Close-packed Metals Research Articles

Unraveling the Atomic Shuffles of Twinning Nucleation in Hexagonal Close-Packed Rhenium Nanocrystals.

Reining in deformation twinning is crucial for the mechanical properties of hexagonal close-packed (HCP) metals and hinges on an explicit understanding of the twinning nucleation mechanism. Unfortunately, it is often suggested rather than conclusively demonstrated that twinning nucleation can be mediated by pure atomic shuffles. Herein, by utilizing in situ high-resolution transmission electron microscopy, we have dissected the atomic shuffling mechanism during the {101̅2} twinning nucleation in rhenium nanocrystals, which revealed the emergence of an intermediate body-centered tetragonal (BCT) structure. Specifically, the double-layered prismatic planes initially shuffle into single-layered {11̅0}BCT planes; subsequently, adjacent {22̅0}BCT planes shuffle in opposite directions to form the basal planes of the twin embryo. This shuffling mechanism is further corroborated by molecular dynamic simulations. The finding provides direct evidence of shuffle-dominated twinning nucleation with atomic details that may lead to better control of this critical twinning mode in HCP metals.

Improved interfacial strength of Ti/Ti ultrasonic welds by introducing α/β/α heterogeneous bonding interface

We investigated ultrasonic welding (USW) of pure α-Ti sheets, which exhibited an insufficient weldability due to the extensive formation of cracks caused by sliding friction during the USW. To overcome this issue, an α/β/α heterogeneous interfacial microstructure was designed by introducing a thin Fe interlayer, a beta stabilizer, between α-Ti sheets. The diffusion of Fe interlayers triggered the formation of a continuous β-Ti thin layer (⁓1–2 μm) without Fe–Ti intermetallics near the bonding interface after USW. The continuous thin β-Ti layer exhibited a significant improvement in the joining of the brittle α-Ti, where the bonding strength was increased from 350 to 1700 N. This study provides a potential approach to design an interfacial microstructure that promotes the formation of a deformable phase, overcoming the inherent brittleness of hexagonal close-packed metals and enabling production of strong joints.

A computational study of how surfaces affect slip family activity

Plastic deformation behavior is most conveniently assessed by characterization on a surface, but whether such observations are representative of bulk properties is uncertain. Motivated by reported inconsistencies in slip resistance probed at different depths, we investigated (i) whether the average slip family activity is affected by the presence of a surface and (ii) how the kinematic nature of available slip families influences a potential surface effect. The slip family activity as a function of distance from the surface was extracted from full-field crystal plasticity simulations of random polycrystalline hexagonal close-packed (HCP) and body-centered cubic (BCC) metals as examples of mixed in contrast to universally-high numbers of slip systems per family. Under certain conditions, a deviation from bulk slip activity is observed up to about two grains from the surface. For the easiest (least slip-resistant) family, a surface effect of decreasing activity with depth emerges if the number of slip systems falls below about six. For harder families, slip activity always increases with depth. These phenomena are explained on the basis of varying constraints with depth in connection with the kinematic properties of slip families in the material.

Dissociation of edge and screw pyramidal I and II dislocations in magnesium

Pyramidal dislocations in magnesium (Mg) and other hexagonal close-packed metals play an important role in accommodating plastic strains along the c-axis. Bulk single crystal Mg only presents very limited plasticity in c-axis compression, and this behavior was attributed to out-of-plane dissociation of pyramidal dislocations onto the basal plane and resulted in an immobile dislocation configuration. In contrast, other simulations and experiments reported in-plane dissociation of pyramidal dislocations on their slip planes. Thus, the core structure and mode of dissociation of pyramidal dislocations are still not well understood. To better understand the dissociation behavior of pyramidal dislocations in Mg at room temperature, in this work, atomistic simulations were conducted to investigate four types of pyramidal dislocations at 300 K: edge and screw Py-I on {101¯1}, edge and screw Py-II on {112¯2} by using a modified embedded atom method (MEAM) potential for Mg and anisotropic elasticity dislocation model. The results show that when energy minimization was performed before relaxation, in-plane dissociation of edge dislocations on respective pyramidal plane could be obtained at room temperature for all four types of dislocation. Without energy minimization, the edge dislocations dissociated out-of-plane onto the basal plane. Calculations of potential energy and hydrostatic stress of individual atoms at the edge dislocation core show that the extraordinarily high energy and atomic stresses in the as-constructed dislocation structures caused the out-of-plane dissociation onto the basal plane. The core structures of all four types of pyramidal dislocation after in-plane dissociation were analyzed by computing the distribution of the Burgers vector.

