Tracing fast phase transitions by electron microscopy

A TEM method is presented which traces a short event in specimen areas ⪆ 1 μm Ø with a resolution of 30 ns, provided the event produces a change in contrast > 20%. An electron pulse initiates the event and generates the bright-field image. Its intensity is amplified by a multiplier and displayed on fast storage oscilloscopes, triggered by a shaped pulse from the bright-field image intensity. The method is used to determine the formation time of crystals in explosively ordering amorphous FeNi films. Lath- and star-like crystals originate with 10 to 20 m/s in films with < 2 at% O2 and N2. Spherulites evolve with a velocity < 1 m/s in films with higher contamination (≈ 7 at%).