic-Scale Characterization of Nanostructured Metallic Materials by HAADF/Z-contrast STEM Eiji Abe Materials Engineering Laboratory, National Institute for Materials Science, Tsukuba , Japan We demonstrate that high-angle annular-dark-field scanning transmission electron microscopy (HAADF-STEM) with a finely-focused electron probe (0:15 nm) is very powerful technique to provide direct information on a local chemistry of nano-materials at atomic scale. This is due to an atomic-number (Z) sensitive nature of the HAADF contrast (Z-contrast). We describe the microstructures of some crystalline, quasicrystalline and amorphous metallic materials with focusing on their local chemical environments. Not only the chemical Z-contrast, HAADF-STEM possesses more substantial advantages compared to the conventional phase-contrast high-resolution imaging, so that it would be one of the most powerful methods for total characterization of nano-structured materials. (Received April 23, 2003; Accepted June 19, 2003) Keywords: scanning transmission electron microscope, high-angle annular-dark-field imaging, atomic-resolution Z-contrast, local chemistry, aluminium- and magnesium-alloys 1. Introduction Nano- or even atomic-scale microstructural control has become a key technique to develop super functional materials. For metallic materials, this fine-scale structure control may be achieved through alloying with a small amount of additional elements into the matrix, which leads to a solid solution, local clustering or nano-scale precipitates that improve the material s properties. This is originated from the solibility or non-soichiometry nature intrinsic for most metallic phases. Besides, there are various structural forms available for metallic phases; crystal, quasicrystal and amorphous states, some of which are realized through a successful non-equilibrium processing. Because of these various flexibilities in terms of matrix structures and chemical environments (alloy compositions), metallic materials are still strongly promising candidates that would realize some superior properties via microstructural controls at extremely fine-scale. For the success of this project, a precise characterization of the local element distribution as well as local atomic configurations is essential to understand the microscopic origins of the alloy properties. Annular dark-field scanning transmission electron microscope (ADF-STEM) provides the atomic-resolution images by effectively illuminating each atomic column one-by-one as a finely focused electron probe (< 0:2 nm) scans across the specimen, generating an intensity map at the annular detector as shown in Fig. 1. 1) When detecting high-angle scatterings, the intensity reaching the detector is almost proportional to the square of the atomic number, Z 2, so that the strong chemical contrast (Z-contrast) can be obtained by high-angle ADF-STEM (HAADF-STEM). A typical HAADF contrast observed in an Al-Ag alloy is shown in Fig. 2. More importantly, the high-angle scattered electrons are basically incoherent to a good approximation, and hence the HAADF-STEM images are the direct representation of the specimen scattering power 1) (weighted by Z 2 ). The incoherent HAADF-STEM image can therefore be directly inverted to the object function representing atomic positions Fig. 1 Schematic illustration showing principle of atomic-resolution imaging with annular-dark-field scanning transmission electron microscope. and species, without the image simulations. This is in sharp contrast to the fact that the conventional coherent highresolution phase-contrast images suffer from the phase problem which means that they cannot be directly converted to the atomic positions; in principle, simulations are necessary for their interpretations. In addition, STEM is the only viable means for obtaining the spectroscopic analysis at near atomic-resolution. Energy-dispesive x-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS)

