Direct Observation of Martensitic Phase-Transformation Dynamics in Iron by 4D Single-Pulse Electron Microscopy

Abstract

The in situ martensitic phase transformation of Iron, a complex solid-state transition Involving collective atomic displacement and Interface movement, Is studied In real time by means of four-dimensional (4D) electron microscopy. The Iron nanofilm specimen Is heated at a maximum rate of -1011 K/s by a single heating pulse, and the evolution of the phase transformation from body-centered cubic to face-centered cubic crystal structure Is followed by means of single-pulse, selected-area diffraction and real-space imaging. Two distinct components are revealed in the evolution of the crystal structure. The first, on the nanosecond time scale, is a direct martensitic transformation, which proceeds In regions heated Into the temperature range of stability of the fee phase, 1185-1667 K. The second, on the microsecond time scale, represents an indirect process for the hottest central zone of laser heating, where the temperature Is initially above 1667 K and cooling Is the ratedetermining step. The mechanism of the direct transformation involves two steps, that of (barrier-crossing) nucleation on the reported nanosecond time scale, followed by a rapid grain growth typically in ∼100 ps for 10 nm crystallites.