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8/10/04

11:31 AM

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Molecular-Dynamics Study of Mechanical Deformation in Nano-Crystalline Aluminum K. KADAU, T.C. GERMANN, P.S. LOMDAHL, B.L. HOLIAN, D. KADAU, P. ENTEL, M. KRETH, F. WESTERHOFF, and D.E. WOLF We report on molecular-dynamics (MD) simulations of tensile loading of nano-crystalline Al modeled by an embedded-atom method (EAM) potential. Usage of two different sample preparation methods of the nano-crystalline material allows us to compare mechanical properties for different sample qualities. A Voronoi-constructed polycrystal exhibits nearly no pores and has different mechanical properties compared to a material that is sintered under pressure and temperature from spherical nanoparticles, resulting in a lower-density sample. We found an inverse Hall–Petch relation for the flow stress for grain sizes smaller than 10 nm. Intergranular fracture was observed for the larger Al grain sizes, but not for nano-crystalline Cu.

I. INTRODUCTION

NANO-CRYSTALLINE metals exhibit physical properties different from ordinary poly-crystalline materials, which make these materials of technological interest.[1] One important example is the increasing hardness with decreasing grain size, attributed to dislocation immobilization at the grain boundaries, and known as the “Hall–Petch effect.”[2,3] However, for grain sizes below a critical value, the hardness decreases with decreasing grain size, i.e., an inverse Hall– Petch effect.[4,5,6] There is still a debate whether this effect is an artifact due to difficulties in sample preparation at the smallest grain sizes.[7] Atomistic simulations identifying an inverse Hall–Petch relation for Cu, Ni, and Pd[8,9] for small grain sizes (d  10 nm, where d  average grain size) exhibit plasticity dominated by intergrain activities, such as rotation, sliding, and growth. Here, we report on large-scale molecular-dynamics (MD)[10] simulations, performed with the SPaSM code,[11] investigating the tensile loading of nano-crystalline Al. The atomic interactions were described by an embedded-atom method(EAM) potential[12] especially designed to model Al.[13] For the grain sizes investigated (d  10 nm), we find an inverse Hall–Petch relation. The deformation processes here are dominated by grain rotation, sliding, and growth—mechanisms that result from the large fraction of grain boundary atoms as seen previously for other face-centered cubic metals.[8,9] In the present case of nano-crystalline Al, we find that intergrain fracture can occur, identified by a sudden drop in the stress-strain plot. We demonK. KADAU, Postdoctoral Research Associate, P.S. LOMDAHL, Deputy Group Leader, and B.L. HOLIAN, Technical Staff Member, are with the Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87545. Contact e-mail: [email protected] T.C. GERMANN, Technical Staff Member, is with the Applied Physics Division, Los Alamos National Laboratory. D. KADAU, Postdoctoral Research Associate, P. ENTEL and D.E. WOLF, Professors, and M. KRETH and F. WESTERHOFF, Ph.D. Students, are with th