Adsorbate-Induced Adatom Formation on Lithium, Iron, Cobalt, Ruthenium, and Rhenium Surfaces.

Recent experimental and theoretical studies have demonstrated the reaction-driven metal-metal bond breaking in metal catalytic surfaces even under relatively mild conditions. Here, we construct a density functional theory (DFT) database for the adsorbate-induced adatom formation energy on the close-packed facets of three hexagonal close-packed metals (Co, Ru, and Re) and two body-centered cubic metals (Li and Fe), where the source of the ejected metal atom is either a step edge or a close-packed surface. For Co and Ru, we also considered their metastable face-centered cubic structures. We studied 18 different adsorbates relevant to catalytic processes and predicted noticeably easier adatom formation on Li and Fe compared to the other three metals. The NH3- and CO-induced adatom formation on Fe(110) is possible at room temperature, a result relevant to NH3 synthesis and Fischer-Tropsch synthesis, respectively. There also exist other systems with favorable adsorbate effects for adatom formation relevant to catalytic processes at elevated temperatures (500-700 K). Our results offer insight into the reaction-driven formation of metal clusters, which could play the role of active sites in reactions catalyzed by Li, Fe, Co, Ru, and Re catalysts.

Twinning-induced plasticity with multiple twinning modes and disclinations in Mg alloys

Twinning-induced plasticity plays a critical role in determining the deformation behaviors of hexagonal close-packed metals (e.g., Mg and Ti), due to the lack of five independent slip systems required for a general deformation. In particular, twinning modes that can accommodate c-axis strain (tensile or compressive) are especially important, which are necessary complementary deformation modes to <a>-type dislocations. However, only limited types of extension twins, e.g., {101¯2} type twin, have been unambiguously identified in the literature, which is theoretically inadequate to accommodate a complex deformation with c-axis extension. Using topological defect theory, here we show that another extension twinning mode, {112¯6} twin, can be formed through reactions between disclinations and {101¯2} twins in Mg-alloys. Based on symmetry of the deformation space, we demonstrate that the {112¯6} twin originates from the inter-connection of correlated deformation paths, which can provide a tensile strain of 5.2% along c-axis and a compressive strain of 4.5% along 〈112¯0〉. {112¯6} twins can be formed through three formation mechanisms, i.e., direct formation, twin-twin intersection, and double twinning, which are validated by a combination of experimental characterizations and phase field simulations. Our work not only suggests a symmetry-based method to analyze twinning-induced plasticity, but also provides a new insight into the deformation mechanisms and mechanical properties of Mg-alloys.

Three-dimensional shape and stress field of a deformation twin in magnesium

While the three-dimensional (3D) shape and stress field of a twin in hexagonal close-packed (HCP) metals have attracted considerable interest in recent years due to their substantial impact on internal stress and mechanical properties, a detailed understanding of their variation with twin size is still lacking. An analytical model that is not restricted by spatial scale is developed in this work by considering the effects of anisotropic twin boundary energy, elastic strain energy and plastic relaxation to predict the 3D shape with the minimum energy, and the stress field, of an isolated ellipsoidal twin of different sizes. The model is applied to Mg with a focus on the {101¯2} twin type. The analytical calculation results show that the nucleation of the nano-sized twin embryos is facilitated by the stress field near structural defects such as dislocations. During the expansion of this nano-sized twin embryo, the interplay between the elastic strain energy and interfacial energy changes the length of the twin along the twin shear (forward) direction from being shorter to longer than that along the lateral direction. In contrast to the current understandings, the maximum shear stress on the twin plane along the twin shear direction occurs at the lateral, rather than the forward, side of the twin. At the forward side, the maximum shear stress occurs at a distance ahead of the twin tip and this distance increases with increasing twin thickness.

