Observation of Compression-Induced Phase Transformations in Zirconium using Ultrafast X-Ray Diffraction*

The mechanisms of phase transitions in bulk materials are analogous to the transition state in the study of chemical reactions. Detailed knowledge of these mechanisms (and the ability to control them) has the potential to revolutionize material synthesis. Studying such phenomena is difficult, however, since the fundamental mechanisms of phase transitions occur on near-atomic (nm) length scales at the speed of sound (1-10 nm/ps for condensed phases), implying picosecond time scales. Using 130 fs x-rays at the LCLS-MEC beamline, we interrogated phase transformation pathways of Zr at the spatial and temporal scales of these fundamental mechanisms. Zr was dynamically compressed to pressures up to 130 GPa, driven by ~120 picosecond duration laser pulses with energies in the range of 2.5-250 mJ. We observed an intermediate body-centered cubic (bcc) β-Zr phase on the transformation path from the hexagonal close-packed (hcp) α-Zr phase towards the P6/mmm ω-Zr phase under rapid compression. At 33 GPa compression, we observed the initial α-Zr phase transform into the β-Zr phase, bypassing the lower-pressure ω-Zr phase. At 130 GPa, we observed direct melting of Zr from the α phase, followed by recrystallization on a longer ns timescale.