Fivefold Twins Research Articles

Formation of quasi-icosahedral structures with multi-conjoint fivefold deformation twins in fivefold twinned metallic nanowires

We show by molecular dynamics simulations that symmetrical quasi-icosahedral structures can be formed in fivefold twinned metallic nanowires (Cu, Au, and Ag) under dynamic tensile loading. The quasi-icosahedral structure, different from the icosahedral nanoclusters found in the past, consists of a twisted original fivefold twinned axis and ten secondary fivefold deformation twins, with five preexisting prismatic and fifteen tetrahedral subunits joined adjacently. Formation of these structures is observed in the necking region during the plastic deformation with successive twinning processes and is found to be independent on the cross-sectional shape as well as the tensile strain rate of the nanowires.

Surface and interface controlled yielding and plasticity in fivefold twinned Ag nanowires

Molecular dynamics simulations are used to investigate the influences of pre-existing microstructural defects on the strength and deformation mechanisms in Ag nanowires under both uniaxial deformation and nanoindentation. In particular, the synergistic effects from both internal and external structural defects, including twin boundaries, surface facets, and a special surface groove, are studied. It is found that the yielding modes vary among Ag nanowires when different microstructures are present. Furthermore, while fivefold twin boundaries are found to cause significant strain hardening under nanoindentation, they can either decrease or increase the initial yield stress of Ag nanowires under uniaxial deformation. The surface groove, in addition, will result in damping behavior of the Ag nanowire that leads to peculiar oscillating load–displacement responses under nanoindentation.

Five-fold twin and surface groove-induced abnormal size- and temperature-dependent yielding in Ag nanowires

Molecular dynamics simulations are used to investigate the influence of pre-existing five-fold twins and surface grooves on the yielding of Ag nanowires under uniaxial deformation. While a five-fold twin together with a surface groove generally lead to strengthening in Ag nanowires under tension, abnormal weakening effects are observed under compression at low temperatures when the nanowire diameter is increased. The peculiar size and temperature effects can be attributed to the toggling of dislocation nucleation between various preferential sites under different loading conditions.

Size dependence and phase transition during melting of fcc-Fe nanoparticles: A molecular dynamics simulation

Continuous melting and cooling of isolated fcc-Fe nanoparticles with 59–9577 atoms are studied by Molecular Dynamics (MD) simulation with Sutton–Chen potential. An energy minimization process was employed to obtain the stable solid structure for simulation of melting. The energy-minimized nanoparticles show lower potential energy and radius compared with the counterparts without energy minimizing. The size dependence of melting point shows perfect linear variation with N−1/3 for particles above a limit of 113 atoms. The bulk melting temperature of 1833.3K, which is close to the experimental data (1811K for bcc and 1800.8K for fcc), has been predicted by a linear relationship. Two different inner structures, including five-fold twinning and lamellar structures, have been found to be the initial stable configurations prior to melting, and both surface premelting and internal defects were verified as the origins for melting behavior.

The crystal structures of sintered copper nanoparticles: A molecular dynamics study

The coalescence of nano-crystals during sintering is often found to result in interesting crystalline structures such as multi-fold twins, and yet the plasticity mechanism accompanying their formation is unclear. In this work, the sintering behavior of two unsupported copper nanoparticles initially at room temperature is investigated by molecular dynamics simulations under the constant-energy ensemble. The results reveal that once the two nanoparticles are brought into contact, they often go through drastic structural changes with the inter-particle grain boundary quickly eliminated, and single- and multi-fold twinning occurs frequently in the coalesced product. Whereas the formation of single twins is found to be via the more usual mechanism of emission of Shockley partials on {111} planes, the formation of fivefold twins, however, takes place via a novel dislocation-free mechanism involving a series of shear and rigid-body rotation processes caused by elastic waves with amplitudes not corresponding to any allowable Burgers vector in the fcc lattice. Such a lattice-wave, dislocation-free twinning mechanism has never been reported before.

Structural evolution of five-fold twins during the solidification of Fe5601 nanoparticle: a molecular dynamics simulation

In the current study, we provide a structural evolution process of isolated Fe nanoparticle with 5601 atoms during solidification. Five-fold twinned structure has been found in the final configuration of the nanoparticle. Furthermore, detailed formation process has been visualized and carefully discussed. During the formation of five-fold twinned structure, two main stages have been identified, including i) liquid-solid phase transition at solidification point, in which the main part of the five-fold twinned structure formed, and ii) structural relaxation, in which twinning boundaries and five-fold axis were transformed to eliminate the dislocation between two adjacent five-fold twins.

