Microscopy Conference 2023

Prof. Dr.

Session chairs

This symposium will present recent progress in the field of metals, alloys, and intermetallic compounds. The material portfolio includes high entropy alloys, chemically complex alloys and metallic glasses and aims at bringing together specialists from materials design, microstructure evolution and material properties. 
Contributed papers are very much welcome.


Small scale model systems for in situ TEM studies: multilayered and alloyed nanowires

MS 2: Metals and alloys

Plenary

Abstract text (incl. figure legends and references) To understand the behavior of complex alloys and composites under mechanical and thermal loading small scale model systems are needed. Only such model systems provide the opportunity to analyze mechanisms of plastic deformation, interdiffusion and phase transformation down to the atomic scale. Tailored nanowires are ideal in this role as they can be directly studied with in situ TEM to elucidate the effect of compositional complexity, multilayered coatings and local ordering. Multilayered coatings prevent the underlying core material from degradation and enhance the mechanical and wear-resistant behavior. Such a coating consists for example of alternating nm-sized layers of metals and metal-oxides. The coating life-time crucially depends on crack initialization and propagation at the interfaces. To further optimize the performance and to prevent early failure like layer delamination, the underlying failure mechanisms on the atomic scale have to be studied. Small scale model systems of such coatings allows characterizing application-limiting factors inside the Transmission Electron Microscope (TEM) with a view to optimize the mechanical stability of the bulk counterpart. In this work, we use a combinatorial process based on advanced Physical Vapor Deposition (PVD) and Atomic Layer Deposition (ALD) to create complex one-dimensional nanowires (NW). These wires are ideal model-systems that enable studying the performance of different coatings. In order to analyze the effect of multilayers on the mechanical behaviour, we compare conventional coatings on metals (Al2O3-Cu) (Figure 1a) with nanolaminates consisting of alternating layers of Al2O3-ZnO (Figure 1b). Defect nucleation at the metal-metal oxide interface as well as crack propagation are studied via in situ mechanical testing inside the TEM. Figure 1c shows a cracked multi-shell NW after mechanical loading. Figure 1: Core-shell NWs as small scale model system for bulk-coatings. Exemplary STEM images. a) Al2O3-Cu core-shell NW. The Cu NW has been subsequently coated with a conformal ALD layer. b) Nanolaminate on Cu NW. The Energy-dispersive X-ray spectroscopy (EDX) map shows the layered Al2O3-ZnO structure. c) Nanolaminate after mechanical loading. Next to metal-metal oxide structures, intermetallic (Cu-Au) nanostructures are used to study the effect of compositional complexity on the material properties as well as diffusion processes at interfaces. By using co-sputtering at elevated temperatures alloyed NWs (Cu3Au/Au3Cu) are directly achieved. Figure 2 a shows an exemplary intermetallic Cu3Au NW with corresponding EDX mapping. The line scan across the NW is shown in figure 2c. The High-Resolution Scanning Transmission Electron Microscopy (HR-STEM) image in figure 2c emphasizes the ordering and figure 2d shows the corresponding diffraction pattern of the NW. In comparison with solid-solutions (of the same composition), such intermetallic model systems are used to characterize the effect of atomic ordering on the mechanical behavior, which is of great interest for the development of complex alloys. Beside the direct creation of the ordered phase via co-sputtering, intermetallic NWs can also be produced by heating core-shell NWs which consist of a copper core covered by a gold layer. These bimetallic nanostructures are ideal systems to study diffusion phenomena and ordering in situ in TEM. Figure 2e shows an EDX mapping of an initial Cu-Au NW before and after heating and figure 2f the corresponding line scans for different time intervals (initial, 50s to 2327s). Based on the Wagner equation, we calculated the interdiffusion coefficients of Cu (8.75· 10-18 m2/s) and Au (3.27·10-18 m2/s). To achieve the intermetallic phase and to observe the ordering, the NWs are annealed at 300°C (see diffraction pattern in figure 2g). Figure 2: Intermetallic NWs as small scale model system for complex alloys. a) Exemplary Cu3Au NW directly obtained via co-sputtering. STEM image with EDX mapping. The corresponding line profile is shown in figure 2b. c) HR-STEM image with Fast Fourier Transform (FFT) and d) diffraction pattern of the NW. Cu-Au core-shell NWs as small scale model system to analyze diffusion processes. e) EDX mapping of Cu-Au NW before and after heating at 400°C. g) Diffraction pattern before and after annealing at 300°C. Reversible formation of an ordered intermetallic phase.

