Femtosecond Visualization of hcp-Iron Strength and Plasticity under Shock Compression
Sébastien Merkel, Sovanndara Hok, C. A. Bolme, Dylan Rittman, Kyle Ramos, Benjamin Morrow, Hae Ja Lee, Bob Nagler, Eric Galtier, E. Granados, Akel Hashim, Wendy L. Mao, A. E. Gleason
Abstract
Iron is a key constituent of planets and an important technological material. Here, we combine in situ ultrafast x-ray diffraction with laser-induced shock compression experiments on Fe up to 187(10) GPa and 4070(285) K at 10^{8} s^{-1} in strain rate to study the plasticity of hexagonal-close-packed (hcp)-Fe under extreme loading states. {101[over ¯]2} deformation twinning controls the polycrystalline Fe microstructures and occurs within 1 ns, highlighting the fundamental role of twinning in hcp polycrystals deformation at high strain rates. The measured deviatoric stress initially increases to a significant elastic overshoot before the onset of flow, attributed to a slower defect nucleation and mobility. The initial yield strength of materials deformed at high strain rates is thus several times larger than their longer-term flow strength. These observations illustrate how time-resolved ultrafast studies can reveal distinctive plastic behavior in materials under extreme environments.