Investigating Iron Material strength Up To 1 Mbar Using Rayleigh-Taylor Growth Measurements

The solid-solid phase transition between the bcc (alpha) and hcp (epsilon) lattice structures in iron is known to occur as the material is compressed. When kept below its melting point, an effective increase in the macroscopic strength of the material accompanies this phase transition. Understanding the material strength of iron throughout the deformation process presents a significant computational challenge, but is important for improving models of planetary structure, including interpretation of seismic measurements taken through our own Earth's core. To explore the strength of iron at high pressures and strain rates, we have developed the IronRT campaign at the OMEGA laser [1]. This laser-driven platform produces pressure greater than 1 Mbar on a thin Fe disk with a sinusoidal ripple pattern imposed on its face. These ripples seed the Rayleigh-Taylor (RT) instability, the growth of which is suppressed by the material strength of the sample. The amplitude of the ripples is diagnosed with high-energy x-ray radiography, and the measured growth is compared to simulations performed with different strength models. By matching the simulations to the low level of growth measured, we infer an average flow stress of greater than 40 GPa over the course of the experiment. This value is in agreement with other dynamic iron strength experiments at pressures greater than 1 Mbar [2].