Cooperative Movement Of Atoms Research Articles

Defect topology and annihilation by cooperative movement of atoms in neutron-irradiated graphite

Graphite has been used as neutron moderator or reflector in many nuclear reactors. The irradiation of graphite in a nuclear reactor results in a complex population of defects. Heating of the irradiated graphite at high temperatures results in annihilation of the defects with release of an unusually large energy, called the Wigner energy. From various experiments on highly irradiated graphite samples from CIRUS reactor at Trombay and ab-initio simulations, we have for the first time identified various 2-, 3- and 4-coordinated topological structures in defected graphite, and provided microscopic mechanism of defect annihilation on heating and release of the Wigner energy. The annihilation process involves cascading cooperative movement of atoms in two steps involving an intermediate structure. Our work provides new insights in understanding of the defect topologies and annihilation in graphite which is of considerable importance to wider areas of graphitic materials including graphene and carbon nanotubes.

Effect of amorphous phase on the plastic deformation mechanism of Mg: A molecular dynamics study

Improving the plasticity of Mg alloys is an important frontier topic in the field of mechanics and materials. The influence of introduction of amorphous phase, grain size and temperature on the deformation mechanisms of nano-polycrystal Mg under tensile loading here is studied by molecular dynamics simulation method. The results indicate that the introduction of amorphous grain can improve the plasticity of the nano-polycrystal Mg due to cooperative movement of atoms in crystalline and amorphous phases. With the decrease of grain size, the plastic deformation of crystal phase in crystal-amorphous Mg-MgAl nanocomposites change from the nucleation of dislocations and growth of tensile twins to the grain boundaries glide and grains rotation, and the plastic deformation mode of amorphous phase change from the shear band dominated deformation to the homogeneous plastic deformation. The results also show that the amorphous grain plays a more and more important role during plastic deformation of Mg-MgAl nanocomposites as grain size decreases, and the deformation behavior of nanocomposites obviously depends on temperature. In addition, some quantified analysis about the deformation mechanism of nanocomposites is also presented.

Two-domain structure and dynamics heterogeneity in a liquid SiO2

Molecular dynamics simulation is employed to investigate the diffusion mechanism, dynamics and structure heterogeneity in a liquid SiO2 at ambient pressure and different temperatures. To explore these phenomena we propose the link-cluster and init-bond functions. The simulation shows that during a moderately long time the liquid has a two-domain structure that consists of separate mobile and immobile domains. These types of domains differ strongly in the rate of atomic rearrangement and fraction of defective atoms. The simulation also reveals a strong correlation between mobility of atoms and init-bond function which evidences the bond-breaking mechanism and that the reactions are non-uniformly distributed in the space. The liquid exhibits dynamics heterogeneity which is accompanied with the structure heterogeneity and cooperative movement of atoms via like-molecules SixOy.

Rotator side chains trigger cooperative transition for shape and function memory effect in organic semiconductors

Martensitic transition is a solid-state phase transition involving cooperative movement of atoms, mostly studied in metallurgy. The main characteristics are low transition barrier, ultrafast kinetics, and structural reversibility. They are rarely observed in molecular crystals, and hence the origin and mechanism are largely unexplored. Here we report the discovery of martensitic transition in single crystals of two different organic semiconductors. In situ microscopy, single-crystal X-ray diffraction, Raman and nuclear magnetic resonance spectroscopy, and molecular simulations combined indicate that the rotating bulky side chains trigger cooperative transition. Cooperativity enables shape memory effect in single crystals and function memory effect in thin film transistors. We establish a molecular design rule to trigger martensitic transition in organic semiconductors, showing promise for designing next-generation smart multifunctional materials.

