Atomistic Studies of Intrinsic Crack-Tip Plasticity
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MRS BULLETIN/MAY 2000
scale simulations to be performed in much shorter computing times than ab initio treatments of the crack-tip region. The cracks are first stabilized near the Griffith condition, and the process of crack advance and/or dislocation emission from the crack tip is studied. The behavior of a semi-infinite or double-ended crack can be studied under Mode I loading for different crack-tip geometries. The crack is embedded in a simulation cell, with periodic boundary conditions along the direction parallel to the crack front, and fixed boundary conditions along the periphery of the simulation cell, given by continuum theory. These boundary conditions bridge the length scales necessary to study the fracture process. In the examples we show, dislocations emitted are, in some cases, observed to move far away from the tip. However, in other cases, the dislocations emitted remain in the immediate vicinity of the crack tip after emission. We also see that the atomistic configurations of the tip region are different in the presence of a highangle grain boundary than in the bulk. In many of the cases studied, the fracture process occurs as a combination of dislocation emission and microcleavage portions. The simulation studies and observations can be used to establish certain trends and correlations. As an example of such a trend, we discuss the relationship of ductility and ordering energy in intermetallic alloys. These alloys are very attractive, particularly for structural applications in the aerospace and aeronautic industries, but the possible applications are restricted by their limited ductility at ambient temperatures.4 Further improvement of the unique mechanical properties of intermetallics requires the development of a fundamental knowledge of microscopic mechanisms of plastic deformation
and fracture in these materials. Atomistic computer simulations can be very useful in understanding these fundamental mechanisms and guiding alloy designers in the search for compositions and processes that result in improved ductility.
Simulation Technique The atomistic simulations of fracture shown in this article were carried out using a simple energy-minimization technique. The anisotropic elastic displacement field of a sharp crack5 was used to introduce a Mode I semi-infinite crack in a perfect crystal. The crack-tip region is then relaxed and allowed to find the minimum energy configuration under the given stress-intensity level. The stress intensity is varied to simulate the equilibrium configuration of the tip as the crack advances. Several different orientations of the crack tip can be studied by changing the Miller indices of the crack faces as well as the crack front. In the simulation, the crack may emit a dislocation on an inclined slip plane, if one is available, or it may cleave directly ahead. In addition, the crack may also deflect and propagate on an inclined plane. For simulations at the atomistic scale, the stress-intensity field can be approximated by using that of a semi-infinite crack, re
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