Spall behavior in iron: microstructural effects on strength, failure, and phase transitions

Spall response of pure iron was studied using high power pulsed laser experiments at the Jupiter Laser Facility and Dynamic Compression Sector. Thin iron foils of varying initial microstructures were subjected to peak pressures of 60 GPa and strain rates ranging from 10⁶ s^-1 – 10⁷ s^-1. Simultaneous time-resolved free surface velocity measurements, diffraction, and recovery techniques were used to investigate spall strength, failure mechanisms, and phase transitions. These uniaxial strain experiments yielded strengths between 5 and 10 GPa for nanocrystalline and single crystal iron, respectively. Post-shock characterization and Molecular Dynamics simulations verify that this difference is due to void initiation sites. Grain boundaries in nano and polycrystalline iron are favorable sites for voids nucleation and will consequently cause failure to occur along grain boundaries perpendicular to the shock direction. In contrast, the formation and interaction of twin boundaries in single crystal iron are the cause for void initiation, growth, and coalescence that ultimately cause ductile failure. The complete α-ϵ phase transition during compression was observed, followed by a rapid transformation back to α-Fe. The grain structure during compression and release/spall failure was found to show strong single crystalline character. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.