Close-packed Metals Research Articles

Disconnection-Mediated Twin Embryo Growth in Mg

While deformation twinning in hexagonal close-packed metals has been widely studied due to its substantial impact on mechanical properties, an understanding of the detailed atomic processes associated with twin embryo growth is still lacking. Conducting molecular dynamics simulations on Mg, we show that the propagation of twinning disconnections emitted by basal-prismatic interfaces controls the twin boundary motion and is the rate-limiting mechanism during the initial growth of the twin embryo. The time needed for disconnection propagation is related to the distance between the twin tips, with widely spaced twin tips requiring more time for a unit twin boundary migration event to be completed. Thus, a phenomenological model, which unifies the two processes of disconnection and twin tip propagation, is proposed here to provide a quantitative analysis of twin embryo growth. The model fits the simulation data well, with two key parameters (twin tip velocity and twinning disconnection velocity) being extracted. In addition, a linear relationship between the ratio of twinning disconnection velocity to twin tip velocity and the applied shear stress is observed. Using an example of twin growth in a nanoscale single crystal from the recent literature, we find that our molecular dynamics simulations and analytical model are in good agreement with experimental data.

Competitive Hydrogenation between Linear Alkenes and Aromatics on Close-Packed Late Transition Metal Surfaces

Selective hydrogenation of linear alkenes in the presence of aromatics is desired to prevent gum formation in pyrolysis gasoline (PYGAS) upgrading. To examine the competitive hydrogenation between linear alkenes and aromatics, we investigate ethylene and benzene competitive hydrogenation on different catalysts. Through density functional theory (DFT) calculations, we show the adsorption energies of benzene and ethylene correlate on monometallic close-packed surfaces, with benzene binding stronger for the same C to surface metal atom ratio. DFT calculations demonstrate Bronsted–Evans–Polanyi and scaling relationships hold, and these are fed into microkinetic modeling to predict the rate of ethylene and benzene hydrogenation with only ethylene and hydrogen binding energies as the surface descriptors. Due to stronger binding, benzene adsorption will dominate the surface. Higher barriers for benzene hydrogenation versus ethylene hydrogenation lead to benzene poisoning at temperatures at which ethylene hydroge...

Mechanism for the Formation of Partial Stacking Faults in Magnesium

Anomalous basal stacking (SFs) have been frequently observed inside {10-11} and {10-12} twins in hexagonal close-packed metals. These SFs were first described by Song and Gray as partial stacking faults in the sense that only every other basal plane is displaced by the SFs. To further understand the formation mechanism of these SFs, in this work, we performed lattice correspondence analysis in the simulations of these two twinning modes. The corresponding plane of the parent to the prismatic plane of the twin was pre-selected and tracked before and after twinning. The atomic positions were closely examined to reveal the stacking sequence change in the twin due to the formation of the basal SFs. The results show that, indeed, only half of the atoms are displaced by the presence of the basal SFs, indicating that no global displacement vector can be defined and no dislocation activities are involved in the formation of the SFs. A special configuration of SFs was observed, which has limited mobility via coordinated atomic shuffles.

Dislocation absorption and transmutation at [formula omitted] twin boundaries in deformation of magnesium

How matrix dislocations, i.e. basal, prismatic and pyramidal, interact with {101¯2}101¯1¯ twin boundaries in hexagonal close-packed metals has been discussed extensively in the literature. However, so far no systematic investigation has been reported. In this work, we performed atomistic simulations to study interaction between matrix dislocations in pure Mg with a {101¯2} twin boundary. Our results show that for the basal and the prismatic slip, when the Burgers vector is parallel to the zone axis of the twins, a matrix basal dislocation can be transmuted to a twin prismatic dislocation and vice versa. However, when the Burgers vector of the matrix dislocation is non-parallel to the zone axis, no transmutation occurs and the dislocation is absorbed by the twin boundary which acts as a dislocation sink. For a matrix pyramidal dislocation, the dislocation is absorbed by the twin boundary and no transmutation occurs either. It appears that if the product dislocation is a real slip system, transmutation may occur during twin-slip interaction. Otherwise the matrix dislocation will be shredded by atomic shuffling and then absorbed by the twin boundary. If the core structure of the product dislocation is complex, transmutation may not occur as well and dislocation absorption will occur. Lattice correspondence in deformation twinning was applied in explaining the interaction mechanisms. Our results can be well correlated to macroscopic experimental observations which show twin-slip interaction only contributes negligibly to work hardening in deformation of hcp metals.

