The grain boundary network of nanocrystalline Cu foils was modified by the application of cyclic loadings and elevated temperatures. Broadly, the changes to the boundary network were directly correlated with the applied temperature and accumulated strain, including a 300% increase in the twin length fraction. By independently varying each treatment variable, a matrix of grain boundary statistics was built to check the plausibility of hypothesized mechanisms against their expected temperature and stress/strain dependences. These comparisons allow the field of candidate mechanisms to be significantly narrowed. Most importantly, the effects of temperature and strain on twin length fraction were found to be strongly synergistic, with the combined effect being ∼150% that of the summed individual contributions. Looking beyond scalar metrics, an analysis of the grain boundary network showed that twin related domain formation favored larger sizes and repeated twin variant selection over the creation of many small domains with diverse variants.

The need to understand fast, complex physical phenomena through direct in situ observation has spurred the development of high-time-resolution transmission electron microscopy (TEM). Two complementary approaches have emerged: the single-shot and stroboscopic techniques. Single-shot TEM has advanced through the development of dynamic transmission electron microscopy (DTEM) and, more recently, by the advent of movie-mode DTEM, which enables high-frame-rate in situ TEM experimentation by capturing nanosecond-scale sequences of images or diffraction patterns. Previous DTEM studies produced only single snapshots of fast material processes. Movie-mode DTEM provides the ability to track the creation, motion, and interaction of individual defects, phase fronts, and chemical reaction fronts, providing invaluable information on the chemical, microstructural, and atomic-level features that govern rapid material processes. This article discusses movie-mode DTEM technology, its application in the study of reaction dynamics in Ti–B-based reactive nanolaminates, and future instrumentation.

We have measured the transient events of the α-β martensitic transformation in nanocrystalline Ti films via single shot electron diffraction patterns with 1.5 ns temporal resolution. This was accomplished with a newly constructed dynamic transmission electron microscope (DTEM), which combines pulsed laser systems and pump-probe techniques with a conventional TEM. The DTEM thereby enables studies of transformations that are (1) far too fast to be captured by conventional bulk techniques, and (2) difficult to study with current ultrafast electron diffraction (UED) instruments (which typically require an accumulation of multiple shots for each diffraction pattern). Martensitic transformations in nanocrystalline materials meet both criteria, with their rapid nucleation, characteristic interface velocities ∼1 km/s, and significant irreversible microstructural changes. Free-standing 40-nm-thick Ti films were laser-heated at a rate of ∼1010 K/s to a temperature above the 1155 K transition point, then probed at various time intervals with a 1.5-ns-long intense electron pulse. Diffraction patterns show an almost complete transition to the β phase within 500 ns. Post-mortem analysis (after the sample is allowed to cool) shows a reversion to the α phase coupled with substantial grain growth, lath formation, and texture modification. The cooled material also shows a complete lack of apparent dislocations, suggesting the possible importance of a "massive" short-range diffusion mechanism.

In view of a planar processing technique to develop a micro-actuator, irradiation experiments on plastically deformed martensitic TiNi thin films were performed using 5 MeV Ni++, with various fluences and temperatures, to study the dependence of microstructure and transformation characteristics on irradiation parameters. TEM observation and X-ray diffraction spectra show that irradiation resulted in a sharply defined continuous amorphous matrix, extending to the projected ion range, that contained both monoclinic and body centered cubic nanocrystals. The irradiation mechanisms and films' temperature-displacement are discussed in terms of the observed microstructure.