The Coupled Effects of Geometry and Surface Orientation on the Mechanical Properties of Metal Nanowires

C. Ji and H.S. Park
Nanotechnology 2007; 18:305704

Abstract

We have performed atomistic simulations of the tensile loading of <100> and <110> copper nanowires to investigate the coupled effects of geometry and surface orientation on their mechanical behavior and properties. By varying the nanowire cross section from square to rectangular, nanowires with dominant surface facets are created that exhibit distinct mechanical properties due to the different inelastic deformation mechanisms that are activated. In particular, we find that non-square nanowires generally exhibit lower yield stresses and strains, lower toughness, elevated fracture strains, and a propensity to deform via twinning; we quantify the links between the observed deformation mechanisms due to non-square cross section and the resulting mechanical properties, while illustrating that geometry can be utilized to tailor the mechanical properties of nanowires.

This paper is available in PDF form .

Molecular Dynamics Simulations of Stretched Gold Nanowires - The Relative Utility of Different Semiempirical Potentials

Q. Pu, Y. Leng, L. Tsetseris, H.S. Park, S.T. Pantelides and P.T. Cummings
Journal of Chemical Physics 2007; 126:144707

Abstract

The mechanical elongation of a finite gold nanowire has been studied by molecular dynamics (MD) simulations using different semiempirical potentials for transition metals. These potentials have been widely used to study the mechanical properties of finite metal clusters. Combining with density functional theory (DFT) calculations along several atomic-configuration trajectories predicted by different semiempirical potentials, we conclude that the second-moment approximation of the tight-binding potential (TB-SMA) is the most suitable one to describe the energetics of finite Au clusters. We find that for the selected geometries of Au wires studied in this work, the ductile elongation of Au nanowires along [001] direction predicted by the TB-SMA potential does not depend on temperature in the range of 0.01~298 K. The elongation leads to the formation of monatomic chains, as has been observed experimentally. The calculated force-versus-elongation curve is remarkably consistent with available experimental data.

This paper is available in PDF form .

Characterizing the Elasticity of Hollow Metal Nanowires

C. Ji and H.S. Park
Nanotechnology 2007; 18:115707

Abstract

We have performed atomistic simulations on solid and hollow copper nanowires to quantify the elastic properties of the hollow nanowires (nanoboxes). We analyze variations in the modulus, yield stress and strain for <100> and <110> nanoboxes by varying the amount of bulk material that is removed to create the nanoboxes. We find that while <100> nanoboxes show no improvement in elastic properties as compared to solid <100> nanowires, <110> nanoboxes can show enhanced elastic properties as compared to solid <110> nanowires. The simulations reveal that the elastic properties of the nanoboxes are strongly dependent on the relative strength of the bulk material that has been removed, as well as the the total surface area of the nanoboxes, and indicate the potential of ultralight, high-strength nanomaterials such as nanoboxes.

This paper is available in PDF form .

Surface Composition Effects on Martensitic Phase Transformations in Nickel Aluminum Nanowires

H.S. Park and V. Laohom
Philosophical Magazine 2007; 87:2159-2168.
(Invited paper: Special Issue on Nanowires).

Abstract

Atomistic simulations are utilized to quantify the effects of surface composition on stress-induced B2 to body-centered tetragonal (BCT) martensitic phase transformations in intermetallic nickel aluminum (NiAl) nanowires. The simulations show that the phase transformation is observed in all considered cases, regardless of the material composition of the transverse {100} surfaces of the initially B2 wires. The results indicate that, for <100> oriented B2 wires with {100} transverse surfaces, the {100} orientation and not the material composition of the {100} surfaces is the dominant factor in controlling the ability of NiAl alloys to undergo martensitic phase transformations at nanometer scales.

This paper is available in PDF form .

The Effect of Defects on the Mechanical Behavior of Silver Shape Memory Nanowires

C. Ji and H.S. Park
Journal of Computational and Theoretical Nanoscience 2007; 4:578-587.

Abstract

We present atomistic simulations of the uniaxial tensile deformation of silver shape memory nanowires to investigate the effects of initial defects on the resulting thermomechanical behavior. In particular, the focus of the work is on investigating the unique atomistic deformation mechanisms that are observed during the tensile loading as a result of the initial defects, while correlating that behavior to the measured mechanical properties of the shape memory nanowires. In particular, wires with initial defects show a non-constant stress state during the <110>/{111} to <100>/{100} reorientation due to the presence of multiple propagating twin boundaries, as well as reductions in transformation stresses and strains due to the presence of the initial defects. Under most circumstances, the wires with initial defects still tend to exhibit complete reversibility between the <110>/{111} and <100>/{100} orientations, and thus the shape memory effect. Comparisons are made to defect-free shape memory nanowires to illustrate the relative mechanical performance of each structure.

This paper is available in PDF form .

Geometric Effects on the Inelastic Deformation of Metal Nanowires

C. Ji and H.S. Park
Applied Physics Letters 2006; 89:181916
(Also selected for publication in the Virtual Journal of Nanoscale Science and Technology, Nov. 13, 2006.)