Formation of 60° [formula omitted] boundaries between [formula omitted] twin variants in deformation of a magnesium alloy

Interrupted tensile testing along the normal direction of a rolled AZ31 magnesium plate was conducted at various strain levels from 6% to 15.5%. Prior to deformation the specimens were annealed for grains to coarsen such that multiple twin variants in one grain can be activated and interaction between variants can be better resolved. The grain structure and texture evolution were then examined via electron backscatter diffraction. The results reveal that multiple primary {101̅2}P twin variants inside individual grains grow and impinge, forming profuse boundaries of 60°〈011̅0〉 orientation relationship. These boundaries are close to {112̅2}〈112̅3̅〉 twin relationship, however, they are not formed by twin nucleation and are only a product of interaction between the primary twins. The morphology of these special boundaries are highly irregular. Atomistic simulations were performed to understand such interaction, and the results show that such 60° 〈011̅0〉 boundaries have limited mobility. Thus, a primary twin variant is able to grow at the expense of other primary twin variants. Our experimental results also show that {101̅2} twinning is active throughout the deformation until the specimens are fractured. As the strain increases, secondary {101̅2}S twins are activated inside the primary twins. As the strain further increases to near fracture, {101̅1}S secondary twins are activated inside the primary twins, followed by activation of tertiary {101̅2}T twins inside the {101̅1}S secondary twins, a process known as {101̅1}−{101̅2} double twinning. The formation of irregular 60°〈011̅0〉 boundaries and the sequential twinning at large strains shed new light on the twinning mechanism in Mg and other hexagonal close-packed metals.

Three-Dimensional Sandwich Nanocubes Composed of 13-Atom Palladium Core and Hexakis-Carbocycle Shell.

Coordination of cyclic unsaturated hydrocarbons to transition metal generally gives bis-ligated sandwich complexes, which are a fundamental class of organometallic compounds. This sandwich structure may be extended to a higher-order three-dimensional one when more than two carbocyclic ligands surround an agglomerate of many transition metal atoms. Here, we report synthesis of three-dimensional sandwich nanocube compounds containing a close-packed transition metal cluster core. Reduction of a [Pd3(μ3-C7H7)2]2+ with tetraarylborate under neat conditions afforded [Pd13(μ4-C7H7)6]2+ containing a cuboctahedral Pd13 core with an octahedral ligand-shell.

Symmetry-Induced Structuring of Ultrathin FeO and Fe₃O₄ Films on Pt(111) and Ru(0001).

Iron oxide films epitaxially grown on close-packed metal single crystal substrates exhibit nearly-perfect structural order, high catalytic activity (FeO) and room-temperature magnetism (Fe3O4). However, the morphology of the films, especially in the ultrathin regime, can be significantly influenced by the crystalline structure of the used support. This work reports an ultra-high vacuum (UHV) low energy electron/synchrotron light-based X-ray photoemission electron microscopy (LEEM/XPEEM) and electron diffraction (µLEED) study of the growth of FeO and Fe3O4 on two closed-packed metal single crystal surfaces: Pt(111) and Ru(0001). The results reveal the influence of the mutual orientation of adjacent substrate terraces on the morphology of iron oxide films epitaxially grown on top of them. On fcc Pt(111), which has the same mutual orientation of adjacent monoatomic terraces, FeO(111) grows with the same in-plane orientation on all substrate terraces. For Fe3O4(111), one or two orientations are observed depending on the growth conditions. On hcp Ru(0001), the adjacent terraces of which are ‘rotated’ by 180° with respect to each other, the in-plane orientation of initial FeO(111) and Fe3O4(111) crystallites is determined by the orientation of the substrate terrace on which they nucleated. The adaptation of three-fold symmetric iron oxides to three-fold symmetric substrate terraces leads to natural structuring of iron oxide films, i.e., the formation of patch-like magnetite layers on Pt(111) and stripe-like FeO and Fe3O4 structures on Ru(0001).