2 2036 E. Abe Fig. 2 Chemically sensitive Z-contrast clearly reveals the Z-shaped 0 - Ag 2 Al platelet precipitates in the Al-Ag alloy. Since the atomic number of Ag (Z=47) is much larger than that of Al (Z=13), Ag-rich regions reveal significant Z-contrast. Particles showing relatively weak Z-contrast are the sphere-shaped GP zone of Ag atoms embedded in the Al matrix. measurements of local compositions and electronic structures are possible by locating the atomic-sized electron probe on an individual atomic column. 2) Because of these substantial advantages compared to conventional high-resolution transmission electron microscopy (HRTEM), atomic-resolution STEM would be one of the most powerful methods for total characterization of nanostructured materials to understand the microscopic origins of their physical and chemical properties. 2) In this paper, we briefly describe some of our recent results that demonstrate the powerful use of Z-contrast to investigate the local chemistry at very high-spatial resolution. 2. Scanning Transmission Electron Microscope For atomic-resolution HAADF-STEM observations, we used a 200 kv field-emission transmission electron microscope (JEOL: JEM-2010F) equipped with a scanning unit. An objective lens is designed as an ultrahigh resolution type pole-piece (a spherical aberration coefficient Cs ¼ 0:5 mm), providing a minimum probe of approximately 0:15 nm with a convergence angle of 10 mrad. A resolution of STEM is determined by a probe size, since the incoherent image can be represented, to a good approximation, by a simple convolution between the projected potential and the probe intensity profile. For the HAADF imaging, the annular detector was set to collect the electrons scattered at angles larger than mrad, sufficiently high to reveal a chemical sensitive Z-contrast. 3. Results and Discussions 3.1 Al-Li-Cu alloy In Al-Li-Cu alloys, two types of precipitates occur, which are a famous Guinier-Preston (GP) zone of Cu atoms and a 0 - Al 3 Li with L1 2 structure. 3) In spite of the fact that these two precipitates are technologically important to control the desired mechanical properties, their precipitation mechanisms are still not fully understood. One of the reasons is due to a lack of structure characterization of these precipitates in atomic level. Recently, the early stages of precipitation behaviors of aged Al-Li-Cu alloys have been investigated using both the HRTEM and HAADF-STEM. 3) Here we describe the microstructural details revealed by atomicresolution Z-contrast. Figure 3(a) shows the HAADF-STEM image of the Al- 6.0 at%li-1.3 at%cu alloy, taken along the [100] axis of the Al matrix. One clearly notices the strong Z-contrast from the GP-I zone structure, a monolayer of Cu atoms lying on the {100} planes of Al crystal. 4) It may also be noticeable that the background contrast is somewhat weaker around the GP-I zones, such as indicated by arrows. This is due to 0 -Al 3 Li precipitates formed in these regions. In the corresponding atomic-resolution Z-contrast image (Fig. 3(b)), the Al, 0 - Al 3 Li and Cu monolayer structures are definitely identified from the contrast. The intensity profile (Fig. 3(c)) generates fairly well the alternate (002) planes of Al lattice and (001) planes of 0 -Al 3 Li with L1 2 structure. Note that the nearestneighbor lattice planes next to the Cu monolayer can be deduced to be Al layers from the sequence of alternate Al and (Al+Li) layers in Z-contrast, although they are not resolved due to a spread background effect of strong contrast at the Cu layer. Anti-phase relationship between the L1 2 superlattice domains across the Cu layer 3) can be also recognized as the relative shift of alternate (020) lattice planes with a sequence of Al and (Al+Li) layers, as indicated by arrows in the Z- contrast image of Fig. 3(d). From these results, the corresponding local atomic configuration can be constructed (Fig. 3(e)) without any help of image simulation. Note that try-and-error comparisons between the simulation of the model structure and observed image have been essentially necessary, if one attempts to derive the atomic configuration around the GP zone from phase-contrast HRTEM imaging that is based on many-beam interference. 5) This is because that various kinds of image contrast occur depending on imaging conditions, for which the defocus values and the specimen thickness are the dominant factors. It is also noted that local strain field around the GP zone may seriously affect and hamper the clear atomic-resolution imaging in conventional HRTEM, while the ADF-STEM is expected to be much less sensitive to the strain field effects when detecting sufficient high-angle scatterings. 3.2 Mg-Zn-Y nanocrystalline alloy A nanocrystalline Mg-1 at%zn-2 at%y alloy (denoted as Mg 97 Zn 1 Y 2 ) was successfully developed by warm extrusion of rapidly solidified powders at 573 K, showing excellent mechanical properties including maximum tensile yield strength 600 MPa and elongation 5% at room temperature. 6,7) HRTEM investigation of the microstructure showed a formation of a novel long-period hexagonal structure, 7) which is believed to play an important role in strengthening the alloy. However, details of atomic configurations and especially the distribution of Zn and Y in the novel structure