Deformation-induced grain rotation and grain boundary formation achieved through dislocation-disclination reactions in polycrystalline hexagonal close-packed metals

Grain boundary mediated plasticity (e.g., grain rotation and grain boundary sliding) plays a critical role in determining the deformation behavior of polycrystals, which is especially important for nanocrystalline metals or plastic deformation at elevated temperatures. However, due to the lack of a theoretical framework, the mechanism of deformation-induced grain rotation has not been well understood. By introducing a disclination-based description, we show that crystal rotation can be captured conveniently through the characteristic rotational vector of a disclination (i.e., the Frank vector). Under the framework of topological defect theory, grain boundary mediated plasticity can be treated as topological reactions between dislocations and disclinations, leading to rigorous predictions of grain/subgrain rotation through a pseudo-inverse solution for the reformulated Frank-Bilby equation. Systematic classifications of grain/subgrain rotation and grain boundary formation are raised for hexagonal close-packed (HCP) metals, which has been validated by Electron-Backscattered Diffraction characterizations in Mg alloys. Our work not only suggests a theoretical approach to investigate grain/subgrain rotation based on the topological correlation between dislocations and disclinations, but also provides a new insight into the plastic behaviors of polycrystalline HCP metals.

The strain rate sensitive and anisotropic behavior of rare-earth magnesium alloy ZEK100 sheet

To overcome the limitation in formability at room temperature, manufacturers have developed magnesium alloys with remarkable properties by adding rare-earth elements. The rare-earth magnesium alloys behave differently from the conventional alloys, especially with respect to their coupled anisotropic and strain rate sensitive behavior. In the current work, such behavior of the rare-earth Mg alloy ZEK100 sheet at room temperature is investigated with the aid of the elastic viscoplastic self-consistent polycrystal plasticity model. Different strain rate sensitivities (SRSs) for various deformation modes are employed by the model to simulate the strain rate sensitive behaviors under different loading directions and loading rates. Good agreement between the experiments and simulations reveals the importance and necessity of using different SRSs for each deformation mode in hexagonal close-packed metals. Furthermore, the relative activities of each deformation mode and the texture evolution during different loadings are discussed. The anisotropic and strain rate sensitive behavior is ascribed to the various operating deformation modes with different SRSs during loading along different directions.

Synthesis of superconducting hcp- ZrH3 under high hydrogen pressure

Zirconium trihydride and, for comparison, trideuteride with hexagonal close-packed (hcp) metal lattices were synthesized at a hydrogen/deuterium pressure of 9 GPa and a temperature of 873 K using toroid-type high-pressure chambers. After a rapid cooling (quenching) to 100 K and lowering the pressure to atmospheric, the quenched sample was removed from the chamber and studied by hot extraction analysis, powder x-ray diffraction, and ac magnetic susceptometry. The hot extraction showed that thermal decomposition of both hcp-${\mathrm{ZrH}}_{3}$ and hcp-${\mathrm{ZrD}}_{3}$ in vacuum begins after heating at a rate of 10 K/min to \ensuremath{\sim}200 K and almost ceases at 270 K, leaving tetragonal $\ensuremath{\varepsilon}\text{\ensuremath{-}}{\mathrm{ZrH}}_{2}/\ensuremath{\varepsilon}\text{\ensuremath{-}}{\mathrm{ZrD}}_{2}$ as a result. The x-ray diffraction study at 85 K gave lattice parameters $a=3.443(2)\phantom{\rule{0.16em}{0ex}}\AA{}$ and $c=5.964(4)\phantom{\rule{0.16em}{0ex}}\AA{}$ for hcp-${\mathrm{ZrH}}_{3}$ and somewhat smaller values of $a=3.430(5)\phantom{\rule{0.16em}{0ex}}\AA{}$ and $c=5.947(5)\phantom{\rule{0.16em}{0ex}}\AA{}$ for hcp-${\mathrm{ZrD}}_{3}$. The measurements of magnetic susceptibility revealed superconductivity with an onset at 11.6(1) K for hcp-${\mathrm{ZrH}}_{3}$ and 9.5(2) K for hcp-${\mathrm{ZrD}}_{3}$.