Invited talk

Heat-induced alloying in individual Au@Ag core-shell nanoparticles as a function of size, shape, and presence of defects

IM 7: In situ/operando electron microscopy

Abstract text (incl. figure legends and references) The recent and ongoing development of advanced colloidal synthetic techniques has allowed scientists to routinely produce bimetallic nanoparticles of various shapes, sizes, composition, and elemental distribution. Novel synthetic routes enable precise control of specific optical properties based on surface plasmon resonance. Thus, nanoplasmonic-based materials are of great interest for the utilization in biosensing, photocatalysts, medicine, data storage, etc. However, operation in real conditions (e.g. at elevated temperatures) may cause particle reshaping and redistribution of metals between the core and shell of the particle, which in turn gradually alter the properties of nanoplasmonics. Hence, a deeper understanding of the influence of the size, shape, and presence of defects on the nature of such processes is essential for the further development of nanoplasmonic-based technologies. The recently developed combination of fast tomography based on High Angle Annular Dark Field Scanning Transmission Electron Microscopy with in situ heating holders has enabled the investigation of heat-induced processes at the single nanoparticle level with high spatial resolution in 3 dimensions (3D). Using this approach, we evaluated the influence of various parameters (size, shape, defect structure) on heat-induced elemental redistribution in Au@Ag core-shell nanoparticles qualitatively and quantitatively. In more detail, the elemental redistribution at high temperature in single-crystalline Au@Ag nanocubes with similar size and composition, but with different shapes of the core (truncated octahedra and sphere), was shown to be uniform along all directions (Figure 1c-d, upper rows). By performing 3D simulations of diffusion based on Fick"s law, similar diffusion constants were found (Figure 1a-b and Figure 1c-d, lower rows). Additionally, our investigation indicated faster alloying kinetics for pentatwinned Au@Ag nanorods in comparison to single-crystalline nanorods of comparable sizes and composition. This may be related to the presence of twinning planes, which cause the formation of distortions and vacant sites in the crystal lattice, facilitating diffusion of atoms. Finally, the influence of the core shape was demonstrated to be negligible in the case of two pentatwinned nanorods with different Au cores (rod and bipyramid). In conclusion, using our fast tomography approach we gained a fundamental insight into the nature of elemental diffusion at high temperature and its dependence on various factors, e.g. size, shape, and presence of twinning defects. We believe, that detailed knowledge of thermal stability and/or thermal degradation of bimetallic nanoplasmonic materials will drastically improve the application of these materials in the future. The project has received funding from European Research Council (ERC Consolidator Grant 815128, REALNANO) and European Commission (grant 731019, EUSMI). Figure 1. (a) – (b) Comparison of the alloying progress of single-crystalline nanocubes (with octahedral and spherical core, respectively) to diffusion simulations performed for different diffusion coefficients. (c) – (d) The upper rows show slices through the experimentally determined 3D elemental distribution at different stages of alloying of single-crystalline nanocubes (with octahedral and spherical core, respectively). The lower rows display slices through the simulated 3D elemental distribution using the optimal diffusion coefficient.

Abstract talk

Prof. Dr. rer. nat. habil.

