Examination of the cerium α-ε phase transition under dynamic loading with x-ray diffraction

Abstract

Recent examination of the cerium Hugoniot with pyrometry and x-ray diffraction (XRD) has revealed a narrow solid-liquid two-phase region. It has been suggested that nonequilibrium melting may be occurring along the Ce Hugoniot, with either melt kinetics or a sluggish \ensuremath{\alpha}-\ensuremath{\epsilon} transition impeding the transition. In particular, the kinetics of the \ensuremath{\alpha}-\ensuremath{\epsilon} is unknown and the location of the phase boundary is in dispute. Static measurements suggest a nearly vertical phase boundary that intersects the Hugoniot at 6--7 GPa. This lies in direct conflict with dynamic measurements along the Hugoniot observing \ensuremath{\alpha}-Ce through incipient melt. This work presents dynamic experiments using XRD to examine the behavior of the \ensuremath{\alpha}-\ensuremath{\epsilon} phase transition. The results show that the \ensuremath{\alpha}-\ensuremath{\epsilon} phase transition occurs through a tetragonal distortion, with the transition beginning at temperatures below the solid Hugoniot. Following the initial deviation from an ideal fcc structure, the $c/a$ ratio is found to gradually increase with no steady value observed in the \ensuremath{\epsilon} phase within the range of these experiments (below 17 GPa). Multiple diffraction patterns captured during the peak stress state show no significant change in $c/a$ ratio prior to uniaxial release, upon which Ce reverts to an fcc structure. The results indicate that the \ensuremath{\alpha}-\ensuremath{\epsilon} transition occurs rapidly, both on loading and release. An examination of the $c/a$ ratio with increasing temperatures suggests 11.5 GPa as a lower bound for the location of the \ensuremath{\alpha}-\ensuremath{\epsilon}-liquid triple point.