本文以分子動力學模擬探討鎳奈米柱及鐵奈米柱在不同外加機械條件下的結構演化。鎳奈米柱及鐵奈米柱的尺寸均為直徑約為6.0 nm及長度約為12.0 nm。鎳奈米柱及鐵奈米柱皆分別經過拉伸、壓縮、非一致性拉伸及彎曲四種過程。所有的模擬過程均在定溫300 K下進行。
在鎳奈米柱受到四種不同外加機械條件下，內禀性疊差均先在鎳奈米柱中產生。{111}面的部分滑移導致內禀性疊差於鎳米柱形成。{111}面的部分滑移也導致之後局部的雙晶結構或錯位原子層在鎳米柱中形成。
當鐵奈米柱收到拉伸時，雙晶結構在鐵奈米柱中形成。隨著鐵奈米柱繼續被拉伸，雙晶結構慢慢被破壞，擬體心立方晶結構變成鐵奈米柱的主要結構。而鐵奈米柱在非一致性拉伸的過程中，經過降伏之後，擬體心立方晶即在鐵奈米柱中形成。局部的六方晶結構會在受到壓縮的鐵奈米柱內部形成。當鐵奈米柱受到彎曲，雙晶結構沒有在拉伸側形成，局部六方晶結構的形成在壓縮側並不顯著。

We investigated structural deformation of Ni nanopillar and Fe nanopillar under loading through molecular dynamics simulation. Ni nanopillar and Fe nanopillar have the consistent geometry with approximately 6.0 nm in diameter and 12.0 nm in length. The loading process includes tension, compression, non-uniform tension, and bending. All loading processes are performed under the constant temperature, 300 K.
Formation of intrinsic stacking fault is observed during the initial deformation of Ni nanopillar under all four loading processes. Existence of intrinsic stacking fault indicates partial slip of {111} plane in Ni nanopillar. Local twins or faulted layers form afterwards in Ni nanopillar under respective loading, which is also related with partial slip of {111} plane.
In the tensile-loaded Fe nanopillar, twinning is observed. With further tension, destruction of twin is accompanied by formation of local quasi body-centered cubic structure. In the non-uniformly tensile-loaded Fe nanopillar, local quasi body-centered cubic structure is the dominant structure after yielding. Local hexagonal closed-packed structure is observed in Fe nanopillar under compression. Twinning is not observed on the tensile side and formation of local hexagonal closed-packed structure is not obviously found on the compressive side of the bent Fe nanopillar, which may be associated with the applied condition of the bending.

1 Gall, K., Diao, J. K. & Dunn, M. L. The Strength of Gold Nanowires. Nano Lett. 4, 2431-2436 (2004).
2 Greer, J. R. & Nix, W. D. Nanoscale Gold Pillars Strengthened Through Dislocation Starvation. Phys. Rev. B 73, doi:245410 (2006).
3 McDowell, M. T., Leach, A. M. & Gall, K. Bending and Tensile Deformation of Metallic Nanowires. Modelling and Simulation in Materials Science and Engineering 16, doi:045003 (2008)
4 Huang, D., Zhang, Q. & Qiao, P. Z. Molecular Dynamics Evaluation of Strain Rate and Size Effects on Mechanical Properties of FCC Nickel Nanowires. Computational Materials Science 50, 903-910 (2011).
5 Diao, J. K., Gall, K., Dunn, M. L. & Zimmerman, J. A. Atomistic Simulations of the Yielding of Gold Nanowires. Acta Materialia 54, 643-653 (2006).
6 Rabkin, E., Nam, H. S. & Srolovitz, D. J. Atomistic Simulation of the Deformation of Gold Nanopillars. Acta Materialia 55, 2085-2099 (2007).
7 Lin, Y. C. & Pen, D. J. Analogous Mechanical Behaviors in (100) and (110) Directions of Cu Nanowires Under Tension and Compression at a High Strain Rate. Nanotechnology 18, doi:395705 (2007).
8 Koh, S. J. A. & Lee, H. P. Molecular Dynamics Simulation of Size and Strain Rate Dependent Mechanical Response of FCC Metallic Nanowires. Nanotechnology 17, 3451-3467 (2006).
9 Wen, Y. H., Zhu, Z. Z., Shao, G. F. & Zhu, R. Z. The Uniaxial Tensile Deformation of Ni Nanowire: Atomic-scale Computer Simulations. Physica E 27, 113-120 (2005).
10 Wu, H. A. Molecular Dynamics Study of the Mechanics of Metal Nanowires at Finite Temperature. Eur. J. Mech. A-Solids 25, 370-377 (2006).
