We discuss the grain-size measurements made during shock compression using in situ x-ray diffraction. Our experiments have shown unambiguously that single-crystal iron shock loaded above 13 GPa along the [100] direction will transform from the ambient $\alpha$ phase (bcc) to a highly ordered polycrystalline $ϵ$ phase (hcp). Here, we present a detailed shape analysis of the diffraction peaks using a modified Warren-Averbach method to quantify the microstructure of shock-compressed high-pressure iron. The $ϵ$ phase was determined through this method to have grain sizes between 2 and 15 nm, in reasonable agreement with results from large-scale molecular-dynamics simulations. We conclude that single-crystal iron becomes nanocrystalline in shock transforming from $\alpha$ to $ϵ$ phase.

• Received 13 November 2008

DOI:https://doi.org/10.1103/PhysRevB.78.220101