Probing the lattice structure of dynamically compressed and released single crystal iron through the alpha to epsilon phase transition

Experiments using broadband Laue x-ray diffraction (XRD) were used to examine the lattice structure of dynamically compressed [100]-oriented single crystal iron samples at the Dynamic Compression Sector at the Advanced Photon Source. These experiments used 1 mu m thick iron single crystal samples sandwiched between a polyimide ablator and a polycarbonate window. A 100J, 10ns duration laser pulse incident on the polyimide ablator was used to shock compress the iron samples to initial stresses greater than 25GPa, exceeding the similar to 13GPa alpha (body-centered-cubic or bcc structure) to epsilon (hexagonal-close-packed or hcp structure) phase transition stress. XRD measurements were performed at various times relative to the shock wave entering the iron sample: early times, <<similar to> 150ps while the initial shock waves propagated through the iron; intermediate times, after the iron equilibrated with the ablator and window reaching a plateau stress state (12 or 17GPa) lasting several nanoseconds; and late times, during uniaxial strain release. The early time measurements show that in < <similar to> 150ps, the high-pressure hcp phase is relaxed with a c / a ratio of 1.61, contrary to previous laser shock experiments where a c / a ratio of 1.7 was inferred. In the plateau stress state and partially released states, XRD measurements showed that the hcp structure retained a c / a ratio of 1.61 with no observable changes in the microstructure. Upon stress release at similar to 1GPa/ns release rate, the reverse phase transition (hcp to bcc) to the original single crystal orientation (implying a transformation memory effect) was observed to reach completion somewhere between 13 and 11GPa, indicating little stress hysteresis under rapid uniaxial strain release. A similar memory effect for the reverse hcp to bcc transformation has been previously observed under hydrostatic compression. However, the bcc/hcp orientation relationships differ somewhat between dynamic and static compression experiments, implying that the transformation pathway under uniaxial dynamic strain differs from the Burgers mechanism.