We study the high-pressure melting behavior of titanium using laser-driven shock compression with in situ femtosecond x-ray diffraction and molecular-dynamics simulations based on a machine-learned interatomic potential. The MD simulations predict the solid-liquid coexistence on the Hugoniot in the $\sim 111-124\mathrm{GPa}$ range. Experimentally, we observe the first evidence of liquid at 86 GPa. We also observe pronounced microstructural changes with pressure, with strong grain refinement associated with the emergence of liquid, within the solid-liquid coexistence $(\sim 110-126\mathrm{GPa})$. Above 126 GPa, we observe the persistence of residual levels of highly textured crystalline Ti to $\sim 180\mathrm{GPa}$, well above the expected melt completion pressure. We discuss the accuracy that current laser-shock experimental platforms have at determining the melt onset and completion pressures.

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• Phase transitions
• Shock waves
• Molecular dynamics
• X-ray diffraction