Shock-induced melting – new insights through continuum and x-ray diffraction measurements

Despite the long and extensive history of melting studies, experiments to determine the dependence of melting on crystal orientation are lacking. To address this need, we performed experiments to measure the longitudinal sound speeds in aluminum single crystals shock-compressed along 〈100〉 and 〈110〉 to 168 GPa. Copper impactors were used to impact targets consisting of Al single crystal buffers bonded to Al single crystals of the same orientation and backed by LiF 〈100〉 optical windows. Wave profile measurements obtained using laser interferometry showed a single jump followed by a constant peak state before the arrival of the smooth release wave from the back of the impactor. For Al 〈100〉, 120 GPa represents the melting onset and is complete at 140 GPa. In contrast, for Al 〈110〉, ~100 GPa represents the melting onset, and the completion of melting is >163 GPa. Our results show that the melting transition, both onset and completion, is strongly anisotropic. The present findings demonstrate the need to consider the role of deformation induced lattice defects on the melting transition and provide a general approach to examine melting anisotropy in crystalline solids.