Partners with Energy

PSC collaborations with the Department of Energy

Improved Actions for Staggered Quarks

Magnetism in the Solar Dynamo:
University of Chicago

Rocket Science:
University of Illinois Urbana-Champaign

Shock Waves in Gas:
California Institute of Technology

Turbines and Turbulence:
Stanford University

A New Picture of How Metals Deform

The Edge of Reality

A New Picture of How Metals Deform

Electron density of the "easy core" screw dislocation in tantalum as calculated by Arias and Ismail-Beigi. A sea of electrons (blue) of medium density moves through the metal except in voids surrounded by low density regions (magenta). Other electrons at high density (gold) remain attached to the atoms. Download a larger version of this image (461KB).

Elastic energy density in the "easy core" configuration of tantalum. High energy regions (red) surround the four dislocations in this 90-atom system. Vectors indicate the displacement of atoms (strain) from their ideal positions. Download a larger version of this image (607KB).

"Our calculations at PSC are turning the last 30 to 40 years of thinking about the origins of plasticity in a large class of transition metals on its head," says physicist Tomás Arias of Cornell University. He and MIT graduate student Sohrab Ismail-Beigi are using PSC's CRAY T3E to study fundamental properties of molybdenum and tantalum, both of which are "transition metals," a subgroup of elements related in atomic structure that hold promise for high-strength, high-temperature technological applications.

Their PSC work represents the first calculations applying density-functional theory — a quantum-theory based approach to representing the interactions between atoms and electrons — to "screw dislocations" in these metals. Dislocations are microscopic structures that govern how the metal deforms, and accepted thinking has been that a dislocation in these materials must have a particular atomic structure — known as a symmetry breaking core.

The calculations at PSC appear to overturn this view. They show that for the screw dislocation in molybdenum and tantalum symmetry isn't broken. They also improve on the accepted theoretical view by indicating better agreement with experiments on the amount of stress required to deform the metals. "Our results," says Arias, "indicate a screw dislocation core without broken symmetry and with energy scales in much better accord with the experimental data."

Arias and Ismail-Beigi also computed the energies and electronic structures associated with two different configurations of the screw dislocation in molybdenum and tantalum. For both metals, there's a low energy, relatively stable configuration known as the "easy core" and a higher energy, less stable state known as the "hard core." PSC calculations show the energy difference between the two states to be two to three times less than computed with the accepted models.

"This information is critical," says Arias, "to constructing large-scale theories of dislocation motion. These results are changing our understanding of the basic physics of these systems, and they provide important information for the experimental part of the ASCI program."

RETURN: Projects in Scientific Computing, 1999