X-ray diffraction measurements across the melt line in shocked nickel*

We have investigated elemental bulk nickel at high shock pressures and temperatures using in situ x-ray diffraction and the velocimetry technique, VISAR, with the goal of bracketing the onset of melt under shock conditions. We observe a solid compressed fcc-Ni phase up to pressures as high as 500 GPa. In the range of approximately 300–375 GPa, we see evidence for the onset of melt, with full melt achieved above 500 GPa. The experiments were conducted at the Matter in Extreme Conditions endstation at the SLAC National Accelerator Laboratory using a flat-top laser drive. Experimental results were supported by 1D hydrodynamic simulations, providing particle velocity, transit time, pressure, and density data that agree well with experiment. 

 

Our work presents the first in situ x-ray diffraction data on shock-compressed nickel up to ~500 GPa and is particularly relevant to computational and experimental researchers studying nickel and other dense metals. The pressures we reached are some of the highest ever reported for shocked nickel, and our identification of a solid compressed phase up to 500 GPa is significantly higher than expected by the majority of melt lines that have been proposed for nickel in the literature.