Abstract

This letter addresses the direct effect that geometry has in controlling the mechanisms of inelastic deformation in metal nanowires. By performing atomistic simulations of the tensile deformation of <100>{100} hollow copper nanowires (nanoboxes), we find that the nanoboxes deform in an unexpected twinning-dominated mode; the non-square wall geometries of the nanoboxes biases the deformation by allowing the larger transverse {100} surfaces to reduce their area through twinning by reorienting to a lower energy {111} surface. Additional analyses on solid nanowires with non-square cross sections confirm that geometry can be utilized to engineer the mechanical behavior and properties of nanomaterials.

This paper is available in PDF form .

Deformation of FCC Nanowires by Twinning and Slip

H.S. Park, K. Gall and J.A. Zimmerman
Journal of the Mechanics and Physics of Solids 2006; 54 (9):1862-1881.

Abstract

We present atomistic simulations of the tensile and compressive loading of single crystal FCC nanowires with <100> and <110> orientations to study the propensity of the nanowires to deform via twinning or slip. By studying the deformation characteristics of three FCC materials with disparate stacking fault energies (gold, copper and nickel), we find that the deformation mechanisms in the nanowires are a function of the intrinsic material properties, applied stress state, axial crystallographic orientation and exposed transverse surfaces. The key finding of this work is the first order effect that side surface orientation has on the operant mode of inelastic deformation in both <100> and <110> nanowires. Comparisons to expected deformation modes, as calculated using crystallographic Schmid factors for tension and compression, are provided to illustrate how transverse surface orientations can directly alter the deformation mechanisms in materials with nanometer scale dimensions.

This paper is available in PDF form .

On the Thermomechanical Deformation of Silver Shape Memory Nanowires

H.S. Park and C. Ji
Acta Materialia 2006; 54 (10): 2645-2654.

Abstract

We present an analysis of the uniaxial thermomechanical deformation of single crystal silver shape memory nanowires using atomistic simulations. We first demonstrate that silver nanowires can show both shape memory and pseudoelastic behavior, then perform uniaxial tensile loading of the shape memory nanowires at various deformation temperatures, strain rates and heat transfer conditions. The simulations show that the resulting mechanical response of the shape memory nanowires depends strongly upon the temperature during deformation, and can be fundamentally different from that observed in bulk, polycrystalline shape memory alloys. The energy and temperature signatures of uniaxially loaded silver shape memory nanowires are correlated to the observed nanowire deformation, and are further discussed in comparison to bulk, polycrystalline shape memory alloy behavior.

This paper is available in PDF form .

Stress-Induced Martensitic Phase Transformation in Intermetallic Nickel Aluminum Nanowires

H.S. Park
Nano Letters 2006; 6 (5): 958-962.

Abstract

Atomistic simulations are utilized to demonstrate a stress-induced martensitic phase transformation in intermetallic nickel aluminum (NiAl) nanowires. The martensitic phase transformation occurs by the propagation and annihilation of {101} twinning planes, and transforms the initially B2 NiAl nanowires to a body centered tetragonal (BCT) phase. The instability of the resulting BCT phase allows pseudoelastic recovery of inelastic strains on the order of 40 percent at all deformation temperatures.

This paper is available in PDF form .

Stable Nanobridge Formation in <110> Gold Nanowires under Tensile Deformation

H.S. Park and J.A. Zimmerman
Scripta Materialia 2006; 54 (6): 1127-1132.

Abstract

We present atomistic simulations of <110> oriented gold nanowires under tensile deformation. We find that <110> gold nanowires tend to form elongated, stable nanobridges upon necking, which is in agreement with previous experimental observations. In addition, the simulations reveal that the formation of a high strength multishell lattice structure during the plastic deformation of the <110> wires may account for the stability of the elongated nanobridges observed experimentally.

This paper is available in PDF form .

Shape Memory and Pseudoelasticity in Metal Nanowires

H.S. Park, K. Gall and J.A. Zimmerman
Physical Review Letters 2005; 95:255504.
(Also selected for publication in the Virtual Journal of Nanoscale Science and Technology, Dec. 25, 2005.)

Abstract

Structural reorientations in metallic fcc nanowires are controlled by a combination of size, thermal energy and the type of defects formed during inelastic deformation. By utilizing atomistic simulations, we show that certain fcc nanowires can exhibit both shape memory and pseudoelastic behavior. We also show that the formation of reversible defect free twins, a process related to the material stacking fault energy, nanometer size scale and surface stresses is the mechanism that controls the ability of fcc nanowires of different materials to show a reversible transition between two crystal orientations during loading and thus shape memory and pseudoelasticity.

This paper is available in PDF form .

Modeling Inelasticity and Failure in Gold Nanowires

H.S. Park and J.A. Zimmerman
Physical Review B 2005; 72:054106.

Abstract

We present numerical simulations of gold nanowires under tensile loading at various strain rates and wire sizes at room temperature. The simulations were performed using molecular dynamics modeling the gold nanowires using various forms of the embedded atom method (EAM), and concentrated on investigating the yield and fracture properties of the nanowires. It is clearly demonstrated that the accurate modeling of stacking fault and surface energies is critical in capturing the fundamental deformation behavior of gold nanowires. By doing so, phenomena which have been observed both experimentally and numerically in first principles calculations such as the formation of atom-thick chains (ATC) prior to fracture, zigzag, helical rotational motion of atoms within the ATC, structural reorientation of the ATC to a hexagonal crystal structure and (111) faceting of the nanowire in the yielded neck region by the ATC are accurately captured.

This paper is available in PDF form .