Lawrence Livermore National Laboratory (LLNL) Scientists have stooped down to the atomic scale to solve every "jiggle and wiggle" of atomic motion that forms the basis of metal strength.
Tantalum crystal can flow like a viscous fluid while remaining a stiff and strong metal and retaining its ordered lattice structure. This snapshot depicts a dense network of lattice defects developing in the flowing crystal. Credit: Lawrence Livermore National Laboratory
In a first-of-its-kind series of computer simulations focused on metal tantalum, the research team predicted that, on achieving specific crucial conditions of straining, metal plasticity (the capability to change shape under load) matches its limits. One limit is reached when crystal defects called dislocations are not able to relieve mechanical loads anymore, and another mechanism – twinning or the sudden reorientation of the crystal lattice – is activated and takes over as the leading mode of dynamic response.
The research work appears in the September 27
th edition of the Nature journal as an Advance Online Publication.
Plasticity and strength properties of a metal are defined by line defects, dislocations in the crystal lattice whose motion results in material slippage along crystal planes. The theory of crystal dislocation was first developed in the 1930s, and since then, much research has focused on dislocation interactions and their role in metal hardening, in which continued deformation increases the metal's strength (much like a blacksmith pounding on steel with a hammer). The same simulations strongly suggest that the metal cannot be strengthened forever.
We predict that the crystal can reach an ultimate state in which it flows indefinitely after reaching its maximal strength. Ancient blacksmiths knew this intuitively because the main trick they used to strengthen their metal parts was to repeatedly hammer them from different sides, just like we do in our metal kneading simulation (link is external).
Vasily Bulatov, Lead Author of the paper, LLNL
Because of strict limits on time scales and accessible length, for a long time it was thought that it was not possible or even thinkable to use direct atomistic simulations to predict metal strength. Taking complete advantage of LLNL's world-leading high-performance computing facilities via a grant from the Laboratory's Computing Grand Challenge program, the Scientists showed that such simulations are possible. They offer a host of critical observations on elementary mechanisms of dynamic response and quantitative parameters required to define strength models important to the Stockpile Stewardship Program. SSP ensures the reliability, security and safety of nuclear weapons without testing.
We can see the crystal lattice in all details and how it changes through all stages in our metal strength simulations. A trained eye can spot defects and even characterize them to an extent just by looking at the lattice. But one's eye is easily overwhelmed by the emerging complexity of metal microstructure, which prompted us to develop precise methods to reveal crystal defects that, after we apply our techniques, leave only the defects while completely wiping out the remaining defect-less (perfect) crystal lattice.
Bulatov , L ead A uthor of the paper, LLNL
The research team developed the first fully dynamic atomistic simulations of plastic strength response of single crystal tantalum exposed to high-rate deformation. Contrasting from computational approaches to strength prediction, atomistic molecular dynamics simulations depend solely on an interatomic interaction potential to resolve every "jiggle and wiggle" of atomic motion and reproduce material dynamics in complete atomistic detail.
The other Livermore Researchers are Luis Zepeda-Ruiz and Tomas Oppelstrup, and former LLNL Postdoc Alexander Stukowski, presently at Darmstadt University in Germany.