Andreas Kulovits

Approaches for ultrafast imaging of transient materials processes in the transmission electron microscope.

The growing field of ultrafast materials science, aimed at exploring short-lived transient processes in materials on the microsecond to femtosecond timescales, has spawned the development of time-resolved, in situ techniques in electron microscopy capable of capturing these events. This article gives a brief overview of two principal approaches that have emerged in the past decade: the stroboscopic ultrafast electron microscope and the nanosecond-time-resolved single-shot instrument. The high time resolution is garnered through the use of advanced pulsed laser systems and a pump-probe experimental platforms using laser-driven photoemission processes to generate time-correlated electron probe pulses synchronized with laser-driven events in the specimen. Each technique has its advantages and limitations and thus is complementary in terms of the materials systems and processes that they can investigate. The stroboscopic approach can achieve atomic resolution and sub-picosecond time resolution for capturing transient events, though it is limited to highly repeatable (>10(6) cycles) materials processes, e.g., optically driven electronic phase transitions that must reset to the materials ground state within the repetition rate of the femtosecond laser. The single-shot approach can explore irreversible events in materials, but the spatial resolution is limited by electron source brightness and electron-electron interactions at nanosecond temporal resolutions and higher. The first part of the article will explain basic operating principles of the stroboscopic approach and briefly review recent applications of this technique. As the authors have pursued the development of the single-shot approach, the latter part of the review discusses its instrumentation design in detail and presents examples of materials science studies and the near-term instrumentation developments of this technique.

Revealing the transient states of rapid solidification in aluminum thin films using ultrafast in situ transmission electron microscopy

Using high time resolution transmission electron microscopy, we have observed rapid solidification dynamics in 80 nm thick Al thin films after pulsed laser melting. The nanometer spatial and 15 nanosecond temporal resolution of the dynamic transmission electron microscope (DTEM) at Lawrence Livermore National Laboratory allowed us to study the morphology and dynamics of the transformation front moving at speed of 0.1–10 m/s during rapid solidification. Additionally, we used an automated orientation imaging system in the TEM for the post-mortem analysis of grain orientations of the solidified microstructure near the position of the solid liquid interface at the start of solidification.

Simultaneous determination of highly precise Debye-Waller factors and structure factors for chemically ordered NiAl

Accurate Debye-Waller (DW) factors and several low-index structure factors of chemically ordered β-NiAl at different temperatures have been measured using an off-zone-axis multi-beam convergent-beam electron diffraction method. The temperature dependences of DW factors of Ni and Al atoms are compared with previous experimental measurements and theoretical calculations. The temperature below which the DW factor of Ni becomes smaller than that of Al was found to be lower than previously reported. Structure factors are determined with an accuracy of 0.05% and compared with prior reports.

Simultaneous determination of highly precise Debye-Waller factors and multiple structure factors for chemically ordered tetragonal FePd.

Accurate Debye-Waller (DW) factors and low-index structure factors up to 222 of chemically ordered FePd have been measured at 120 K. Ordered FePd has a simple tetragonal unit cell (tP2, P4/mmm) with Fe and Pd atoms at 0, 0, 0 and at ½, ½, ½, respectively, requiring the measurement of four different DW factors. It was possible to simultaneously determine all four DW factors and several low-order structure factors using different, special off-zone-axis multi-beam convergent-beam electron diffraction patterns with high precision and accuracy. The different diffraction conditions exhibit different levels of sensitivity to changes in DW and structure factors. Here the sensitivity of different off-zone-axis convergent-beam electron diffraction patterns with respect to changes in DW factors and structure factors is discussed.

Determination of Debye-Waller factor and structure factors for Si by quantitative convergent-beam electron diffraction using off-axis multi-beam orientations.

Debye-Waller (DW) factors and structure factors have been measured for Si using convergent-beam electron diffraction (CBED) experiments with a transmission electron microscope equipped with a field-emission gun and a post-column energy-filtering device. Si has been used here to evaluate the accuracy of multi-beam near-zone-axis orientations for the simultaneous refinement of DW factors and multiple structure factors. Strong dynamic interactions among different beams are obtained by tilting the crystal to specific four- or six-beam orientations near major zone axes, which provide sufficient sensitivity to determine accurate DW factors and structure factors. The DW factors of Si were measured using four-beam conditions near the [001] zone axis for temperatures ranging from 96 to 300 K. A comparison of the multi-beam near-zone-axis orientations with other CBED methods for DW and structure factor F(g) refinement is presented.

