Molecular Dynamics Study of the Effect of Dopant Atoms on Grain Boundary Sliding
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Molecular Dynamics Study of the Effect of Dopant Atoms on Grain Boundary Sliding Paul C. Millett, R. Panneer Selvam, Ashok Saxena Computational Mechanics Laboratory, BELL 4190 University of Arkansas, Fayetteville, AR 72701 Email: [email protected] ABSTRACT Molecular dynamics simulations are used to study grain boundary sliding in pure and doped Cu bicrystals using both Lennard-Jones and Embedded-Atom Method potentials. Two tilt [100] grain boundaries are considered: the coincident site lattice Σ5 interface and a random high angle interface. Shear stress between 0.69 GPa and 1.61 GPa was applied to the bicrystals for a duration of 10 ps at ambient temperature (300K) and high temperature (800K). For the pure bicrystals, the sliding of the Σ5 interface with respect to the random interface was lower at 800K and higher at 300K. For the doped bicrystals, interstitial dopant atoms and substitutional dopant atoms with larger atomic radius were effective in retarding grain boundary sliding. These simulations will aid further work to determine how segregated dopant atoms alter the tensile properties of nanocrystalline metals. INTRODUCTION For many years, grain boundary sliding (GBS) has been known to be an important process in creep deformation of polycrystalline materials particularly for temperatures greater than 40% of the absolute melting temperature in which the shear strength of the grain boundaries is lower than the crystalline regions. GBS is defined as the rigid translation of one grain relative to a neighboring grain in a direction parallel to their common boundary. It is independent of diffusional processes such as Nabarro-Herring and Coble creep; however, GBS typically accompanies these mechanisms. The microstructural consequence of GBS is the nucleation of voids that are either wedge-shaped (forming at triple junctions) or spherical (forming at steps and/or inclusions in grain boundaries) which will subsequently grow via diffusion. Coalescence of these voids leads to the onset of tertiary creep which ultimately leads to macroscopic failure. During the last decade, experimental [1] and theoretical [2,3] research suggests that the role of GBS becomes even more pronounced at lower temperatures and higher strain rates in nanocrystalline materials (NCMs) in which the average grain size, d, is < 30 nm. The rationalization for this phenomenon is the large surface-to-volume ratio resulting in a significantly higher density of grain boundaries in addition to the fact that Frank-Read sources for dislocation nucleation and emission cannot operate within grains that are smaller in diameter than their pinning points. This transition in deformation mechanisms explains the observed departure from the conventional Hall-Petch relationship in which the strengthening rate due to decreasing d reduces to zero and in some cases becomes negative. In addition, poor tensile ductility [4] and creep deformation at ambient temperatures [5] have been observed for materials with d < 30 nm. Interestingly, a few recent papers invo
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