Shock compression of silver to 300 GPa: Wave profile measurements and melting transition

Recent in-situ x-ray diffraction (XRD) measurements on laser-shock compressed Ag foils demonstrated a face-centered-cubic to body-centered-cubic transformation at \ensuremath{\sim}150 GPa and melting between 172 and 197 GPa [Phys. Rev. Lett. 124, 235701 (2020)]. As a complement to the XRD work, we conducted plate impact experiments to obtain shock velocity and wave profile measurements on thicker Ag samples shock-compressed to peak stresses between 30 and 300 GPa. The shock velocity--particle velocity results were fitted very well by a linear relation over the entire stress range, providing an accurate determination of the Ag Hugoniot (locus of the stress-volume states achieved under shock compression). For peak stresses below 187 GPa and above 210 GPa---corresponding to the solid and liquid phases, respectively---the wave profiles show clean single waves. No wave profile features related to the fcc-bcc transformation at \ensuremath{\sim}150 GPa were observed, implying minimal volume change for the transformation. For stresses between 187 and 210 GPa, an initial jump was followed by a time-dependent increase in the particle velocity (20--80 ns risetime) to the peak state---corresponding to the solid-liquid mixed phase response. Unlike the solid and liquid response, the mixed-phase response cannot be readily analyzed analytically. Instead, numerical simulations incorporating an accurate multiphase equation of state for Ag---not currently available---are required to analyze the wave profiles measured at 187--210 GPa stresses. The present work shows the potential for using wave profile measurements to examine the melting transition under shock compression.