单晶铜纳米线弯曲、扭转的变形机制的分子动力学研究

Molecular dynamics simulations are used to investigate the mechanical behaviors of <001>/{100} and <110>/{111} single-crystalline Cu nanowires under bending and torsion. In the numerical simulations of <001>/{100} Cu nanowires subjected to bending, some fivefold deformation twins are observed at large bending angles. It is found that a reciprocal phase transformation from atoms of other 12-coordinate lattice to hcp lattice plays an important role in forming the fivefold deformation twins. Therefore, the formation process is distinct from that reported by Cao and Wei [Appl. Phys. Lett. 89, 041919 (2006)] in nanocrystalline Cu systems. However, for the <110>/{111} counterparts under bending, fivefold deformation twin can not be observed during the whole deformation process. In addition, we reveal that the emission of full dislocations from the two ends of the wire is the major deformation mechanism for <001>/{100} Cu nanowires to torsion, while for <110>/{111} Cu samples, the nucleation of full dislocations from the side surfaces is responsible for the torsion plastic deformation.

TEM preparation method for site- and orientation-specific sectioning of individual anisotropic nanoparticles based on shadow-FIB geometry

Abstract For the comprehensive characterization of nanoparticles cross-sectional investigation on the atomic scale by analytical and high-resolution transmission electron microscopy (TEM) is indispensable. Cross-sectioning is especially important for anisotropic nanoparticles to gain information on structure and chemistry along all important projections. We present a focused ion beam (FIB) method for site- and orientation-specific cross-sectioning of arbitrary nanoparticles that are dispersed on a substrate. By adopting a shadow geometry originally developed for thin sensitive films' protection of the specimen by a platinum layer is avoided. This enables simultaneous observation (from the front side) by the electron beam and ion-beam sectioning (from the back side of the supporting substrate) of individually selected particles with excellent accuracy on the nanometer scale. The feasibility and general applicability of the method is demonstrated by site-specific sectioning and cross-section HRTEM investigation of two types of anisotropic nanostructures: silver nanorods with five-fold twin structure and Janus-type silver patchy particles.

Role of Five-fold Twin Boundary on the Enhanced Mechanical Properties of fcc Fe Nanowires

The role of 5-fold twin boundary on the structural and mechanical properties of fcc Fe nanowire under tension is explored by classical molecular dynamics. Twin-stabilized fcc nanowire with various diameters (6-24 nm) are examined by tension tests at several temperatures ranging from 0.01 to 1100 K. Significant increase in the Young's modulus of the smaller nanowires is revealed to originate from the central area of quinquefoliolate-like stress-distribution over the 5-fold twin, rather than from the surface tension that is often considered as the main source of such size-effects found in nanostructures. Because of the excess compressive stress caused by crossing twin-boundaries, the atoms in the center behave stiffer than those in bulk and even expand laterally under axial tension, providing locally negative Poisson's ratio. The yield strength of nanowire is also enhanced by the twin boundary that suppresses dislocation nucleation within a fcc twin-domain; therefore, the plasticity of nanowire is initiated by strain-induced fcc→bcc phase transformation that destroys the twin structure. After the yield, the nucleated bcc phase immediately spreads to the entire area, and forms a multigrain structure to realize ductile deformation followed by necking. As temperature elevated close to the critical temperature between bcc and fcc phases, the increased stability of fcc phase competes with the phase transformation under tension, and hence dislocation nucleations in fcc phase are observed exclusively at the highest temperature in our study.

Atomistic exploration of deformation properties of copper nanowires with pre-existing defects

Free to read full-text Based on the embedded atom method (EAM) and molecular dynamics (MD) method, in this paper, the tensile deformation properties of Cu nanowires (NWs) with different pre-existing defects, including single surface defects, surface bi-defects and single internal defects, are systematically studied. In-depth deformation mechanisms of NWs with pre-existing defects are also explored. It is found that Young's modulus is insensitive to different pre-existing defects, but yield strength shows an obvious decrease. Defects are observed influencing greatly on NWs' tensile deformation mechanisms, and playing a role of dislocation sources. Besides of the traditional deformation process dominated by the nucleation and propagation of partial dislocations, the generations of twins, grain boundaries, fivefold deformation twins, hexagonal close-packed (HCP) structure and phase transformation from face-centred cubic (FCC) structure to HCP structure have been triggered by pre-existing defects. It is found that surface defect intends to induce larger influence to yield strength than internal defect. Most importantly, the defect that lies on slip planes exerts larger influence than other defects. As expected, it is also found that the more or longer of the defect, the bigger influence will be induced.

