The Role Played By Two Parallel Free Surfaces in the Deformation Mechanism Of Nano-crystalline Metals: A Molecular Dynam
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The Role Played By Two Parallel Free Surfaces In The Deformation Mechanism Of Nanocrystalline Metals: A Molecular Dynamics Simulation P. M. Derlet and H. Van Swygenhoven Paul Scherrer Institute, CH-5253 Villigen PSI, Switzerland Abstract Former molecular dynamics computer simulations of polycrystalline Ni and Cu metals with mean grain sizes ranging between 3 and 12 nm demonstrated a change in deformation mechanism as a function of grain size: at the smallest grain sizes all deformation is accommodated in the grain boundaries. In this paper we report on the influence of the presence of two free surfaces on the deformation behaviour. The purpose of this simulation is to study which phenomena observed in in-situ tensile experiments performed in the electron microscope can be expected to be intrinsic properties of the deformation process and which phenomena are due to the presence of two free surfaces separated by a very small distance. Introduction With the reduction in grain size to the nanometre regime and a corresponding increase in the percentage of grain boundary atoms, the traditional view of dislocation driven plastic deformation has had to be reconsidered [1-5]. For nano-grain materials an alternative inelastic deformation process, driven primarily by grain boundary activity is now believed to be an important contribution. Extensive molecular dynamics has been performed by us (HVS) [6-10] on high angle randomly orientated bulk Cu and Ni nano-grain material in which the grain boundary region has a high degree of structural order. These simulations have demonstrated that the mechanism for plastic deformation changes from an intra-granular mechanism (dislocations) to an inter-granular mechanism (sliding), as the average grain size decreases. TEM and HREM constitute important experimental techniques in the analysis of static and dynamic nanocrystalline boundary structure (see for example [11]). More recently, in situ tensile deformation experiments have been undertaken [12,13]. The requirement of thin samples for optimal contrast and resolution calls into question the assumption of a bulk structure. The current work investigates if the presence of the surface leads to additional grain boundary dynamics and thus a change or modification of the nanocrystalline plastic deformation process. Sample Preparation and Relaxation Nanocrystalline samples where created by constructing nanograins at random locations and crystallographic orientations until the grains overlap each other according to the Voronoi construction [14]. A more detailed description of this procedure can be found in [8]. Two samples with similar microstructure but differing mean grain sizes of 5nm and 12nm where used. The surface is introduced by removing the periodic boundary conditions in one direction of the (bulk) equilibrated sample. The samples were then relaxed under zero applied stress conditions for about 40ps, allowing the surface structure to find a more equilibrium configuration. In the present work all MD was performed at constant temperature
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