Phase transitions in aluminum under shock-ramp compression

Aluminum 6061 alloy has been used extensively as an electrode material in dynamic-compression experiments at the Z machine. Such experiments would benefit from improved understanding of aluminum's equation of state in the multi-megabar pressure range. Previous theoretical works suggest that aluminum has three structural phases in the solid with an equilibrium triple-point between fcc (ambient), hcp, and bcc structures in the vicinity of 200-300 GPa and 2000-4000 K, but with significant discrepancies between results. Previous X-ray diffraction measurements have detected the hcp phase above ~200 GPa on the room-temperature isotherm under static compression in a diamond-anvil cell, and both the hcp and bcc phases above ~220 GPa and ~320 GPa, respectively, near the principal isentrope under laser-based dynamic compression. To better constrain the triple-point pressure, a series of shock-ramp experiments is underway at Sandia's Z machine to measure compressibility along elevated-temperature quasi-isentropes. I will review the experimental method and present preliminary results from analyses of velocimetry data.