Dynamic Transmission Electron Microscopy for Non-Equilibrium Microstructure Evolution

The dynamic transmission electron microscope (DTEM) at Lawrence Livermore National Laboratory (LLNL) was developed to enable imaging of rapidly evolving, far-from-equilibrium materials processes with nanometer spatial and nanosecond temporal resolutions. Movie-mode DTEM (MM-DTEM) provides multiple image acquisitions across temporal ranges from hundreds of nanoseconds to microseconds, yielding frame rates that are on the order of 10⁶ times higher than conventional in situ TEM frame rates. MM-DTEM employs a laser based on an arbitrary waveform generator (AWG) to shape laser pulses to square temporal profiles, producing a user-defined (pulse duration and spacing) pulse train that is delivered to the instrument’s photocathode to generate an electron pulse train. Each electron pulse captures an image at a specified delay time. A high-speed electrostatic deflector beneath the specimen directs each image to a different segment of a CCD camera and the multi-frame image (i.e., a movie) is read out at the end of the experiment. A brief overview of MM-DTEM instrumentation and operation will be provided.<br/><br/>Examples of application of MM-DTEM to non-equilibrium microstructure evolution will focus on two types of studies: laser-induced rapid alloy solidification and high-strain-rate plastic deformation of polycrystalline metals. Development of time-resolved, in situ imaging techniques capable of capturing solid-liquid interfacial evolution in metallic alloys with high spatial and temporal resolution under diverse solidification conditions are relevant for applications ranging from conventional directional solidification, crystal growth, and casting to welding and additive manufacturing. These experiments enable direct visualization of transient behaviors that would otherwise remain unknown, providing unique insights into the physics that impact microstructure and defect development and strategies for their control and mitigation, respectively. Understanding microstructural evolution and the characteristics that form under various solidification conditions to impact the properties and performance of metallic alloys is essential for the development of multiscale, experimentally informed predictive modeling, which will be highlighted here by solidification simulations that utilize in situ DTEM measurements of solidification dynamics. Plastic deformation of metals at high strain rate is controlled by the nucleation, propagation, and interaction of defects in the microstructure. This is typically estimated based on dynamic mechanical measurements coupled with ex situ microstructure investigations, which are fundamentally limited in their ability to characterize transient mechanisms. Here, experimental observations of the nucleation, motion, and interaction of defects and cracks during deformation of metals at strain rates between 10³ - 10⁴ s^-1 will be presented. Experimental capabilities that enable these measurements will be discussed.<br/><br/>Prepared by LLNL under Contract DE-AC52-07NA27344.