Kai Zweiacker

Determination of crystal growth rates during rapid solidification of polycrystalline aluminum by nano-scale spatio-temporal resolution in situ transmission electron microscopy

In situ investigations of rapid solidification in polycrystalline Al thin films were conducted using nano-scale spatio-temporal resolution dynamic transmission electron microscopy. Differences in crystal growth rates and asymmetries in melt pool development were observed as the heat extraction geometry was varied by controlling the proximity of the laser-pulse irradiation and the associated induced melt pools to the edge of the transmission electron microscopy support grid, which acts as a large heat sink. Experimental parameters have been established to maximize the reproducibility of the material response to the laser-pulse-related heating and to ensure that observations of the dynamical behavior of the metal are free from artifacts, leading to accurate interpretations and quantifiable measurements with improved precision. Interface migration rate measurements revealed solidification velocities that increased consistently from ∼1.3 m s−1 to ∼2.5 m s−1 during the rapid solidification process of the Al th...

Quantitative Phase Analysis of Rapid Solidification Products in Al-Cu Alloys by Automated Crystal Orientation Mapping in the TEM

There has been significant interest in laser-based-processing methods for the manufacturing of complex components (e.g. additive manufacturing) or post-processing/repair work (e.g. laser welding). Since laser processing tends to result in the formation of rapidly solidified microstructures, it becomes increasingly important to understand the microstructural development under such non-equilibrium conditions. Pulsed-laser-based-processing methods have been foci of investigations on rapidly solidified microstructures. This method has been shown to produce unique microstructures and micro-constituents in metallic thin films at the nanometer-scale [1].

Quantitative Determination of Thermal Fields and Transformation Rates in Rapidly Solidifying Aluminum by Numerical Modeling and In-situ TEM

The dynamic transmission microscope (DTEM) offers incomparable nano-scale spatiotemporal resolution that is advantageous for characterizing irreversible transient processes in various material systems [1]. Previously, we utilized bright field imaging and diffraction in DTEM with nano-second temporal resolution to characterize the dynamics of rapid solidification (RS) in pure Al and Al-Cu alloy thin films in single-shot, single-image acquisition mode [2-4]. Recent developments in DTEM instrument enable a single-shot, multiple-image acquisition mode (Movie-mode) [5], which significantly reduces potential uncertainty and experimental error when measuring parameters such as interface velocity during rapid solidification process. However, it is inherently challenging to measure the temperature evolution during the RS transformation cycle while performing in-situ DTEM observations. Knowledge of the thermal field evolution in correlation to the solidification front velocities would greatly benefit quantitative understanding of RS process in material systems.

Capturing Dynamics of Pulsed Laser Induced Melting and Rapid Solidification in Aluminum Polycrystals with Nanoscale Temporal Resolution In-situ TEM

The unprecedented spatiotemporal resolution offered by the dynamic transmission electron microscope (DTEM) facilitates unique studies of irreversible transitions in condensed matter [1]. In prior work we utilized the DTEM for single-shot nano-second temporal resolution bright field imaging and diffraction studies of rapid solidification (RS) in Al, Cu, Ag and Al-Cu alloy thin films [2-5]. Recent upgrades to the DTEM instrument enable single-pump/multiple-image-acquisition experiments (image series), which hold prospect to greatly reduce quantitative uncertainties in measurements, e.g. transformation front velocity, during RS, and increase by about one order of magnitude the data density obtained from direct observations of the dynamics of the irreversible transformation processes [6]. Here we report and discuss first results of in-situ TEM investigations of the dynamics of pulsed-laser induced melting and RS in Al thin films performed with the upgraded DTEM. Thin Al films, 80nm-160nm thick, were deposited by e-beam evaporation on square windowed TEM grids with 50 nm thick amorphous Si3N4 membranes and melted with a single 15 ns, 1064 nm laser pulse, defining time t0 (Fig. 1). Image series comprised of nine frames have been acquired with preselected 20ns exposures per frame and 0.2s time steps between frames and also for 50ns exposures and inter-frame time steps of 2s. The resulting image series spanning at least ~1.8s (e.g. Fig. 1) and up to ~20.4s have been acquired for systematically varied pre-delay times, t, in the range of -0.2s to 40μs. Dynamics of the morphological and structural changes in the nano-crystalline Al thin films and the kinetics during the pulsed laser induced sequences of melting and subsequent RS have been determined from direct observations. Bright field imaging showed (e.g. Fig. 2) that the laser pulse induced irreversible transformation involves the preferential melting along grain boundaries and development of a ‘mushy’ two-phase region during the melting sequence. Subsequently solidification initiates with a transformation interface that is rough on the scale of the adjacent crystalline grains. Driven by the thermal gradient the solidification interface transitions to a planar morphology, which supports growth of columnar crystals to the center of the melt pool. Effects from the Si frame support of the TEM transparent square windows, which represents a heat sink, on the solidification velocity measurements in the DTEM have been studied systematically. Kinetic details of the Al thin film melting and the solidification sequences under the driven conditions are analyzed quantitatively and discussed. Post-mortem TEM analysis have been performed to evaluate crystallographic details of the directional solidified Al grains relative to the original film.

