Atomistic Studies of Plasticity in Nanophase Metals
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Atomistic Studies of Plasticity in Nanophase Metals H. Van Swygenhoven 1, P. Derlet 1 , A. Caro 2, D. Farkas3, M. Caturla4, and T. Díaz de la Rubia4 (1) Paul Scherrer Institute, CH-5232 Villigen-PSI, Switzerland (2) Centro Atmico Bariloche, 8400 Bariloche, Argentina (3) Dept. of MS & E, Virginia Polytechnic Inst. & State Univ. Blacksburg, VA (4) Lawrence Livermore National Laboratory, Livermore, CA Abstract Molecular dynamics computer simulation of nanocrystalline Ni and Cu with mean grain sizes ranging from 5 to 20 nm show that grain boundaries in nanocrystalline metals have structures similar to most grain boundaries found in conventional polycrystalline materials. Moreover, the excess enthalpy density in grain boundaries and triple junctions appears to be independent of grain in both, computer generated and experimental measured samples. Simulations of deformation under constant uniaxial stress demonstrate a change in deformation mechanism as function of grain size: at the smallest grain sizes all deformation is accommodated in the grain boundaries, at higher grain sizes, intragrain deformation is observed Introduction The reduction of the grain size down to the nm regime opened new avenues for research in several aspects of materials science, including mechanical properties. At the lower end of the grain size range obtainable nowadays, half of the atoms belong to, or are affected by, the interfaces. Grain boundaries are believed to play a predominant role in plastic deformation of such materials, although the details of how precisely deformation occurs are still uncertain. The relation between yield stress and grain size has been the subject of intensive research in recent years due to the complex behavior observed in nanophase materials. Most of the results confirm the validity of the classical Hall-Petch (H-P) relation down to grain sizes of the order of a few tens of nanometers, eventually with a different slope in the sub-micron range but keeping the classical exponent d–1/2. Overviews of experimental data are given in [1-3]. At smaller grain sizes (below 20 nm) the results are controversial: whereas some results indicate a yield stress independent of grain size, or even a reverse H-P relation, others confirm an increasing yield stress with decreasing grain size [4,5]. At these small grain sizes the difficulty in the sample preparation plays an increasing role. It has recently been shown that controversial results can in some cases be ascribed to sample imperfections that have particular influence depending on the measurement technique [6]. Samples obtained from compaction of nano-powder particularly suffer from grain boundary (GB) imperfections. For instance, a reverse HP relation is obtained in Cu nanophase synthesised by inert gas condensation, when yield stress is obtained from tensile experiments, whereas an increase in yield stress is obtained from hardness measurements and compressive stress-strain curves. Another uncertainty in the results on yield values is the measurement of the grain size and the
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