Dynamic strength measurement of iron at 450 GPa using direct laser-driven Rayleigh-Taylor instability

Iron (Fe) is one of the most abundant elements in the universe and is present in a wide range of astronomical objects. The equation of state, phase transitions and melting behavior of iron have been primarily investigated across a broad pressure-temperature ranges to understand the formation and evolution of rocky planets with iron-rich cores. However, its rheological properties under extreme conditions remain elusive. Here, we present laser-driven Rayleigh-Taylor (RT) instability experiments to measure the dynamic strength of iron. A physics package with a ripple interface between a lighter epoxy and a heavier iron sample is directly accelerated by a high-power laser up to 450 GPa and 5.5 kK at 10^-8 /s. The RT growth of the ripple amplitude is measured using in-situ face-on x-ray radiography. These experimental results are compared with hydrodynamic simulations to extract the pressure, temperature, flow stress, and strain profiles during acceleration. We report the dynamic strength of two iron samples, initially single-crystal a-Fe in BCC structure with [100] and [111] orientations. A pronounced strength anisotropy is observed, which is attributed to microstructural evolution during the BCC-to-HCP (or a-to-e) transition under dynamic loading, as elucidated by molecular dynamics simulations.

* This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.