Direct observation of the hcp-bcc phase transition and melting along the principal Hugoniot of Mg

Time-resolved x-ray diffraction was used to examine the phase evolution of polycrystalline Mg shocked along the principal Hugoniot to states that span multiple phase boundaries into the liquid. The diffraction data indicate that the hcp-bcc phase boundary lies above 27 GPa, with the hcp-bcc transition occurring within nanoseconds of loading. Experiments at 55 GPa reveal a mixture of bcc and liquid, indicating incipient melt occurring 10--15 GPa lower than previously reported, with complete melting observed at 63 GPa.