Analysis of stacking disorder in ice I using pair distribution functions

Stacking-disordered materials display crystalline order in two dimensions but are disordered along the direction in which layered structural motifs are stacked. Countless examples of stacking disorder exist, ranging from close-packed metals, ice I and diamond to open-framework materials and small-molecule pharmaceuticals. In general, the presence of stacking disorder can have profound consequences for the physical and chemical properties of a material. Traditional analyses of powder diffraction data are often complicated by the presence of memory effects in the stacking sequences. Here it is shown that experimental pair distribution functions of stacking-disordered ice I can be used to determine local information on the fractions of cubic and hexagonal stacking. Ice is a particularly challenging material in this respect, since both the stacking disorder and the orientational disorder of the water molecules need to be described. Memory effects are found to contribute very little to the pair distribution functions, and consequently, the analysis of pair distribution functions is the method of choice for characterizing stacking-disordered samples with complicated and high-order memory effects. In the context of this work, the limitations of current structure-reconstruction approaches are also discussed.

The surface nucleation of tension twin via pure-shuffle mechanism: The energy landscape sampling and dynamic simulations

The surface nucleation of tension twin in magnesium monocrystalline is investigated by potential energy landscape sampling and dynamic simulations. Surface nucleation via pure-shuffle mechanism is proved without any other pre-existing defects or defect interactions as a trigger event. The stress-dependent reaction pathways and activation potential energies of a tension twin nucleus are reported. The growth of a tension twin nucleus in a submicron-sized sample is simulated by molecular dynamics. During the growth, a twin nucleus with basal-prismatic interfaces as its dominant boundaries evolves into a twin grain which is characterized by coherent twin boundaries. A better understanding of shuffle-induced plasticity in hexagonal close-packed metals is pursued.

Evidence of new twinning modes in magnesium questioning the shear paradigm

Twinning is an important deformation mode of hexagonal close-packed metals. The crystallographic theory is based on the 150-year-old concept of simple shear. The habit plane of the twin is the shear plane; it is invariant. This article presents electron backscatter diffraction observations and crystallographic analysis of a millimetre-size twin in a magnesium single crystal whose habit plane, unambiguously determined both in the parent crystal and in its twin, is not an invariant plane. This experimental evidence demonstrates that macroscopic deformation twinning can be achieved by a mechanism that is not a simple shear. This unconventional twin is often co-formed with a new conventional twin that exhibits the lowest shear magnitude ever reported in metals. The existence of unconventional twinning introduces a shift of paradigm and calls for the development of new crystallographic theories of displacive transformations.

Constitutive Model for the Thermo-viscoplastic Behavior of Hexagonal Close-Packed Metals with Application to Ti–6Al–4V Alloy

In this paper, a new physically based constitutive model is developed for hexagonal close-packed metals, especially the Ti–6Al–4V alloy, subjected to high strain rate and different temperatures based on the microscopic mechanism of plastic deformation and the theory of thermally activated dislocation motion. A global analysis of constitutive parameters based on the Latin Hypercube Sampling method and the Spearman’s rank correlation method is adopted in order to improve the identification efficiency of parameters. Then, an optimal solution of constitutive parameters as a whole is obtained by using a global genetic algorithm composed of an improved niche genetic algorithm, a global peak determination strategy and the local accurate search techniques. It is concluded that the proposed constitutive modal can accurately describe the Ti–6Al–4V alloy’s dynamic behavior because the prediction results of the model are in good agreement with the experimental data.

Properties of Moving Discrete Breathers in Beryllium

Discrete breathers (DBs) have been described among pure metals with face-centered cubic (FCC) and body-centered cubic (BCC) lattice, but for hexagonal close-packed (HCP) metals, their properties are little studied. In this paper, the properties of standing and moving DBs in beryllium HCP metal are analyzed by the molecular dynamics method using the many-body interatomic potential. It is shown that the DB is localized in a close-packed atomic row in the basal plane, while oscillations with a large amplitude along the close-packed row are made by two or three atoms, moving in antiphase with the nearest neighbors. Dependences of the DB frequency on the amplitude, as well as the velocity of the DB on its amplitude and on parameter δ, which determines the phase difference of the oscillations of neighboring atoms, are obtained. The maximum velocity of the DB movement in beryllium reaches 4.35 km/s, which is 33.7% of the velocity of longitudinal sound waves. The obtained results supplement our concepts about the mechanisms of localization and energy transport in HCP metals.