3 Atomic-Scale Characterization of Nanostructured Metallic Materials by HAADF/Z-contrast STEM 2037 Fig. 3 (a) HAADF-STEM image obtained from the Al-6.0 at%li-1.3 at%cu alloy aged at 473 K for 2 days. (b) Enlargement of the rectangle area in (a), showing the atomic-resolution Z-contrast image of the precipitates. (c) One-dimensional Z-contrast intensity profile across X-Y obtained by integrating over the framed area in (b). (d) Fourier-filtered image of (b), obtained by low-pass filter mask to remove the background noise. (e) Atomic model directly derived from the incoherent Z-contrast image. were not clarified. Figures 4(a) and (b) show many-beam bright-field TEM and HAADF-STEM images of the fine-lamellar grains composed of the novel long-period stacking structure. Some extra diffraction spots show up along the c -direction with strong streaks (Fig. 4(a)), revealing a correlation length of 6 d (002)Mg as marked by an arrow in the diffraction pattern. In the HAADF-STEM image, one finds an interesting feature where the lamellae appear as bright lines due to the significant Z-contrast, directly representing the distributions of the heavier constituent elements in the fine-lamellar grain. 8) That is, Zn and/or Y atoms are located not randomly as a solid solution in the whole Mg crystal, but at the specific positions of the lamellar structure. In fact, the bright Z- contrast lamellae are arrayed with an interval of approximately 1.56 nm (¼ 6 d (002)Mg ) at some places, which means a certain chemical order with this correlation-length in the long-period structure. Further structural details of the long-period six-layer structure can be derived from the atomic-resolution HRTEM and HAADF-STEM images (Figs. 4(c)-(e)). 8) The HRTEM image, in which bright dots represent projected atomic columns under this observation condition (Fig. 4(c)), shows that the long-period six-layer structure is basically constructed by a stacking sequence of ABCBCB (6H-type) with accompanying lattice distortion from an ideal 6H-type. The corresponding atomic-resolution HAADF image clearly reveals strong Z-contrast at two close-packed planes (Fig. 4(b)) that are A- and one of the B-layers in the ABCBCB stacking-structure inserted in Fig. 4(e). This directly shows a chemical order configuration, and the present long-period ordered structure is represented as ABCBCB 0 where A and B 0 layers are remarkably enriched by Zn and/or Y. It is noteworthy here that, in the HRTEM image of Fig. 4(a), the brightest spots occur at different layers from those in the Z- contrast, so that it is difficult to deduce a possible chemical order configuration solely based on the HRTEM image. Since the Z-contrast method cannot distinguish whether both of the Zn and Y or only one of them enrich the significant bright Z-contrast layers, EDS analysis with the sub-nanometer (0:3 nm) probe was performed for the regions X, Y in Fig. 4(d). 8) It was found that both the Zn and Y are significantly enriched at the bright Z-contrast layers, leading to the atomic structural model shown in Fig. 4(f). The results demonstrate that the additional elements of a few atomic percent to Mg lead to formation of a long-period chemical-ordered as well as stacking-ordered structure, as directly revealed by a unique Z-contrast method. 3.3 Al-Ni-Co quasicrystalline compound The Al 72 Ni 20 Co 8 decagonal phase is thermodynamically stable at 1100 K and well characterized as a nearly-perfect, ideal quasiperiodic structure. Since the most quasicrystalline phases form as an intermetallic compound with a chemicallyordered structure, Z-contrast is effective for specifying the heavy atom position in the structure. 9) Z-contrast images of

