From Atomistic Simulations To Crash Simulations

March 05, 1998

Materials scientists are currently untangling the elementary atomistic processes of the deformation and fracture of materials. Recent atomistic simulations (Science, 279, March 6 issue, 1998) treat one of the last and most complicated cases, namely the intersection of two dislocations.

Continuum mechanical techniques like the finite element method are exclusively used for engineering simulations of metal-forming processes such as deep-drawing or rolling or the crash simulations used to assess the deformation characteristics of car bodies. These continuum models require the specification of the governing constitutive equations, that is, laws that relate the response of a material to an applied stress. Currently, the necessary parameters are empirically adjusted to fit experiments, but simulations could in the near future allow these constitutive equations to be determined from physically based simulations.

Fundamental atomistic simulations can not directly be applied because the typical feature size in plastically deformed materials is far too large and would involve far too many atoms to be handled by a computer. However, six years ago, a French research group (Solid State Phenom. 29, 456, 1992) first demonstrated that such constitutive equations can, at least in principle, be obtained from simulations on the basis of discrete, straight dislocation segments. These discrete dislocation dynamic (DDD) simulations, however, need to be fed with mobility laws and explicit rules for the short range defect interactions, which can indeed be obtained from atomistic simulations.

Because of the tantalising prospect that it now seems possible to predict macroscopic large-scale deformation behaviour from atomic scale processes, several groups around the world and among them one group at the Max Planck Institute for Metals Research in Stuttgart are currently focusing on the atomistic input to the DDD. Geometrically simpler problems that only concern one specific crystal defect like a dislocation or a crack have been studied atomistically for some years. With interatomic potentials recently reaching a sufficient level of sophistication, these studies are becoming more and more quantitative even for complex materials like intermetallic alloys. However, geometrically complicated problems that involve more than one type of crystal defect, like dislocation intersection and junction formation are being tackled now for the first time. These atomistic simulations encompass millions of atoms and generate such a vast amount of information that one of the most important steps is to learn how to intelligently discard most of it. What is left is an atomistic picture of the dislocation intersection and some quantitative information about the stresses required to break the junction.

Properly quantified, these atomistic simulations may finally connect macroscopic deformations with the behaviour of individual atoms. Of course, there is still a long way to go to capture the full complexity of all the dislocation interactions even in the simplest model materials and it still seems unrealistic to think about complicated technical alloys. Nevertheless, the present developments hold great promise that a link can eventually be achieved.
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Max-Planck-Gesellschaft

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