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.
Journal
Science