Precipitation of Icosahedral Qusicrystal Phase in Mg-Zn-Y(ZW61) alloy

Precipitation of the ordered icosahedral quasicrystal in Mg-6wt%Zn-1wt%Y alloy has been characterized by transmission electron microscopy observations. The lamellar-type icosahedral qusicrystal phases (I-phase) with the face-centered icosahedral (FCI) structure are observed in alloy after solution treatment at <TEX>$550^{\circ}C$</TEX>. In the alloy annealed at <TEX>$400^{\circ}C$</TEX>, polygon-shaped I-phases are observed in the <TEX>${\alpha}$</TEX>-Mg matrix. The interfaces of the I-phase with the matrix are facetted and the facets are on five-fold and two- fold plane of the I-phase. The orientation relationship of the I-phase with the matrix is determined to be <TEX>$[I5]_I//[001]_{Mg}$</TEX>, <TEX>$(2f)_I//(2\overline{1}0)_{Mg}$</TEX> and <TEX>$[I2]_I//[311]_{Mg}$</TEX>, <TEX>$(5f)_I//(0\overline{1}1)_{Mg}$</TEX>. The icosahedral grains are occasionally found to be twinned with one of the five-fold axis as the twin axis. The twin boundaries appear to be fairly straight and perpendicular to the fivefold twin axis. The icosahedral twin can be expressed as a rotation of <TEX>$63.4^{\circ}$</TEX> or <TEX>$116.62^{\circ}$</TEX> around two fold zone axis.

Homogeneous ice nucleation from supercooled water

Homogeneous ice nucleation from supercooled water was studied in the temperature range of 220-240 K through combining the forward flux sampling method (Allen et al., J. Chem. Phys., 2006, 124, 024102) with molecular dynamics simulations (FFS/MD), based on a recently developed coarse-grained water model (mW) (Molinero et al., J. Phys. Chem. B, 2009, 113, 4008). The calculated ice nucleation rates display a strong temperature dependence, ranging from 2.148 ± 0.635 × 10(25) m(-3) s(-1) at 220 K to 1.672 ± 0.970 × 10(-7) m(-3) s(-1) at 240 K. These rates can be fitted according to the classical nucleation theory, yielding an estimate of the effective ice-water interface energy γ(ls) of 31.01 ± 0.21 mJ m(-2) for the mW water model. Compared to experiments, our calculation underestimates the homogeneous ice nucleation rate by a few orders of magnitude. Possible reasons for the discrepancy are discussed. The nucleating ice embryo contains both cubic ice Ic and hexagonal ice Ih, with the fraction of each structure being roughly 50% when the critical size is reached. In particular, a novel defect structure containing nearly five-fold twin boundaries is identified in the ice clusters formed during nucleation. The way such defect structure is formed is found to be different from mechanisms proposed for the formation of the same defect in metallic nanoparticles and thin film. The quasi five-fold twin boundary structure found here is expected to occur in the crystallization of a wide range of materials with the diamond cubic structure, including ice.

Fivefold Twin Center in CVD Diamond Film

The microstructure and crystallographic characteristics of diamond film deposited with the chemical vapor deposition method was investigated using transmission electron microscopy (TEM). It is found that fivefold twin center does not always behave as growth nuclei of diamond grain. For those behaving as growth nuclei, there are a large amount of defects in every block of fivefold twin. However, for those not behaving as growth nuclei, two blocks of fivefold twin contain large quantities of defects, the other three are nearly clear. High resolution TEM analysis shows stacking faults coexist with microtwins in the two vast-defects-contained blocks. The effect of the mismatch angle (7.5°) in fivefold twins on microtwins, and the coexistence mechanism of both stacking faults and microtwins are also discussed.

Formation of fivefold deformation twins in an ultrafine-grained copper alloy processed by high-pressure torsion

Fivefold deformation twins (DTs) are formed in an ultrafine grain, with a size of ∼100 nm, of a Cu–16 at.% Al alloy processed by high-pressure torsion. Formation occurs through the sequential emission of partial dislocations from grain boundaries and other multiple twin boundaries. These observations confirm the emission of partial dislocations from multiple-fold nodes which can be attributed to increased twinnability via the introduction of Al atoms which lower the driving force for partial emission.

