Molecular dynamics simulations of surface reconstruction at the edges of a crack in ruthenium aluminum
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Using molecular dynamics computer simulations and interatomic potentials derived partly by Voter and Chen1 and Rifkin et al.,2 we studied the surface reconstruction taking place on free surfaces of arrays of RuAl. Surface reconstruction appears to be very important on {111} and {110} types of planes and almost nonexistent on {100} type of planes. Cracks oriented so that their crack planes were either {111} types or {110} types exhibit on their internal free surface important surface reconstruction. It is believed that this effect may have some contribution in the brittle versus ductile behavior of the crack.
I. INTRODUCTION During molecular dynamics simulations of crack propagation in RuAl (CsCl structure with a lattice parameter of 3.03 A), it was observed that surface reconstruction was occurring at the free surfaces of the crack for specific orientations. This localized deformation close to the crack tip can result in extra strains, and modify the existing strain field (due to the crack tip). As the brittle versus ductile behavior of a crack depends upon the surrounding strain field, it is expected that surface reconstruction will influence the cracking properties of a material. To characterize this phenomenon, simulations were performed on perfect RuAl arrays with free surfaces. Different planes were chosen to be parallel to the free surfaces. The interatomic potentials for RuAl used in the simulations were partly derived by Voter and Chen,1 and partly by Rifkin,2 both using the Embedded Atom Method (EAM).
molecular dynamics algorithm. For the arrays containing a crack, the crack was simulated by displacing the atoms in an otherwise perfect array, according to the plane strain linear elastic displacement field for a cracked anisotropic solid.3 A more detailed description of the scheme adopted to simulate the crack is given in Ref. 4. The crack was 25 A long and various tensile loads were applied. After the crack has been "created", the atoms are allowed to relax for 2000 steps according to the molecular dynamics algorithm. To prevent the crack from closing, the atoms of two outer layers on three of the four free surfaces (later referred to as "frozen layers") are held fixed (Fig. 1). The stress is then increased (after 2000 steps) by applying a continuously increasing displacement field to the "frozen layers". The crack then propagates, or dislocations are emitted or both. As can be seen in Fig. 1, the atoms on the surface containing the crack opening are not held fixed (there are no free surfaces in the Z direction due to the repeating boundary conditions). All simulations were performed at 1 K.
II. CONDITIONS OF THE SIMULATIONS The simulated arrays of RuAl contained approximately 8000 particles with periodic boundary conditions along the Z direction. Different orientations were studied that are represented by the directions parallel to the X and the Y axes:
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