4 2038 E. Abe Fig. 4 (a) Many-beam bright-field TEM image together with the corresponding electron diffraction pattern and (b) HAADF-STEM image obtained from the fine-lamellar grains formed in the nanocrystalline Mg 97 Zn 1 Y 2 alloy. Atomic-resolution (c) HRTEM and (d) HAADF- STEM images of the novel long-period ordered structure. (e) Fourier-filtered Z-contrast image of (b), obtained by applying a low-pass filter mask to remove the background noise level. (f) Atomic model of the long-period chemically-ordered as well as stacking-ordered Mg-(Zn, Y) structure. the Al 72 Ni 20 Co 8 structure successfully revealed a striking feature of transition metal (TM; Ni or Co) arrangements that breaks tenfold symmetry of the decagonal cluster, 10 13) which is a structural unit to construct the entire quasiperiodic structure and had been believed to be tenfold symmetric. This discovery of local symmetry-breaking lead to a new paradigm of the quasi-unit-cell picture. 12) Apart from the chemical Z-contrast, ADF-STEM may provide information on a local thermal vibration anomaly at atomic-scale, 14) because high-angle scattering is dominated by phonon scattering events, namely the thermal diffuse scattering (TDS) 1) that is sensitive to the Debye-Waller (DW) factor. We describe the direct observation of local anomalies of the DW factor in the Al 72 Ni 20 Co 8, through an in-situ hightemperature ADF-STEM observation. 14) In the observation at room temperature, 300 K (Fig. 5(a)), the ADF-STEM image highlights the TM positions relative to the Al due to the Z- contrast. Some decagonal clusters with a diameter of 2 nm are outlined in Fig. 5(a). When the sample is heated and held at approximately 1100 K, we find a remarkable change in the relative contrast; compare Fig. 5(a) with Fig. 5(b). It is evident that a significant enhancement in contrast appears at some specified places that can be well represented by the pentagonal quasiperiodic lattice (tiling) with an edge-length of 2 nm, as drawn in Fig. 5(b). Further, we notice that the anomalous contrast occurs at cores of the decagons, as shown in Figs. 5(c) and (d). Viewing carefully the interior contrast of the decagons, increase of intensities is found to be significant at the positions indicated by arrowheads, which are the Al sites in the decagonal cluster model (Fig. 5(e)). We confirmed that this temperature-dependence contrast change is reversible; that is, after cooling down to 300 K from 1100 K, the anomalous contrast regions become darker again. This temperature-dependence of ADF-contrast is fairly well explained by assuming the anomalous DW factor at the specific Al atomic sites, demonstrating the first direct observation of a local vibration anomaly in a solid. The local DW anomalies imply significant anharmonicity at these specific Al sites, providing an important hint on the issue of phason - the phason is an extra elastic degree of freedom specific to the quasicrystals (in addition to the usual phonons in crystals) and has been theoretically predicted to cause

5 Atomic-Scale Characterization of Nanostructured Metallic Materials by HAADF/Z-contrast STEM 2039 Fig. 5 Atomic-resolution ADF-STEM images of Al 72 Ni 20 Co 8 along the tenfold symmetry axis, taken at (a) 300 K and (b) 1100 K. Decagonal clusters with a diameter of about 2 nm, whose image contrast at (c) 300 K and (d) 1100 K. (e) Atomic model of the cluster. localized fluctuations. The results described here demonstrate the additional capabilities and advantages of ADF-STEM - by doing the angle-resolved 14) and/or in-situ heating/cooling experiments, it now can be used to see not only the atomic structure but also the significant local fluctuations, which is caused by slight atomic displacement with a order of 0:01 nm. 3.4 Al-Ni-Cu-Ce amorphous alloy Al-base amorphous alloys in the Al-TM-RE (TM: transition metal, RE: rare-earth metal) systems possess excellent mechanical properties including maximum strength 1000 MPa. 15,16) Interestingly, their strength can be further improved by having nano-scale precipitation of fcc-al crystals within the amorphous matrix. The early stage of Al nanocrystal formation was investigated to understand why a very high density of nanosrystals can be formed, and it was proposed that heavy RE atom behaviors may determine the growth and thermal stability of the nanocrystals. 