Often a material’s macroscopic behavior under external stimuli is described through the observation of its microstructural features and dynamical behavior. Materials models and computer simulations that are used to predict material behavior in different environments, e.g., phase transformation kinetics under high pressures and temperatures, typically require experimental data for validation or interpretation of simulated quantities. However, most materials dynamics are extremely rapid, making it difficult to capture their transient, fine-scale features, especially on the length and time scale relevant for most mesoscale models. In effort to meet the need for studying fast material processes, we have constructed a nanosecond dynamic transmission electron microscope (DTEM) at Lawrence Livermore National Laboratory to improve the temporal resolution of in-situ TEM observations.
The DTEM consists of a modified JEOL 2000FX transmission electron microscope that provides access for two pulsed laser beams. One laser drives the photocathode to produce the brief electron pulse and nanosecond exposure times. The other strikes the sample, rapidly heating, for example, an amorphous NiTi film to initiate crystallization. A series of pump-probe experiments with varying time delays enable, for example, the reconstruction of the average behavior and events occurring during rapid phase transformations. This presentation will discuss the core aspects of the DTEM instrument with particular focus on how it has been used to study the “superheated” crystallization processes in amorphous NiTi films. In particular, we will show how the DTEM has been used to quantify the nucleation rates and crystallization kinetics at high temperatures far above those used in conventional techniques, e.g. differential scanning calorimetry. The crystallization rates observed under pulsed laser heating were anomalously high as compared to slow-heating DSC crystallization experiments.
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Security, LLC, Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.