Crystal Growth Mode Changes during Pulsed Laser Induced Rapid Solidification in Nanoscale Thin Films of Al-Cu Eutectic

Pulsed laser induced rapid solidification is a promising processing method to produce unique microstructures and micro-constituents in metallic thin films at the nano-scale [1]. Recent in-situ dynamical transmission electron microscopy (DTEM) studies on the rapid solidification of hypo-eutectic Al-Cu alloys have identified various growth mode changes with modulation of the microstructural features [2]. Elucidating crystallographic orientation changes during growth mode transition are of fundamental importance in understanding rapid solidification processes [3-5]. Due to the nano-crystalline nature of the resulting microstructure, acquiring statistically significant and representative data sets is rather difficult with electron backscatter pattern based orientation image mapping (OIM) by scanning electron microscopy. With the recent development of transmission electron microscopy (TEM) based OIM, which relies on automated acquisition and indexing of precession electron diffraction (PED) patterns, changes in crystal orientation and crystal structure can be detected with nanometer spatial resolution [6-8].

Atom Probe Tomography and analytical Scanning Transmission Electron Microscopy of Rapid Solidification Microstructures in Al-Cu Alloy Thin Films

Laser processing of alloys results in formation of rapid solidification (RS) microstructures that can exhibit refined length scale, extreme solute segregation, formation of meta-stable phases, and unusual textures [1]. Recent nano-scale spatio-temporal resolution in situ dynamical transmission electron microscopy (DTEM) studies of pulsed laser (PL) induced RS in multi-component alloys of hypoeutectic Al-Cu thin film specimens have identified characteristic morphological modulations of the multi-phase microstructures with solidification interface velocity [1, 2]. Elucidating constitutional effects during RS crystal growth in multi-component systems is of fundamental importance in understanding laser-assisted materials processing [1-5]. However, due to the nano-scale of the resulting microstructural features, acquiring quantitative analytical data with the appropriate spatial-resolution is challenging with scanning and transmission electron microscopes (SEM and TEM). A promising alternative analytical technique is atom probe tomography (APT). APT offers sub-nanometer spatial resolution, and provides statistically significant representative data sets for elemental composition mapping, but is limited to relatively small analytical volumes. The current work presents complementary aberration-corrected scanning TEM (STEM) based energy dispersive X-ray spectroscopy (EDXS) mapping to study constitutional effects in RS microstructure evolution of hypo-eutectic Al-Cu alloys.

Composition and Crystal Orientation Mapping of nano-scale multi-phase Rapid Solidification Microstructures in hypo-eutectic Al-Cu Alloy Thin Films

1. Department of Mechanical Engineering and Materials Science, Swanson School of Engineering, 636 Benedum Hall, 3600 O’Hara Street, University of Pittsburgh, Pittsburgh, PA 15261, USA. 2. Materials Science Division, Physical and Life Science Directorate, Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA, 94551, USA 3. Now at EMPA, Swiss Federal Laboratories for Materials Science and Technology Überlandstrasse 129, CH-8600 Dübendorf, Switzerland.

Rapid Solidification in Thin-Film Al-Cu Alloys: Capturing the Dynamics with Time-Resolved In Situ TEM

Rapid solidification of metals and alloys has been widely recognized as a viable processing route to obtain unique microstructures with potentially advantageous properties that are not obtainable with conventional solidification processes. Previously, the dynamics of rapid solidification in pure Al [1] and Al-Cu alloy [2] thin films were characterized using the dynamic transmission electron microscope (DTEM) in its singleshot, single-image acquisition mode. Recent upgrades to the DTEM instrument enable a single-shot, multiple-image acquisition mode (so-called movie mode) [3], which allows study of microstructural evolution and kinetics in far greater detail. Movie-mode DTEM provides higher data throughput and a reduction in uncertainty and experimental error while following complex, irreversible processes in time.