Origins and dissociation of pyramidal dislocations in magnesium and its alloys

Alloying magnesium (Mg) with rare earth elements such as yttrium (Y) has been reported to activate the pyramidal <c + a> slip systems and improve the plasticity of Mg at room temperature. However, the origins of such dislocations and their dissociation mechanisms remain poorly understood. Here, we systematically investigate these mechanisms using dispersion-inclusive density-functional theory, in combination with molecular dynamics simulations. We find that <c + a> dislocations form more readily on the pyramidal I plane than on the pyramidal II plane in Mg. The addition of Y atoms in Mg facilitates the dissociation of <c + a> dislocations on pyramidal II, leading to the easier formation of the pyramidal II than pyramidal I in Mg-Y alloy. Importantly, in pyramidal II slip plane, a flat potential-energy surface (PES) exists around the position of stable stacking fault energy (SFE), which allows cooperative movement of atoms within the slip plane. Alloying Mg with Y atoms increases the range of the PES, and ultimately promotes different sliding pathways in the Mg-Y alloy. These findings are consistent with experimentally observed activation of the pyramidal II <c + a> slip system in Mg-Y alloys, and provide important insight into the relationship between dislocation structure and macroscopic enhancement of plasticity.

The Role of Twinned and Detwinned Structures on Memory Behaviour of Shape Memory Alloys

Shape memory alloys have a peculiar property to return to a previously defined shape or dimension when they are subjected to variation of temperature. Shape memory effect is facilitated by martensitic transformation governed by changes in the crystalline structure of the material. Martensitic transformations are first order lattice-distorting phase transformations and occur with the cooperative movement of atoms by means of lattice invariant shears in the materials on cooling from high temperature parent phase region. The material cycles between the deformed and original shapes on cooling and heating in reversible shape memory effect. Thermal induced martensite occurs as twinned martensite, and the twinned martensite structures turn into detwinned structures by deforming the material in the martensitic condition. Deformation of shape memory alloys in martensitic state proceeds through a martensite variant reorientation. The deformed material recovers the original shape on first heating over the austenite finish temperature in reversible and irreversible shape memory cases. Meanwhile, the parent phase structure returns to the twinned structure in irreversible shape memory effect on cooling below to martensite finish temperature and to the detwinned structure in reversible shape memory effect. Therefore, the twinning and detwinning processes have great importance in the shape memory behaviour of the materials. Copper based alloys exhibit this property in metastable β-phase region, which has bcc-based structures at high temperature parent phase field, and these structures martensitically turn into layered complex structures with lattice twinning following two ordered reactions on cooling.

Phase Transitions and Elementary Processes in Shape Memory Alloys

Shape memory effect is a peculiar property exhibited by certain alloy systems, and shape memory alloys are recognized to be smart materials. These alloys have important ability to recover the original shape of material after deformation, and they are used as shape memory elements in devices due to this property. The shape memory effect is facilitated by a displacive transformation known as martensitic transformation. Shape memory effect refers to the shape recovery of materials resulting from martensite to austenite transformation when heated above reverse transformation temperature after deforming in the martensitic phase. These alloys also cycle between two certain shapes with changing temperature.Martensitic transformations occur with cooperative movement of atoms by means of lattice invariant shears on a {110} - type plane of austenite matrix which is basal plane of martensite.Copper based alloys exhibit this property in metastable β-phase field. High temperature β-phase bcc-structures martensiticaly undergo the non-conventional structures following two ordered reactions on cooling, and structural changes in nanoscale level govern this transition cooling. Atomic movements are also confined to interatomic lengths due to the diffusionless character of martensitic transformation.

Brittle fracture in structural steels: perspectives at different size-scales.

This paper describes characteristics of transgranular cleavage fracture in structural steel, viewed at different size-scales. Initially, consideration is given to structures and the service duty to which they are exposed at the macroscale, highlighting failure by plastic collapse and failure by brittle fracture. This is followed by sections describing the use of fracture mechanics and materials testing in carrying-out assessments of structural integrity. Attention then focuses on the microscale, explaining how values of the local fracture stress in notched bars or of fracture toughness in pre-cracked test-pieces are related to features of the microstructure: carbide thicknesses in wrought material; the sizes of oxide/silicate inclusions in weld metals. Effects of a microstructure that is 'heterogeneous' at the mesoscale are treated briefly, with respect to the extraction of test-pieces from thick sections and to extrapolations of data to low failure probabilities. The values of local fracture stress may be used to infer a local 'work-of-fracture' that is found experimentally to be a few times greater than that of two free surfaces. Reasons for this are discussed in the conclusion section on nano-scale events. It is suggested that, ahead of a sharp crack, it is necessary to increase the compliance by a cooperative movement of atoms (involving extra work) to allow the crack-tip bond to displace sufficiently for the energy of attraction between the atoms to reduce to zero.

