Dynamic Reflectance Changes In High-Purity Zirconium When It Is Shocked From The Alpha To Omega Phase

Flash lamp-illuminated integrating spheres have been used to measure the dynamic reflectance at the interface between a shocked metal target and a window maintaining the shock stress. In conjunction with radiance measurements, this method was recently used to study the pressure versus temperature phase boundary of the beta phase to body-centered-tetragonal transition of tin at around 13 GPa and, in separate experiments, to show a relationship between the reflectance and the phase fraction. We have used this technique to measure the ratio, R, of the dynamic reflectance relative to the ambient value for a high-purity zirconium sample attached to a sapphire window to study the alpha to omega phase transition. Nine experiments cover the range of stress, s, from 4 to 16 GPa at six wavelengths from 500 to 1550 nm. The two phases in zirconium differ in volume by < 1%, so a phase change between them can be difficult to identify from anomalies in the particle velocity measured on the shock Hugoniot. The phase boundary is believed to cross the Hugoniot around 3 GPa, but dynamically it is observed at much higher stresses > 6 GPa. Our results show that the relative reflectance decreases upon initial shock in the alpha state but increases as the material transforms to its omega state. The slope dR/d sigma changes the sign around the phase transition. For shocks below 6 GPa, dR/d sigma < 0, and for sigma above 10 GPa, dR/d sigma > 0. In the stress region between 6 and 10 GPa, dR/ds varies somewhat, perhaps due to the band structure or because the transition is incomplete there. (C) 2018 Author(s).