Morphology and grain structure evolution during epitaxial growth of Ag films on native-oxide-covered Si surface

Epitaxial nanocrystalline Ag films were grown on initially native-oxide-covered Si(001) substrates using radio-frequency magnetron sputtering. Mechanisms of grain growth and morphology evolution were investigated. An epitaxially oriented Ag layer (∼5 nm thick) formed on the oxide-desorbed Si surface during the initial growth phase. After a period of growth instability, characterized as kinetic roughening, grain growth stagnation, and increase of step-edge density, a layer of nanocrystalline Ag grains with a uniform size distribution appeared on the quasi-two-dimensional layer. This hierarchical process of film formation is attributed to the dynamic interplay between incoming energetic Ag particles and native oxide. The cyclic interaction (desorption and migration) of the oxide with the growing Ag film is found to play a crucial role in the characteristic evolution of grain growth and morphology change involving an interval of grain growth stagnation.

Synthesis of Fe–Pd and Fe–Pd∕Ta magnetic nanocomposites by severe plastic deformation

Severe plastic deformation (SPD) and cyclic codeformation were used to prepare bulk magnetic nanocomposite of ordered L10Fe-Pd phase and soft α-Fe following an atomic ordering and precipitation reaction. Enhanced coercivity and remanence have been achieved with this method. Layering of Ta foils with the Fe–34at.%Pd foils was explored in an effort to minimize nanocomposite grain size by confinement. Faster kinetics and improvement in the remanence resulted from Ta layering.

Validation of density functionals for transition metals and intermetallics using data from quantitative electron diffraction

Accurate low-order structure factors (Fg) measured by quantitative convergent beam electron diffraction (QCBED) were used for validation of different density functional theory (DFT) approximations. Twenty-three low-order Fg were measured for the transition metals Cr, Fe, Co, Ni, and Cu, and the transition metal based intermetallic phases γ-TiAl, β-NiAl, and γ1-FePd using a multi-beam off-zone axis QCBED method and then compared with Fg calculated by ab initio DFT using the local density approximation (LDA) and LDA + U, and different generalized gradient approximations (GGA) functionals. Different functionals perform very differently for different materials and crystal structures regarding prediction of low-order Fg. All the GGA functionals tested in the paper except for EV93 achieve good overall agreement with the experimentally determined low-order Fg for BCC Cr and Fe, while EV93 performs the best for FCC Ni and Cu. The LDA and GGA functional fail to predict accurately the low-order Fg for β-NiAl and γ1-FePd. The LDA + U approach, through tuning of U, can achieve excellent matches with the experimentally measured Fg for all the metallic systems investigated in this paper. The use of experimentally accessible low order Fg as an additional set of metrics in approaches of validation of DFT calculations is discussed and has potential to assist in and to stimulate development of improved functionals.

Nano-Structuring of 316L Type Steel by Severe Plastic Deformation Processing Using Two-Dimensional Linear Plane Strain Machining

Using a novel plastic deformation technique, termed linear plane-strain machining, large shear strains up to ~2.3 have been imparted to 316L stainless steel at rates of up to 1700/s. Combinations of hardness and magnetic measurements, X-ray diffraction (XRD) and transmission electron microscopy (TEM) experiments were used to monitor the microstructural and mechanical property changes for the room temperature plastic deformation processing. Grain refinements to the ultra-fine grained and even the nanocrystalline size regime have been achieved without formation of significant volume fractions of strain-induced martensite. The mechanical strength enhancements in the linear plane-strain machined 316L have been attributed to grain refinement and stored strain. The suppression of martensite formation has been correlated to significant adiabatic heating of the 316L during high strain rate plastic deformation processing.

Comparison of convergent beam electron diffraction methods for simultaneous structure and Debye Waller factor determination.

The measurement of accurate and precise structure factors and Debye Waller (DW) factors by quantitative convergent beam electron diffraction (QCBED) permits experimental determination of the electron density distribution and probing of interatomic bonding in crystals. The three QCBED methods used successfully for high precision measurements of low order structure factors to date, namely the zone axis pattern (ZAP) method, the excited row ER method and the multi-beam off-zone axis (MBOZA) technique, differ from each other regarding the crystal orientation relative to the incident electron beam. Consequently, the details of their respective dispersion surface representations differ regarding the number, relative amplitudes and phases of excited Bloch wave branches. Under the same experimental setup conditions, the factors most important to the degree of accuracy and precision achievable in electron density determination for crystals with QCBED methods ultimately depend on the sensitivity of the excited Bloch wave branches and the resultant contrast in the respective CBED patterns to changes in both structure and DW factors. In general, a QCBED pattern will be more sensitive to changes in both structure and DW factor, if it contains more and stronger excited Bloch wave branches, as dynamic interactions of the Bloch waves increase the sensitivity of the pattern. In this work we analyzed Bloch wave excitation and dispersion surfaces for the three most popular QCBED methods. The analysis indicates, that the QCBED patterns obtained using the MBOZA orientation generally contain more and stronger excited Bloch wave branches. Hence, MBOZA diffraction patterns are more sensitive than the ZAP and the ER patterns to changes in both DW and structure factors and therefore allow in differences to the other two methods simultaneous refinements effectively and robustly.