New computer simulations by LLNL researchers show that 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. Image: LLNL.Researchers at Lawrence Livermore National Laboratory (LLNL) have dived down to the atomic scale to resolve every ‘jiggle and wiggle’ of atomic motion that underlies metal strength.
In a first-of-its-kind series of computer simulations focused on the metal tantalum, the researchers predicted that on reaching certain critical conditions of straining, the plasticity (the ability to change shape under load) of tantalum meets its limits. One limit is reached when crystal defects known as dislocations are no longer able to relieve mechanical loads. This activates another mechanism – twinning, or the sudden reorientation of the crystal lattice – which takes over as the dominant mode of dynamic response. This research is reported in a paper in Nature.
The strength and plasticity properties of a metal are defined by dislocations, line defects in the metal’s crystal lattice whose motion causes material slippage along crystal planes. The theory of crystal dislocation was first advanced in the 1930s; much research since then 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). These 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," said Vasily Bulatov, LLNL lead author of the paper. "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."
Due to severe limits on accessible length and time scales, it was long thought impossible or even unthinkable to use direct atomistic simulations to predict metal strength. Taking full advantage of LLNL's world-leading high-performance computing facilities through a grant from LLNL’s Computing Grand Challenge program, the researchers demonstrated that not only are such simulations possible, but they can deliver a wealth of important observations on the fundamental mechanisms of dynamic response. This includes the quantitative parameters needed to define strength models important to the Stockpile Stewardship Program, which ensures the safety, security and reliability 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," Bulatov said. "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. "
The researchers developed the first fully dynamic atomistic simulations of the plastic strength response of single crystal tantalum subjected to high-rate deformation. Unlike computational approaches to strength prediction, atomistic molecular dynamics simulations rely purely on an interatomic interaction potential to resolve every ‘jiggle and wiggle’ of atomic motion and reproduce material dynamics in full atomistic detail.
This story is adapted from material from Lawrence Livermore National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.