The microstructure and properties of a material depend on dynamic processes such as defect motion, nucleation and growth, and phase transitions, which commonly occur on the nanosecond to microsecond timescale. Modern in situ transmission electron microscopy (TEM) can spatially resolve these nanoscale phenomena but lacks the time resolution to observe the processes directly. The Dynamic TEM (DTEM) has been developed to address this gap in characterization capability by using a short photoemitted electron pulse to probe dynamic events with "snap-shot" diffraction and imaging with 15-nanosecond resolution.

This fast in situ capability is highlighted here in a study of Al/Ni reactive multilayer foils (RMLF), which are used as localized heat sources for joining in microelectronics and as rapid fuses. The layered foil geometry contains many interfaces where the atoms can mix to react with each other, forming intermetallic compounds. Once the reaction is initiated by a laser, the formation reaction releases more heat and becomes self-sustaining as it propagates through the remaining material at ∼10 m/s. This dynamic mixing reaction is referred to as self-propagating high-temperature synthesis (SHS). The reaction front velocity and nanoscale geometry make direct observation of the reaction front impossible by any other existing technique.

This work describes direct observations of single-pulse nanosecond scale TEM data in both diffraction and imaging modes, which are necessary to study the propagation and behavior of energetic nanolaminates in situ. The pulsed electron interrogation method reveals that transient morphologies are produced and attributed to a brief phase separation during the SHS. The features appear as lines of mass-thickness contrast due to cellular phase formation of liquid and an ordered B2 NiAl phase. At high formation temperatures of ∼1700 K, these materials are now known to exhibit transverse self-ordering.

It is significant and exciting to find spontaneous, rapid formation of ordered structures in materials far from equilibrium, which is also an important step for essential comprehension of RMLF performance in applications. These observations provide insight into the mechanisms driving the dynamic process and give perspective on the application of the DTEM technique to other materials.