11 Park, H. S., Gall, K. & Zimmerman, J. A. Deformation of FCC Nanowires by Twinning and Slip. J. Mech. Phys. Solids 54, 1862-1881 (2006).
12 Wen, Y. H., Zhang, Y., Wang, Q., Zheng, J. C. & Zhu, Z. Z. Orientation-dependent Mechanical Properties of Au Nanowires Under Uniaxial Loading. Computational Materials Science 48, 513-519 (2010).
13 Kondo, Y. & Takayanagi, K. Gold Nanobridge Stabilized by Surface Structure. Physical Review Letters 79, 3455-3458 (1997).
14 Toimil Molares, M. E., Balogh, A. G., Cornelius, T. W., Neumann, R. & Trautmann, C. Fragmentation of Nanowires Driven by Rayleigh Instability. Appl. Phys. Lett. 85, 5337-5339 (2004).
15 Dai, X. D., Kong, Y., Li, J. H. & Liu, B. X. Extended Finnis-Sinclair Potential for BCC and FCC Metals and Alloys. J. Phys.-Condes. Matter 18, 4527-4542 (2006).
16 Berendsen, H. J. C., Postma, J. P. M., Vangunsteren, W. F., Dinola, A. & Haak, J. R. Molecular Dynamics with Coupling to an External Bath. J. Chem. Phys. 81, 3684-3690 (1984).
17 Finnis, M. W. & Sinclair, J. E. A Simple Empirical N-body Potential for Transition Metals. Philisophical Magazine A 50, 45-55 (1984).
18 Kittel, C. Introduction to Solid State Physics. (Wiley, 1996).
19 Lide, D. R. Handbook of Chemistry and Physics. (CRC Press, 2000).
20 Zhou, M. A New Look at the Atomic Level Virial Stress: On Continuum-molecular System Equivalence. Proc. R. Soc. London Ser. A-Math. Phys. Eng. Sci. 459, 2347-2392 (2003).
21 Stukowski, A. Visualization and Analysis of Atomistic Simulation Data with OVITO-the Open Visualization Tool. Modelling and Simulation in Materials Science and Engineering 18, doi:015012 (2010).
22 Honeycutt, J. D. & Andersen, H. C. Molecular Dynamics Study of Melting and Freezing of Small Lennard-Jones Clusters. J. Phys. Chem. 91, 4950-4963 (1987).
23 Vitek, V. Thermally Activated Motion of Screw Dislocations in BCC Metals. Physica Status Solidi 18, 687-701 (1966).
24 Vitek, V. Intrinsic Stacking Faults in Body-Centred Cubic Crystals. Philosophical Magazine 18, 773-786 (1968).
25 Rice, J. R. Dislocation Nucleation from a Crack Tip: An Analysis Based on the Peierls Concept. J. Mech. Phys. Solids 40, 239-271 (1992).
26 Clatterbuck, D. M., Chrzan, D. C. & Morris, J. W. The Ideal Strength of Iron in Tension and Shear. Acta Materialia 51, 2271-2283 (2003).
27 Young, D. A. Phase Diagrams of the Elements. (University of California Press, 1991).
28 Kadau, K., Germann, T. C., Lomdahl, P. S. & Holian, B. L. Microscopic View of Structural Phase Transitions Induced by Shock Waves. Science 296, 1681-1684 (2002).
29 Kalantar, D. H., Belak, J. F., Collins, G. W., Colvin, J. D., Davies H. M., Eggert J. H., Germann, T. C., Hawreliak, J., Holian, B. L., Kadau, K., Lomdahl, P. S., Lorenzana, H. E., Meyers, M. A., Rosolankova, K., Schneider, M. S., Sheppard, J., Stölken, J. S. & Wark, J. S. Direct Observation of the α-ε Transition in Shock-compressed Iron via Nanosecond X-ray Diffraction. Physical Review Letters 95, doi:075502 (2005).
30 Wang, F. M. & Ingalls, R. Iron BCC-HCP Transition: Local Structure from X-ray-absorption Fine Structure. Phys. Rev. B 57, 5647-5654 (1998).
31 Ladd, A. J. C., Moran, B. & Hoover, W. G. Lattice Thermal Conductivity: A Comparison of Molecular Dynamics and Anharmonic Lattice Dynamics. Phys. Rev. B 34, 5058-5064 (1986).
32 Volz, S. G. & Chen, G. Molecular Dynamics Simulation of Thermal Conductivity of Silicon Nanowires. Appl. Phys. Lett. 75, 2056-2058 (1999).