Solvent-Induced Carboxylate Shift and Movement of an Anthryl Side-Group in Single-Crystal to Single-Crystal Structural Dynamics in a Gadolinium Coordination Polymer

Single crystal to single crystal (SC–SC) transformation involving cooperative movement of atoms represents one of the most fascinating phenomena in coordination polymers. Here, we describe a novel two-dimensional coordination polymer {[Gd2(L)3(DMF)2(H2O)2]·(DMF)2·(H2O)5.5}n (1) synthesized from carboxylate-based flexible ligand 5-[(anthracen-9-ylmethyl)-amino]-isophthalic acid and Gd(III) ion by the solvothermal technique. The complex undergoes solvent-induced rearrangement reactions with the cleavage and formation of coordination bonds and substantial movement of the anthracene side groups without losing crystallinity to form the daughter products as {[Gd2(L)3(H2O)4]·(DMF)4·(H2O)1.5}n (1a), {[Gd2(L)3(DMF)2(H2O)]·(DMF)2·(DCM)2·(H2O)5}n (1b), and [Gd(L)2(DEF)]n (1c). These transformations exhibit a crystallographic snapshot of “carboxylate-shift” process which is further supported by IR spectroscopy, elemental analysis, and powder X-ray diffraction patterns. To the best of our knowledge, it is the first ex...

Martensitic Transformation and Microstructural Characteristics in Copper Based Shape Memory Alloys

Martensitic transformations are first order solid state phase transitions and occur in the materials on cooling from high temperature. Shape memory effect is an unusual property exhibited by certain alloy systems, and based on martensitic transformation. The shape memory property is characterized by the recoverability of previously defined shape or dimension when they are subjected to variation of temperature. The shape memory effect is facilitated by martensitic transformation, and shape memory properties are intimately related to the microstructures of the materials. Martensitic transformations occur as martensite variant with the cooperative movement of atoms on {110}β - type plane of austenite matrix. Martensitic transformations have diffusionless character, and the atomic movement is confined to interatomic lengths in the materials. The basic factors which govern the martensitic transformation are Bain distortion and homogeneous shears. Copper based alloys exhibit this property in metastable β-phase field.

Dimerization of a Metal Complex through Thermally Induced Single‐Crystal‐to‐Single‐Crystal Transformation or Mechanochemical Reaction

Single-crystal-to-single-crystal (SCSC) transformations, which involve cooperative movement of atoms in the solid state, have received much attention from chemists. Through SCSC transformation, new complexes, which may not be obtained under conventional conditions, can be obtained in high yield. The SCSC transformation was first reported as a photoreaction in organic complexes, which could be considered as a new organic synthetic method. Solid-state photochemical [2+2] cycloadditions of organic molecules have been well studied. Recently, SCSC transformation has been extended to other complexes, such as metal complexes and coordination polymers (or metal–organic frameworks). In the past decades, many metal–organic frameworks (MOFs) that undergo SCSC transformation have been reported. Most of these reports describe the sliding of layers or breathing of 3D porous MOFs through solvent exchange. SCSC transformations involving breaking and forming bonds in the solid state remain quite rare. The MacGillivray and Foxman groups attained considerable success in solid-state photodimerization of organic groups mediated by metals. Converting a finite metal complex into an infinite coordination network was also reported by the groups of MacGillivray and Vittal. The reported topochemical dimerizations of metal complexes involve the organization of two double or triple bonds in the solid state: the geometric criteria are very important for such SCSC reactions. However, the coordination geometry of the metal ion remains unchanged in these topochemical dimerizations. More recently, zur Loye and co-workers described a 3D cobalt–organic framework exhibiting reversible shrinkage and expansion involving a change in the cobalt coordination geometry through SCSC transformation induced by the removal and re-absorption of guest molecules. Studies on the change in the metal coordination geometry caused by the removal and addition of ligands from the framework itself through SCSC transformation remain less common. Herein, we describe a new type of SCSC dimerization of a mononuclear copper complex, specifically, thermally induced head-to-tail dimerization involving the formation of a new Cu Oligand bond and a change in the metal coordination geometry. The transformation induced a color change from blue to green, which was caused by the change in the copper coordination geometry (Scheme 1). Interestingly, the trans-

