Kelton is working to develop a better understanding of the principles governing phase formation and stability in condensed phases and the relations between phase transitions and atomic structures. Studies are focused on nucleation and growth processes in condensed phases, the relations between local atomic structure and the nucleation barrier, the coupling of phase transitions of different order, the glass transition, and the formation of metallic glasses and their crystallization to consolidated nanostructured materials. A new kinetic theory for nucleation, developed in this group, has led to an improved understanding of nucleation processes where long- range diffusion is important, such as for solid-state precipitation.
The structure and formation of quasicrystals, a unique form of condensed matter discovered in 1984, have long been of interest to this group. Icosahedral quasicrystals are ordered phases that produce sharp diffraction patterns, indicative of long- range order, but have a symmetry that is inconsistent with the translational periodicity that is characteristic of crystal phases. Most of the known titanium-, zirconium-, and hafnium- based quasicrystals have been discovered in Kelton’s laboratory. One of these, Ti45Zr38Ni17, is the only known stable quasicrystal at low temperatures and might be the first case of a quasiperiodic ground state. These quasicrystals can also store significant quantities of hydrogen, making them of potential interest for hydrogen storage and battery applications.
Metallic glasses, like more common silicate glasses, are amorphous, containing no long-range translational order but significant short- and medium-range order. Kelton uses a wide range of experimental techniques to study the order in these glasses and in related transition metal alloy liquids. Working in collaboration with researchers at NASA Marshall Space Flight Center and the Advanced Photon Source, he has recently developed a new technique for studying the structures of reactive metal alloys at temperatures up to 3000 K. This has led to the first proof of a 50-year-old hypothesis linking the liquid structure to the nucleation barrier and has improved understanding of the atomic structures of equilibrium and supercooled liquids. It has also provided new information on liquid-liquid phase transitions. A levitation facility constructed in this group is used to study crystal nucleation kinetics in supercooled metallic liquids, as well as liquid thermophysical and structural properties. A similar levitation facility was constructed for neutron scattering studies. It is now permanently housed at the Spallation Neutron Source at Oak Ridge National Laboratory in Oak Ridge, Tennessee. It was used by Kelton and his collaborators to provide the first experimental evidence for a link between local structure in a supercooled liquid and liquid dynamics.