Loading path effect on the mechanical behaviour and fivefold twinning of copper nanowires

The effect of loading path on the mechanical behaviour of single crystalline copper nanowires is investigated with molecular dynamics simulations. Different loading conditions including pre-tensile torsion and pre-torsional tension at different temperatures are taken into consideration. It is found that elastic pre-loading conditions can induce a distinct weakening on the resistance against plastic deformation under later applied loads. Meanwhile, coupled thermal and pre-loading effect can also facilitate the transformation from elasticity to plasticity. Formations of fivefold twins are observed in copper nanowires subjected to the loading path with tension after pre-torsion. These fivefold twins all form at the necking stage before fracture, and are found to be pre-torsion- and size-dependent but insensitive to the change in temperature and cross-sectional shape. The results reported here indicate that the loading path effect on the mechanical behaviour plays an important role in the formation of some special microstructures such as multiple twins in metallic nanowires.

Investigations on the deformation behavior of polycrystalline Cu nanowires and some factors affecting the modulus and yield strength

This paper consists of two parts. In the first part, the deformation behavior of polycrystalline Cu nanowires under tension, bending and torsion is studied by using molecular dynamics simulations. The results show that both free surfaces and grain boundaries can control the plastic deformation. In detail, for polycrystalline Cu nanowires under tension, the stress-assisted grain growth caused by atomic diffusion and grain boundary migration is found, which is thought to be the reason for necking. Then under the bending effect, full dislocations and deformation twins of polycrystalline Cu nanowires are found in the grains. Moreover, the twin boundaries act as obstacles to dislocation motion. Finally, full dislocations and fivefold deformation twins are detected in polycrystalline Cu nanowires in the torsional state. These phenomena are in good agreement with the experimental observations of Liao et al (2005 Appl. Phys. Lett. 86 103112). The second part investigates the effects of sample shape and crystallographic structure on the modulus and yield strength under tension, bending and torsion. The results demonstrate that the modulus (in particular the bending and torsion modulus) can be significantly influenced by both the effects; however, remarkable difference in yield strength can merely be caused by different crystallographic structures (here, different crystallographic structures refer to polycrystalline and single-crystalline structures).

The enhanced elastic modulus of nanowires associated with multitwins

In this study, we performed molecular static simulations to investigate the enhanced elastic modulus of multiply twinned nanowires (MTNs) that had fivefold twin planes. MTNs have the same size-dependent elastic behavior as twin-free nanowires (TFNs), however, the simulation results reveal stiffer elastic behavior of MTNs when compared with that of TFNs of the same size. This is because atomic rearrangement occurs inside MTNs through the intrinsic elastic deformation as a result of their unique geometry. These results are supported by the elastic-modulus distribution on the cross-sectional area—the elastic modulus increases close to the central part of MTNs—and the intrinsically strained structure that has a highly compressed central portion in the radial direction.

Quasicontinuum study on formation of fivefold deformation twin in nanocrystalline aluminum

The formation of a fivefold deformation twin in nanocrystalline Al is studied by the quasicontinuum method. The study focuses on the microstructural effect on the formation process. A particular low-index grain boundary, which has been investigated in a bicrystal model, is observed in the sample. The dislocation reflection and inhomogeneous stress distribution caused by this grain boundary are discussed. The importance of this microstructure to the formation of a fivefold deformation twin is manifested by the results.

Fivefold annealing twin in nanocrystalline Cu

Fivefold annealing twin has been observed in nanocrystalline Cu at zero external stress. The microstructure of the multifold twins at various annealing temperatures has been examined by transmission electron microscopic in detail. Contrary to the well known sequential twinning mechanism, coherent twin boundaries formed via migration of grain boundary segment were proposed to be the dominant mechanism of fivefold annealing twin, which associated with grain growth during annealing. Moreover, the possibilities of the proposed mechanism may operate in forming fivefold deformation twin were discussed.

Fivefold twin formation during annealing of nanocrystalline Cu

Fil: Bringa, Eduardo Marcial. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentina. Lawrence Livermore National Laboratory; Estados Unidos. Consejo Nacional de Investigaciones Cientificas y Tecnicas; Argentina