Phase transmission electron microscopy with aberration correction based on active defocus modulation: dynamic observation of surface atom movement

AbstractWe have developed a phase transmission electron microscopy system that enables real‐time observation of spherical aberration‐free phase images based on active defocus modulation. With the use of this system, the cooperative movement of atoms on an Au(011) reconstructed surface was clearly observed at an atomic level. Relaxation of stress in a deformed crystal was also observed when slipping of (111) atom planes occurred. The dynamic behaviour of an atom‐sized Au wire that was elongated in the [001] direction was observed at a time resolution of 1/30 s, and its behaviour was different from that of a wire elongated in the [011] direction, which was observed in a previous study. Copyright © 2005 John Wiley &amp; Sons, Ltd.

In-(13&amp;minus;&lt;I&gt;X&lt;/I&gt;) at%Pb-&lt;I&gt;X&lt;/I&gt; at%Sn擬2元系合金のマルテンサイト変態

Changes in both crystal structure and surface relief during the phase transformation in Indium-(13−X) at%Pb-X at%Sn pseudobinary alloys have been studied by means of metallurgical techniques. Lattice parameters at each temperature during the phase transformation show that the alloys undergo two kinds of phase transformation: fct(c⁄a<1)↔fco phase transformation in the alloys containing (0-9) at%Sn and fct(c⁄a<1)↔fct(c⁄a>1) one in the alloys containing (11-13) at%Sn. The characteristic feature of phase transformation becomes more recognizable first-order gradually with increasing tin content. The appearance of a banded surface relief in a low-temperature phase concludes that these phase transformations are martensitic ones taking place by the cooperative movement of atoms. The change in surface relief is reversible in the alloys containing (0-7) at%Sn, but not in others. Corresponding to this, the amount of shape recovery in the former alloys is nearly constant, and the latter alloys show a decrease in shape recovery with increasing tin content. The characteristic features of shape recovery associated with phase transformation on heating is discussed in terms of the crystallographic reversibility in each phase transformation. Furthermore, the fct(c⁄a<1)↔fco phase transformation is analyzed phenomenologically on the basis of the Landau theory, and some physical quantities such as the change in entropy and the elastic constant C′ are calculated. The temperature dependence of the stress-strain relation is also discussed in terms of ferro- and super-elasticity. The analyzed results suggest that the characteristic behavior of the fct(c⁄a<1)↔fco phase transformation depends upon the average atomic size as well as the valence electron concentration.

等原子組成のFe-V合金における&amp;sigma;相の生成挙動

The electrical resistivity and acoustic emission (AE) were measured simultaneously during a cyclic heat treatment from room temperature either to 973 K or 873 K, in order to study the mechanisms of sigma phase formation in an equiatomic Fe-V alloy. During the holding stage for 432 ks either at 973 K or 873 K, the 973 K specimen produced drastic increase both in the resistivity and the AE event count, but the 873 K specimen produced a considerable amount of AE but a slight decrease in the resistivity. The occurrence of the AE took place not continuously but intermittently during the holding stage at those temperatures. Furthermore, examinations by X-ray analysis and transmission electron microscopy showed that the 973 K specimen consisted entirely of the sigma phase containing many lattice defects, and that the 873 K specimen consisted mainly of the ferrite phase and slightly of the sigma phase which was confined to an extremely thin layer on the free surface.On the basis of the concept that the AE occurrence and introduction of lattice defects may be restricted to the diffusionless transformation, it was inferred that the cooperative movement of atoms takes part in the sigma phase formation in the equiatomic Fe-V alloy.