Time Resolved Lattice Kinetics Of Rapidly Compressed Single Crystal Iron Through The alpha-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 micron thick iron single crystal samples sandwiched between a polyimide ablator and a polycarbonate window. A 40 J laser pulse incident on the polyimide ablator was used to shock or ramp compress the iron samples to stresses exceeding the ~13 GPa alpha to epsilon phase transition stress. XRD measurements of the lattice structure were performed at various times relative to the loading wave entering the iron sample. The shock measurements show that in less than ~150 ps 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 ramp case in situ x-ray diffraction measurements show that a mixed alpha/epsilon phase was observed starting at ~13 GPa which persisted for ~2 ns until the peak stress of 18 GPa was reached. Similar to observations in shock-compression experiments, the epsilon phase formed with two dominant variants both with the epsilonphase c-axis orthogonal to the compression direction and a c/a ratio of 1.61. 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.