Utilizing gradient samples to investigate solid state dewetting of bimetallic alloys

MS 7: Ceramics and composites

Abstract text (incl. figure legends and references) Solid-state-Dewetting (SSD) is a typical degradation process of thin metallic films deposited on a substrate when annealed below the melting point. The process is driven by the minimization of total energy through surface- and interfacial energy reduction. While it used to be an undesirable effect, nowadays it is utilized as a powerful tool to fabricate bimetallic alloyed nanostructures for e.g. catalysis [1]. Here, switching from a single component film to binary systems makes the whole process more complex. A detailed understanding of this process is, however, unavoidable in order to fabricate nanostructures with targeted composition, shape and size. In the present study, we present an elegant route for the fabrication of single-gradient (Fig.1 A) and triple gradient (Fig.1 B) samples via e-beam evaporation of Au and Ni on (0001) oriented α-Al2O3. Subsequently, rapid thermal annealing is utilized at 890°C for 120s in reducing atmosphere (Ar/5% H2). The temperature is chosen in order to anneal the samples above the miscibility gap of the Au-Ni system for the production of AuxNi1-x alloys. This type of sample preparation allows i) the preparation of AuNi alloys with fine concentration differences to study the effect of alloying on SSD and ii) to study alloying effects and film thickness effects using a single sample. Ex-situ scanning electron microscopy (SEM) studies of the thermal annealed single-gradient sample are displayed in Fig. 2A. The homogeneous back-scatter contrast in the particles confirm a successful formation of an AuNi solid solution for all compositions ranging from 1 at% Au up to 99 at% Au. Via segmentation of the SEM images, the ratio between the exposed and unexposed are extracted to visualize the Au concentration dependent dewetting kinetics (Fig. 2B). An increased dewetting kinetic can be found in the range of 50 at% to 70 at% which is in good agreement with our recent studies [2]. Using EBSD measurement the texture of the formed particles is investigated (Fig. 2 C). The pole figures show a strong 111 out-of-plane texturing for a wide concentration range. Furthermore, the microstructure is fibre-like with tendencies towards epitaxial orientation relationships of AuNi (111)[1-10] || α-Al2O3 (0001)[10-10]. Interestingly this texture is substanial lost for the sample with 60at% Au leading to a texture free microstructure of the particles. With the presented approach, the influence of a multitude of different factors like atomic size mismatch during alloying onto the SSD of bimetallic alloys can be investigated in just one single sample. Furthermore, we demonstrate, that the concentration can be tuned continuously over a concentration range from 1 at% to 99 at% Au, which allows a systematic investigation of the influence of the misfit between metal particles and ceramic substrate, which is currently part of further investigations. [1] Shahvaranfard et al., CS Appl. Mater. Interfaces 2020, 12, 34, 38211–38221 [2] Dierner et al., unpublished work

Univ. Prof. Dr.-Ing. habil. Dr. h.c.

Unravelling shear band formation and propagation in metallic glasses through 4D-STEM

Abstract text (incl. figure legends and references) Metallic glasses (MGs) are an exciting new class of materials due to their unique mechanical and functional properties. Still, potential applications are hindered by their limited ductility caused by the formation of shear bands leading to catastrophic failure. A fundmental understanding of the shear banding process is key for improving the mechanical properties of MGs. While progress in the understanding of deformation localization was made using colloidal solids or atomistic simulations, highlighting the importance of nanoscale structural variations and local strains for shear banding, direct experimental studies are scarce due to limited characterization techniques. In the current work we study the deformation of metallic glasses at the nanoscale by scanning transmission electron microscopy (STEM) imaging using a JEOL 2200FS equipped with an in-column energy filter. To quantify local density and strain, a map of precession nanodiffraction patterns is recorded using 4D-STEM. Scanning and precession of the electron beam is controlled by a TVIPS universal scanning generator and the nanodiffraction patterns are recorded using a fast CMOS camera (TVIPS XF416) and a direct electron detector (Quantumdetectors MerlinEM 4R). A Cu46Zr46Al8 bulk MG plate is made by suction casting and shear bands (SB) are introduced by uniaxial compression. Cross section specimens are extracted on major SBs using a focused ion beam dual-beam workstation. Precession nanodiffraction enables to map the strain at at sufficient spatial resolution (&lt;2 nm) to quantify atomic density and strain distribution within an individual SB. Figure 1 shows a HAADF image and a local density map, revealing a decrease in atomic density within the SB [1]. In addition, the SB shows density alternations from the atomic scale to the submicron scale. The nanoscale density alternations show good agreement with molecular dynamic simulations indicating that they are linked to the autocatalytic generation of shear transformation zones, while the density alternation at submicron scale results from the progressive propagation of the SB front. The current results demonstrate that a direct quantitative measurement of strain in amorphous nano-volumes is important for revealing fundamental deformation processes in MGs, eventually allowing to tune their mechanical properties in a more controlled fashion. This work was supported by the Austrian Science Fund (FWF): Y1236-N37. [1] H. Sheng, D. Şopu, S. Fellner, J. Eckert, C. Gammer PHYSICAL REVIEW LETTERS 128, 245501 (2022)