Using Brønsted-Evans-Polanyi relations to predict electrode potential-dependent activation energies

Various elementary (de-)hydrogenation reactions at transition metal surfaces in heterogeneous catalysis have shown a linear Brønsted-Evans-Polanyi relation (BEP) between the activation energy and the reaction energy across metal surfaces. Using Density Functional Theory (DFT), we investigate if these BEP relations can be extended to elementary electrochemical reduction reactions involving transfer of a proton-electron pair. We examine the effect of the applied electrode potential on the BEP relations. We focus in particular on elementary electrochemical CH, OH and NH bond formation reactions on 7 close-packed transition metal surfaces, including Ag (111), Au (111), Cu (111), Ni (111), Pt (111), Pd (111) and Rh (111). The potential-dependent activation energies used to construct the BEP relations are calculated using a Marcus theory based approach. The role of interfacial water in the kinetics and the reaction mechanism of CH, OH and NH bond formation is explored. Specifically, two mechanisms are considered, a Tafel-like scheme involving direct surface hydrogenation and a Heyrovsky-like mechanism in which the proton is shuttled via an explicit water molecule. The potential-dependent elementary kinetics across three reduction reaction series, C* → CH4, O* → H2O and N* → NH3, are also discussed.The Heyrovsky-like scheme is the preferred mechanism for CH, OH and NH bond formation on all but two metals. BEP relations hold at the same applied electrode potential for CH, OH and NH formation for both examined mechanisms. However, BEP parameters differ among CH, OH, and NH reactions, indicating that elementary reaction energies alone are not sufficient for comparing the activity of catalysts when electrocatalytic reactions involve such different elementary steps.

A Graph Theory-Based Automated Twin Recognition Technique for Electron Backscatter Diffraction Analysis

The present article introduces a new software, Microstructure Evaluation Tool for Interface Statistics (METIS), that performs high-throughput microstructure statistical analysis from electron backscatter diffraction maps. Emphasis is placed on the detection of twin domains in hexagonal close-packed metals. The numerical framework on which METIS is built leverages graph theory, group structures, and associated numerical algorithms to automatically detect twins and unravel both their intrinsic characteristics features and those pertaining to their interactions. The proposed graphical interface allows for the detection and correction of unlikely twin/parent associations rendering the approach applicable to highly deformed microstructures. Twin statistics and microstructural data are classified and saved in a relational database that can be interrogated via either GUI or SQL requests to reveal a wide spectrum of features of the microstructure. Illustration of the approach is performed in the case of zirconium.

Changes in misorientations of $$ \{ 10\bar{1}1\} $$ { 10 1 ¯ 1 } twin boundaries in deformed magnesium alloy

Deformation twins usually play an important role in plastic deformation and undoubtedly influence the mechanical property of hexagonal close-packed (hcp) metals. Understanding the interfacial features of twinning boundaries and their evolution during the plastic deformation is crucial to hcp materials design, processing and application. In this work, the interfacial characteristics of $$ \{ 10\bar{1}1\} $$ contraction twin in deformed magnesium alloy are investigated by means of electron back-scatter diffraction, transmission electron microscope and high-resolution TEM. The results show that the misorientation angle across the boundaries of $$ \{ 10\bar{1}1\} $$ contraction twin can deviate from the theoretical value $$ \left( {56.2^\circ \left\langle {11\bar{2}0} \right\rangle } \right) $$ , resulting in that the $$ \{ 10\bar{1}1\} $$ twinning planes of the twin crystal and the matrix crystal do not coincide any longer. Multiple dislocation activities within the $$ \{ 10\bar{1}1\} $$ contraction twin and interfacial defects located on the $$ \{ 10\bar{1}1\} $$ twinning plane are also characterized. According to these experimental features, a possible mechanism responsible for such deviation phenomenon is proposed. In addition, the similar deviation phenomenon is also observed in the $$ \{ 10\bar{1}1\} $$ – $$ \{ 10\bar{1}2\} $$ double twin system.

Свойства движущихся дискретных бризеров в бериллии

AbstractDiscrete breathers (DBs) have been described among pure metals with face-centered cubic (FCC) and body-centered cubic (BCC) lattice, but for hexagonal close-packed (HCP) metals, their properties are little studied. In this paper, the properties of standing and moving DBs in beryllium HCP metal are analyzed by the molecular dynamics method using the many-body interatomic potential. It is shown that the DB is localized in a close-packed atomic row in the basal plane, while oscillations with a large amplitude along the close-packed row are made by two or three atoms, moving in antiphase with the nearest neighbors. Dependences of the DB frequency on the amplitude, as well as the velocity of the DB on its amplitude and on parameter δ, which determines the phase difference of the oscillations of neighboring atoms, are obtained. The maximum velocity of the DB movement in beryllium reaches 4.35 km/s, which is 33.7% of the velocity of longitudinal sound waves. The obtained results supplement our concepts about the mechanisms of localization and energy transport in HCP metals.

