College of Engineering News • Iowa State University

Iowa State professor publishes two papers in highly ranked journals

Valery Levitas, Schafer 2050 Challenge Professor and faculty member of aerospace engineering and of mechanical engineering, has published two papers in highly ranked journals on his theory of virtual melting phenomenon, which describes short-term melting of materials followed by immediate recrystallization.

Levitas and Ramon Ravelo, a collaborator from Los Alamos National Laboratory, were published in the Proceedings of the National Academy of Sciences of the United States of America (Levitas and Ravelo, PNAS, 2012), the world’s foremost interdisciplinary journal.

In their paper “Virtual melting as a new mechanism of stress relaxation under high strain rate loading,” the researchers have discovered a new and very unexpected mechanism of plastic, or irreversible, deformation in shock waves. Using a new thermodynamic approach under nonhydrostatic loading, or loading that is different in different directions, as well as simulations at the atomic level, the researchers  proved that in very strong shock waves, material can melt at up to 4,000K below the melting temperature. This melting substitutes traditional mechanisms of plastic deformation due to crystal defects like dislocations and twinning. After melting, non-hydrostatic stresses relax, leading to an undercooled and unstable liquid, which recrystallizes in picosecond time scales to a hydrostatically loaded crystal.

Their findings will be relevant for studying nuclear explosions and meteorite impacts, as well as for planned experiments in large laser facilities such as the National Ignition Facilities at the Lawrence Livermore National Laboratory in the U.S. and LULI (the Laboratoire pour l’Utilisation des Lasers Intenses) in France. This paper is expected to be featured in Nature Materials and .

Levitas’ second paper “Crystal-crystal phase transformation via surface-induced virtual pre-melting” was published as a Rapid Communications piece in Physical Review B (PRB), and encompassed the work he has been doing with an experimental group in China.

Via virtual melting and surface-induced virtual melting, the group justified thermodynamically and confirmed experimentally several new phenomena for crystal-crystal phase transformations in nanofibers with the chemical composition PbTiO3. Utilizing these phenomena and changing the surroundings of an element, one can control the surface energy, and consequently the thermodynamics, kinetics, and mechanism of such transformations.

This work also has provided a way to reduce transformation temperature for solid-solid transformation. The new transformation mechanism can be tailored for other material systems, leading to nanomaterials adopting different crystal structure, size, and shape, which may be useful for mechanical and electronic nanodevice applications.

For a full explanation of Levitas’ research, visit the links above.

Note that virtual melting phenomenon first was revealed as the new mechanism of crystal-crystal phase transformations in explosive material (Levitas et al. Phys. Review Letters, 2004, 92, 235702). Then it was predicted as the mechanism of high pressure amorphization in various electronic, geological, and superhard materials (Levitas. Phys. Review Letters, 2005, 95, 075701) and as the mechanism of sublimation (Levitas & Altukhova. Phys. Review B, 2009, 79, 12101).