Computer Simulation of Misfit Dislocation Mobility in Cu/Ni and Cu/Ag Interfaces
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Computer Simulation of Misfit Dislocation Mobility in Cu/Ni and Cu/Ag Interfaces Richard J Kurtz1, Richard G. Hoagland2 and Howard L. Heinisch, Jr.1 1 Pacific Northwest National Laboratory Richland, WA 99352 2 Los Alamos National Laboratory Los Alamos, NM 87545 ABSTRACT The mobility of misfit dislocations in semicoherent Cu/Ni and Cu/Ag interfaces is determined by molecular dynamics and elastic band simulation methods. Cube-on-cube oriented Cu/Ni and Cu/Ag systems were studied with the interfaces parallel to (010). Core structures of misfit dislocations in semicoherent interfaces are found to be quite different in these systems. In Cu/Ni the misfits have very narrow cores in the plane of the interface. Consequently, the shear stress to move these dislocations is large, ~1.1 GPa. The core width and hence the misfit mobility can be changed by placing the misfit away from the chemical interface. Placement of the misfit oneatom layer into the Cu increased the core width a factor of 1.6 and lowered the threshold shear stress to 0.4 GPa. The misfit dislocations in Cu/Ag interfaces, on the other hand, are wide and therefore are much more mobile. The threshold shear stress for misfit movement in Cu/Ag is very low, ~0.03 GPa.
INTRODUCTION Nanolayered bimetallic composites of semicoherent Cu/Ni and Cu/Ag prepared by codeformation or physical vapor deposition techniques display near theoretical tensile strengths and substantial ductility [1]. Arrays of misfit dislocations exist in semicoherent interfaces to accommodate lattice parameter mismatch between the layers. The strength of the interface is controlled, in part, by the mobility of the misfits and interactions with glide dislocations. In this paper we apply elastic band techniques and molecular dynamics simulation methods to study the properties and attendant mobility of misfit dislocations in Cu/Ni and Cu/Ag interfaces.
COMPUTATIONAL DETAILS The methodology we have used to calculate the atomic arrangements of interfaces have been described in detail previously [2] in connection with calculations of the structures of grain boundaries. For brevity, only an outline of the methodology is provided here. The model consists of a two-part rectangular computational cell. One part, Region 1, contains movable atoms embedded in a semi-rigid part, Region 2. The interface approximately bisects the model as shown in Figure 1. Equilibrium structures at T ~ 0K are obtained via relaxation using molecular dynamics with an energy quench. The two crystals on either side of the interface are free to move and undergo homogenous strain in all three directions. This movement occurs during the relaxation via a viscous drag algorithm, i.e., the velocities and strain rates associated B2.9.1
with such motions are proportional to the net forces acting on each of the two crystals within Region 1. In addition, we employed a mapping scheme whereby average displacements within Region 1 were used to adjust the positions of individual atoms near the interface but within the surrounding Region 2. Period
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