Proliferation of twinning in hexagonal close-packed metals: Application to magnesium

Plastic deformation of metallic alloys usually takes place through slip, but occasionally involves twinning. In particular, twinning is important in hexagonal close packed (HCP) materials where the easy slip systems are insufficient to accommodate arbitrary deformations. While deformation by slip mechanisms is reasonably well understood, comparatively less is known about deformation by twinning. Indeed, the identification of relevant twinning modes remains an art. In this paper, we develop a framework combining a fundamental kinematic definition of twins with large-scale atomistic calculations to predict twinning modes of crystalline materials. We apply this framework to magnesium where there are two accepted twin modes, tension and compression, but a number of anomalous observations. Remarkably, our framework shows that there is a very large number of twinning modes that are important in magnesium. Thus, in contrast to the traditional view that plastic deformation is kinematically partitioned between a few modes, our results suggest that deformation in HCP materials is the result of an energetic and kinetic competition between numerous possibilities. Consequently, our findings suggest that the commonly used models of deformation need to be extended in order to take into account a broader and richer variety of twin modes, which, in turn, opens up new avenues for improving the mechanical properties.

Electron-Phonon Coupling Constant of Metallic Overlayers from Specular He Atom Scattering.

He atom scattering has been shown to be a sensitive probe of electron-phonon interaction properties at surfaces. Here it is shown that measurements of the thermal attenuation of the specular He atom diffraction peak (the Debye-Waller effect) can determine the electron-phonon coupling constant, λ, for ultrathin films of metal overlayers on various close-packed metal substrates. Values of λ obtained for single and multiple monolayers of alkali metals, and for Pb layers on Cu(111), extrapolated to large thicknesses, agree favorably with known bulk values. This demonstrates that He atom scattering can measure the electron-phonon coupling strength as a function of film thickness on a layer-by-layer basis.

Twin-twin interactions and contraction twin formation in an extruded magnesium alloy subjected to an alteration of compressive direction

Twinning and detwinning represent major deformation mechanisms in hexagonal close-packed (hcp) metals. The aim of this study was to identify twin-twin interactions and contraction twin formation in an AZ31 magnesium alloy when the compressive direction was changed from the extrusion direction (ED) to the normal direction (ND) via electron backscatter diffraction (EBSD) and Schmid factor analysis. {101¯2} extension twins of multiple variants were observed after compressive deformation of 4.3% along ED. The detwinning of {101¯2} extension twins occurred along with the formation of {101¯1} and {101¯3} contraction twins, when the compressive direction was changed to ND. The extension twins in some grains almost fully vanished, making the grains back to a twin-free state. A new twin-twin interaction mechanism being different from double twinning (defined as twins within a twin) was identified, due to the impingement of {101¯1} contraction twins nucleated in the matrix grain on a pre-existing {101¯2} extension twin.

Dehydrogenation Selectivity of Ethanol on Close-Packed Transition Metal Surfaces: A Computational Study of Monometallic, Pd/Au, and Rh/Au Catalysts

Ethanol (EtOH) decomposition has been widely studied in recent years. However, the initial dehydrogenation selectivity on catalytic surfaces, which plays a crucial role in EtOH partial oxidation and steam reforming, is not well understood. Here, density functional theory (DFT) was used to calculate the initial dehydrogenation selectivities of EtOH on monometallic and X/Au (X = Pd and Rh) close-packed surfaces. The energy for the initial bond scissions of O–H and α- and β-C–H were calculated on each surface. The binding energy of EtOH is found to be a good reactivity descriptor for the scission of O–H and β-C–H bonds, while the binding energy of CH3CHOH is a good reaction descriptor for α-C–H bond scission. The scaling relationships between the activation energy barriers and binding energies on Pd/Au and Rh/Au surface alloys are significantly different from those of monometallic surfaces. Additionally, the specific atomic ensembles on the Pd/Au and Rh/Au surfaces have different initial dehydrogenation sele...