Deformation mechanisms of face-centered-cubic metal nanowires with twin boundaries

Abstract

This letter addresses the issue of deformation mechanisms and mechanical tensile behavior of the twinned metal nanowires using atomistic simulations. Free surfaces are always the preferential dislocation nucleation sites in the initial inelastic deformation stage, while with further plastic deformation, twin boundary interfaces will act as sources of dislocations with the assistance of the newly formed defects. The smaller the twin boundary spacing, the higher the yielding stresses of the twinned nanowires. Twin boundaries, which serve both as obstacles to dislocation motion and dislocation sources, can lead to hardening effects and contribute to the tensile ductility. This work illustrates that the mechanical properties of metal nanowires could be controlled by tailoring internal growth twin structures. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2721367͔Due to the unique mechanical, electrical, and optical properties, materials with nanometer-sized structure have attracted a great deal of interest in the past few decades [1][2][3] Many researchers have demonstrated, through both experiments and analysis, that the structure and properties of nanowires can be quite different from those of bulk materials due to the effects of larger free surfaces. [4][5][6][7][8][9][10][11][12][13][14] Simulations reported in the previous literatures are generally defect-free and the sizes of the modeled wires are usually smaller than 6 nm. [6][7][8][9][10][11][12][13][14] Meanwhile experimentally, twins, a class of planar defects, are observed most often in single-crystal metal ͑copper, silver, and gold͒ nanowires with a ͓111͔ growth orientation. 15,16 Because twin boundaries ͑TBs͒ in nanowires will strongly affect the physical properties of nanowires, it is of great significance to investigate the details of twin structures and their roles in metal nanowires. Until now, there are still some key issues about twin related deformation which are not quite clear and need fundamental understanding. For example, what is the role of TBs in mechanical deformation? Does it act as grain boundaries or surfaces? In our recent work, the mechanical behavior of fivefold twinned nanowires is studied. 17 We found that the strengthening mechanism in the fivefold twinned nanowires is due to the TBs as obstacles to the dislocation motion. In this letter, we address the effect of TBs on inelastic deformation of the twinned nanowires using atomistic simulations.In this work we focus on twinned Cu nanowires with ͗111͘ growth orientation and nearly square cross section as indicated from the experimental observations. 15,16 The initial configuration of the twinned nanowires is constructed by repeating ⌺3 coherent twins in the ͗111͘ axis orientation. 18 Four samples are prepared for the simulations. The first one consists of two twins, the second consists of four twins, and the third contains five twins. The twin boundary spacings ͑TBSs͒ of the first, second, and third wires are 14 nm ͑sample I͒, 7 nm ͑sample II͒, and 5 nm ͑sample III͒, respect...

Alternating starvation of dislocations during plastic yielding in metallic nanowires

“…More recently, a ''mechanical annealing" test was used to demonstrate that dislocations can be swept out of the samples through the progressive activation and exhaustion of dislocation sources [19]. These promising results raise an important question: is the size effect still operative when the size of the pillar is reduced to under 10 nanometers?Despite many efforts [6][7][8][9][10][11][12][13][14], a quantitative understanding of dislocation flow in nanoscale metals has remained elusive. In particular, the correlation of the dislocation flow with the stress-strain curve is of great interest, for the underlying mechanisms are typically represented by a mechanical response in macroscopic level experiments.…”

“…Due to the unique mechanical, thermal, electrical and optical properties, materials with nanometer-sized structure have attracted a great deal of interest as potential building blocks in nanoelectronic and nanoelectromechanical devices [1]. Many researchers have demonstrated, through both experiments and analysis, that the structure and properties of nanowires can be quite different from those of bulk materials due to the effect of the large surface to volume ratio [2][3][4][5][6][7][8][9][10][11][12][13][14].Recently, Uchic et al [15,16] and Greer et al [17,18] reported that the plastic deformation behavior of single-crystalline sub-micropillars is dependent on the size of the pillar, even without a deformation gradient. More recently, a ''mechanical annealing" test was used to demonstrate that dislocations can be swept out of the samples through the progressive activation and exhaustion of dislocation sources [19].…”

“…Despite many efforts [6][7][8][9][10][11][12][13][14], a quantitative understanding of dislocation flow in nanoscale metals has remained elusive. In particular, the correlation of the dislocation flow with the stress-strain curve is of great interest, for the underlying mechanisms are typically represented by a mechanical response in macroscopic level experiments.…”

“…Many researchers have demonstrated, through both experiments and analysis, that the structure and properties of nanowires can be quite different from those of bulk materials due to the effect of the large surface to volume ratio [2][3][4][5][6][7][8][9][10][11][12][13][14].…”

“…Starting from the equilibrium configuration of the nanowires, uniaxial compressive loading was applied under the simulated quasistatic conditions [7,11,14]. To increase the simulation efficiency, in the first five loading steps all atoms were first displaced with a prescribed uniform compressive strain increment of 1% in the length direction and a 0.2% strain increment was applied in the remaining steps.…”