Electron time correlation microscopy and the stability of the numerical evaluation

IM 5: Quantitative image and diffraction data analysis

Abstract text (incl. figure legends and references) The high spatial and temporal resolution of electron microscopy opens new ways of characterizing matter. Electron Correlation Microscopy (ECM) is a comparably new method analyzing time-intensity correlations in thermodynamic equilibrium [1,2]. For amorphous materials, transmission electron microscopy (TEM) dark-field (DF) images show speckle contrast arising by local structural arrangements. ECM measures basically a speckle lifetime. In the present work, the ECM data evaluation and the resulting accuracy are discussed. Measuring ECM out of thermodynamic equilibrium at room temperature (RT) was shown recently for a metallic glass [3,4]. ECM measurements out of thermodynamic equilibrium are here denoted Electron time Correlation Microscopy (EtCM). This brings additional evaluation criteria, visualized by calculating two-time correlation functions. A schematic of EtCM is shown in Figure 1. An amorphous sample is imaged in TEM-DF mode to detect structural arrangements fulfilling the chosen DF condition leading to speckle contrast. Typically about 50 nm in thickness samples and a beam current density of about 105 e-/nm²s at exposure times of seconds are applied. From the image time series, a correlation function (g2(t)) is calculated, which is fitted to a stretched exponential function ( A*[1-exp(-t/τ)β], see Figure 1 top-right). A β=1 leads overall to a single exponential function, β&lt;1 to a stretched, and β&gt;1 to a compressed exponential function. The β therefore reflects the energy landscape of the underlying processes. Depending on the material, the correlation time at RT can be extremely large (&gt; 10 k seconds). Consequently, the corresponding data quality is limited due to the limited experimental accessible time. One approach is to extrapolate τ and β to infinitely long measurement times [3] by using e.g. a*[1-exp(-x/b)], where a is the value of τ or β for time to infinity, b is a parameter reflecting how fast the data analysis comes to a numerical steady state and x denotes the experimentally determined τ or β for different, finite times. It should be noted, that τ and β also depend on the DF condition as the beam tilt selects the scattering vector magnitude. In Figure 2, an example of β and τ as a function of the evaluated stack length is shown. Evaluating short measurement time leads to numerically unstable values. Evaluating longer times leads to more stable values, underestimating the real values. The dependence of β and τ on the evaluated time are different. However, the data analysis using different fractions of a DF-time series is a robust way to gain insights into the stability of the fitting parameters β and τ. Moreover, by extrapolating β and τ to timeàinfinity allows to estimate the real values (at a given beam current), or at least to estimate a proper measurement time for an EtCM experiment. Based on literature and numerical tests, the τ and β values are reviewed and discussed. [1] L. He at al., Microsc. and Microanal. 21 (2015) 1026-1033. [2] P. Zhang et al., Nature Communications (2018) 9:1129. [3] K. Spangenberg et al., Adv. Funct. Mater. 31 (2021) 2103742. [4] M. Stringe et at., J. Alloys Compd. 915 (2022) 165386. Fig. 1: Schematic EtCM measurement: (left) data acquisition and (right) data analysis including a two-time correlation plot to confirm a quasi-equilibrium condition. Fig. 2: Streching exponent β and correlation time τ obtained by fitting a (fraction of a) dataset.