17) We describe microstructures of an Al 87 Ni 7 Cu 3 Ce 3 (at%) amorphous alloy 18) through a preliminary Z-contrast observation, and test whether or not the heavy atom positions in the amorphous matrix can be detected. Since the atomic number of Ce (Z=58) is much larger than the other constituent elements (Al: Z=13, Ni: Z=28, Cu: Z=29), we would expect sufficient Z-contrast from the Ce atoms even though they are just added to the alloy by a few atomic percent. Figure 6(a) shows the HAADF-STEM image of the Al 87 Ni 7 Cu 3 Ce 3 amorphous structure. In the image, one notices that some brightest spots occur, some of which are indicated by small circles. Note that there are no definite atomic columns in the amorphous structure; in the case of zone axis imaging of crystalline or quasicrystalline materials described previously, all the bright spots represent the projected atomic columns. Therefore, the brightest spots observed in Fig. 6(a) are highly likely to be individual Ce atoms embedded in the amorphous matrix. Some Ce atoms seem to form clusters as indicated by large circles. Under the zone axis illumination condition for the crystalline specimen, a successful atomic-resolution STEM imaging is described by the strong electron channeling effects along the atomic columns. 1,2) Although there are no such columns, we nevertheless obtain atomic-scale Z- contrast images from the amorphous structure composed of a small amount of heavy atoms. In this case, the situation may be similar to the imaging of a single, isolated heavy atom located on an amorphous carbon support. In this sense, projected potential concept can be applied to explain the successful imaging of individual heavy atoms embedded in the amorphous matrix. The advantageous Z-contrast imaging is well demonstrated after nanocrystallization takes place (Fig. 6(b)). Since the nanocrystals precipitate in almost pure fcc-al, 17) these are clearly identified as significantly darker regions in the Z- contrast image of Fig. 6(b), whereas it is rather difficult and not straightforward to find the Al nanocrystals in the HRTEM image of Fig. 6(c). As nanocrystallization proceeds, the

6 2040 E. Abe Fig. 6 HAADF-STEM images of (a) as rapidly-solidified and (b) subsequently annealed (at 553 K for 30 sec) samples of an Al 87 Ni 7 Cu 3 Ce 3 alloy. In this alloy, a high density of Al nanocrystals precipitates in the amorphous matrix by short time annealing. (c) HRTEM phase-contrast image of the annealed Al 87 Ni 7 Cu 3 Ce 3 alloy. amorphous matrix becomes enriched by the solute atoms rejected from the growing Al nanocrystals, and therefore the matrix reveals brighter Z-contrast in Fig. 6(b). Even with this situation, we still find the brightest spots within the amorphous matrix as indicated by small circles in Fig. 6(b); these can be attributed to the Ce atoms. As demonstrated in the present work, atomic-resolution Z- contrast imaging is also powerful to investigate the structure of multi-component amorphous alloys. Further detailed analysis regarding the Ce atom distribution in the Al 87 Ni 7 Cu 3 Ce 3 amorphous and nanocrystalline states is still in progress, and the results will be published elsewhere. 4. Summary We have demonstrated that HAADF-STEM provides direct information about local atomic structure including its chemical environments through the atomic-number sensitive contrast (Z-contrast), which is shown to be effectively applicable for crystalline, quasicrystalline and even amorphous metallic materials. Since there are few possible artifacts for HAADF imaging compared to the phase-contrast imaging, HAADF imaging gives a straightforward interpretation on the atomic structures. Combining with the spectroscopic analysis such as EELS or EDS, ADF-STEM will be one of the most powerful methods for total characterization of nano-structured materials. Acknowledgements This paper is based on the several collaborating projects. The author would like to thank to the following collaborators: Dr. M. Murayama and Dr. K. Hono for the Al-Ag alloy, Prof. T. J. Konno and Prof. K. Hiraga for Al-Li-Cu alloys, Prof. Y. Kawamura and Prof. A. Inoue for Mg-Zn-Y nanocrystalline alloy, Dr. A. P. Tsai and Dr. S. J. Pennycook for Al-Ni-Co quasicrystal, and Dr. A. P. Tsai for Al-Ni-Cu-Ce amorphous alloys. REFERENCES 1) S. J. Pennycook and D. E. Jesson: Acta Metall. Mater. 40 (1992) S149- S159. 2) S. J. Pennycook and P. D. Nellist: Impact of electron and Scanning Probe Microscopy on Materials Research, (Kluwer Academic Publishers, Netherlands, 1999) pp ) R. Yoshimura, T. J. Konno, E. Abe and K. Hiraga: Acta Mater. 51 (2003) ) T. J. Konno, K. Hiraga and M. Kawasaki: Scr. Mater. 44 (2001) ) M. Karlik, B. Joufrrey and S. Belliot: Acta Mater. 46 (1998) ) Y. Kawamura, K. Hayashi, A. Inoue and T. Masumoto: Mater. Trans. 42 (2001) ) A. Inoue, Y. Kawamura, M. Matsushita, K. Hayashi and J. Koike: J. Mater. Res. 16 (2001) ) E. Abe, Y. Kawamura, K. Hayashi and A. Inoue: Acta Mater. 50 (2002) ) E. Abe, H. Takakura and A. P. Tsai: J. Electron Microscopy 50(3) (2001)

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