UC San Diego

The role of microstructure on material properties is a foundational principle in materials science. The processing of a material will change its microstructure which, in turn, will change its properties. Understanding the strength of condensed matter at ultrahigh pressures and strain rates is challenging. The material’s behavior changes unexpectedly and, in some conditions, may be surprisingly insensitive to the initial microstructure, which is contrary to the results of conventional static and quasi-static experiments. Therefore, experiments are needed in these extreme regimes to better understand and model material properties. Iron is a largely used material in many applications but it is also relevant to geophysical phenomena. Accurately quantifying iron strength would aid in geophysical modeling of planet impact and formation, the understanding of more complex and relevant structural iron alloys, and fundamental material modeling at high strain rates. As a material, it is especially interesting in its pure form because it goes through phase transitions that change its mechanical properties. So, by understanding a relatively simple material like iron, one can begin to gain insight into more complex phase-changing materials that are more universally used.Quasi-static and dynamic compression of iron was performed to determine microstructural effects on iron strength at various, increasing strain rates. Quasi-static compression strength experiments obey the well-known Hall-Petch relationship in which strength increases with decreasing grain size. Contrarily, the spall strength under dynamic compression increases with increasing grain size. This is due to spallation being a failure process that is dominated by defects. It has also been found that the timescale of the reverse phase transformation is only 8 ns and completes before spall. Preliminary results of iron strength under the highest pressures achievable suggest that it is higher than models are predicting and insensitive to initial microstructure. The high-pressure phase transformation in iron is thought to be a crucial factor in its high-pressure behavior. A measured reduction in grain size is thought to contribute to this microstructural insensitivity during compression. Hydrodynamic simulations are also used to design and predict results for these various high-pressure laser-driven experiments.