Segregation to creep-induced planar faults in Ni-base SX superalloys

Abstract text (incl. figure legends and references) Ni-base single-crystal (SX) superalloys find application in turbine blades for gas engines due to the high-temperature and high-stress strength originating from the coherent γ/γ" microstructure. It is well-known at sufficiently high stresses, two 1/2 dislocation families with different Burgers vector can react and dissociate into two partial dislocations in γ channels. This allows the leading 1/3[-1-12] Shockley partial dislocation continuously gliding on {111} planes to cut into γ" precipitates where they create planar faults [1]. We study the segregation behaviours of alloying elements across the planar faults by performing the [11-2] (111) creep shear experiments, to intentionally activate the slip system [11-2] (111) with the highest Schmid factor of 1 where the resolved shear stress is exactly equal to loading stress. The creep-deformed specimens are interrupted after 1% and 2% shear strain under 250 MPa at 750°C. The resulting microstructure is investigated using conventional transmission electron microscopy (TEM), analytical scanning TEM (STEM) with energy-dispersive X-ray spectroscopy (EDXS) focussing on structural, physical, and chemical details of the local deformation. Fig. 1 presents the specimen perpendicular to the (111) plane with the [1-10] direction parallel to the electron beam. Numerous stacking faults (SF) are observed after 1% and 2% creep strains. Fringe contrasts under two-beam conditions indicate inclined stacking faults, shown in Fig 1a and 1d, where the 2% strain sample has more planar faults within one γ&quot; precipitate indicating a higher density of planar faults in the 2% sample. High-resolution STEM micrographs illustrate the superlattice extrinsic nature of stacking faults (SESF) in the 1% and 2% strained samples, see Fig1b and 1e. The chemical distributions across SESF are measured by EDXS and the corresponding concentration profile of 1% and 2% samples are shown in Fig. 1c and 1f respectively. Both samples show almost similar segregation tendency, which is that γ forming elements Cr, Co and Re are enriched across the SESF while γ&quot; alloying elements Ni and Al are depleted, which is partly in agreement with theoretical predictions [2]. For these measurements all microscope parameters and sample thickness for EDXS analysis are kept the same to quantitatively find out how creep strain and time affect the evolution of segregation. Figure 1. (S)TEM micrographs of the [1-10] foil normal sample show typical planar faults in Ni-base SX superalloys crept at 750 °C/250 MPa to creep strains of 1% (upper row) and 2% (lower row). TEM bright filed images present fringe contrast of inclined stacking faults under two-beam conditions in (a) g = (00-2) and (d) g = (002) excitation condition. High resolution STEM micrographs indicate the superlattice extrinsic nature of edge-on stacking faults of (b) 1% and (e) 2% strain specimens. Alloying concentration profiles across SESF of (c) 1% and (f) 2% from corresponding EDXS mappings. References [1] Eggeler, Y. M. et al. Annu. Rev. Mater. Res., 2021, 51, 209–240. [2] Zhao, X. et al. Comput. Mater. Sci, 2022, 202, 1–8. [3] Authors gratefully thank Christian Kübel, Di Wang and Yuting Dai (Electron Microscopy and Spectroscopy Laboratory, KIT) for HRSTEM micrographs and EDXS results, and acknowledge the DFG priority program SFB‐TR 103.

MS 2
Metals and alloys

Termin

Session chairs

Beschreibung

Programm

MS 2Metals and alloys

Termin

Session chairs

Beschreibung

Programm

MS 2
Metals and alloys