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Title: Modelling and Simulation of Faceted Boundary Structures and Dynamics in FCC Crystalline Materials
Keywords: grain boundary, facet, twin, dislocation, plastic deformation, molecular dynamics simulation
Issue Date: 10-Aug-2010
Source: WU ZHAOXUAN (2010-08-10). Modelling and Simulation of Faceted Boundary Structures and Dynamics in FCC Crystalline Materials. ScholarBank@NUS Repository.
Abstract: Large scale molecular dynamics (MD) simulations are employed to study faceted grain boundaries' defect structures and dynamics in face-centered cubic (FCC) crystalline metals. In particular, two problems: (1) the plastic deformation of nanotwinned FCC metals; (2) the finite length grain boundary faceting are investigated in detail. The first one is studied through MD simulations employing an embedded-atom method (EAM) potential. Two dislocation-twin interaction mechanisms that explain the observation of both ultrahigh strength and ductility in nanotwinned FCC metals are found. First, the interaction of a 60° dislocation with a twin boundary leads to the formation of a {001}<110> Lomer dislocation which, in turn, dissociates into Shockley, stair-rod and Frank partial dislocations. Second, the interaction of a 30° Shockley partial dislocation with a twin boundary generates three new Shockley partials during twin-mediated slip transfer. The generation of a high-density of Shockley partial dislocations on several different slip systems contributes to the observed ultrahigh ductility while the formation of sessile stair-rod and Frank partial dislocations (together with twin boundaries) explain observations of ultrahigh strength. Furthermore, polycrystalline MD simulations show that plastic deformation of nanotwinned copper is initiated by nucleation of partial dislocations at grain boundary triple junctions. Both dislocations crossing twin boundaries and twin migrations are observed. 60° dislocations frequently cross slip onto {001} planes in twin grains and form Lomer dislocations, constituting the dominant crossing mechanism. We further examine the effect of twin spacing on this mechanism through samples over a wide range of twin spacing. The simulations show a transition in the dominant dislocation mechanism at a small critical twin spacing. While at large twin spacing, cross-slip and dissociation of Lomer dislocations create dislocation locks which restrict and block dislocation motion and thus enhance strength. At twin spacing below the critical size, cross-slip does not occur, steps on the twin boundaries form and deformation is much more planar. These twin steps can migrate and serve as dislocation nucleation sites, thus softening the material. Based on these mechanistic observations, an analytical model for the critical twin spacing is proposed and is shown to be in excellent agreement both with simulations and experiments. This suggests the above dislocation mechanism transition is a source of the observed transition in nanotwinned copper strength. For the problem of finite length grain boundary faceting, symmetrical and asymmetrical aluminium boundary faceting are studied with MD simulations using two EAM potentials. Facets formation, coarsening, reversible phase transition of S3{110} boundary into {112} twin and vice versa are shown and the results are consistent with earlier study. The S11{002}<sub>_1</sub>/{667}<sub>_2</sub> boundary shows faceting into {225}<sub>_1</sub>/{441}<sub>_2</sub> and {667}<sub>_1</sub>/{001}<sub>_2</sub> boundaries and coarsens with a slower rate when compared to S3{112}. However, facets formed by {111}<sub>_1</sub>/{112}<sub>_2</sub> and {001}<sub>_1</sub>/{110}<sub>_2</sub> boundaries from a {116}<sub>_1</sub>/{662}<sub>_2</sub> boundary is stable against finite temperature annealing. In the above faceted boundary, elastic strain energy induced by atomic mismatch across the boundary creates barriers to facet coarsening. The observed finite facet sizes are dictated by facet coarsening kinetics which can be strongly retarded by deep local energy minima associated with atomic matching across the boundary.
Appears in Collections:Ph.D Theses (Open)

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