Heterogeneous fiberous structured Mg-Zn-Zr alloy with superior strength-ductility synergy

• The heterogeneous fiberous structured ZK60 Mg alloy was obtained by conventional extrusion method with high yield strength of ∼ 345 MPa, ultimate tensile strength of ∼ 370 MPa and high tensile strain of ∼ 20.5%, superior than most ZK60 Mg alloys reported so far. • The excellent mechanical properties were mainly attributed to the heterogeneous fiberous structure that was characterized by alternating coarse- and fine-grain layers, nanoscale precipitates and profuse < c + a > dislocations. The novel heterogeneous fiberous structured ZK60 Mg alloy have achieved superior strength-ductility synergy. • A novel design of alternating coarse- and fine-grain layered structure was considered to be a general strategy for promoting the strength-ductility synergy of hexagonal close-packed (hcp) metals. Here we reported a heterogeneous fiberous structured Mg-5.6Zn-0.6Zr (wt%) alloy obtained by conventional extrusion method, which exhibited high yield strength of ∼ 345 MPa, ultimate tensile strength of ∼ 370 MPa, and high tensile strain of ∼ 20.5%, superior to most of the Mg-Zn based alloys reported so far. The extraordinarily high mechanical properties were mainly attributed to the heterogeneous fiberous structure consisting of alternating coarse- and fine-grain layers. Grains in the different layers grew into the neighboring layers, ensuring a good layer bonding. A high Schmid factor and geometric compatibility factor for pyramidal slip led to full slip transfer between the neighboring coarse grains and fine grains, which could help to release the stress concentration and avoid early fracture. The profuse activated <c + a> glide dislocations could render the unprecedented high tensile strain. The constraint by the hard fine-grain domains made the soft coarse-grain domains strong like the hard fine-grain domains, as well as the nanoscale precipitates pinning dislocations, contributed to the high strength. The heterogeneous microstructure design was shown to have synergistic improvement in strength-ductility balance, which could be an inspiring strategy to improve mechanical properties of hexagonal close-packed (hcp) metals.

N. A. Bulienkov’s Simplicial-Modular Design As a Basis for Modeling Metal Clusters

The author considers the main principles of Bulienkov’s simplicial-modular design of metal clusters (face-centered cubic (FCC), body-centered cubic (BCC), hexagonal close-packed (HCP) metals). Models of clusters are presented along with algorithms for their construction based on operations of twinning (bar {1}, 2, m) to simplices and modules of the corresponding metals.

Thermal self-reduction of metal hydroxide acrylate monolayer nanoparticles leads formation of nanoparticulate and porous structured alloys.

Chemical and physical designs of alloy nanomaterials have attracted considerable attention for the development of highly functional materials. Although polyol processes using ionic precursors are widely used to synthesise alloy nanoparticles, the reduction potential of polyols limits their chemical composition, making it difficult to obtain 3d transition metals. In this study, we employed pre-synthesized metal hydroxide salt monolayer nanoparticles as precursors to obtain alloy nanoparticles. Simultaneous dehydroxylation of the hydroxide moiety and decomposition of the organic moiety allowed the formation of stable face-centred cubic metals passing through the metal carbide and metastable hexagonal close-packed metal phases. This self-reduction process enabled the formation of nanoparticulate bimetallic alloys and macroporous/mesoporous-structured bimetallic alloys by compositing hard/soft templates with pre-synthesized metal hydroxide salt nanoparticles. We believe that the strategy presented in this study can be used to design nanostructures and chemical compositions of multimetallic alloy nanoparticles as well as bimetallic systems.

Geometric compatibility measure m′ for twin transmission: A predictor or descriptor?

In this work, the geometric compatibility factor m’ is critically analyzed to assess whether it can be used to interpret/predict twin transmission (TT) across grain boundaries (GBs). This geometric measure is widely used to relate the likelihood of TT to the misalignment of both the shear and plane-normal directions within a twin set (i.e., incoming and outgoing twin). Using a large set of electron back scattering diffraction (EBSD) data, a detailed statistical analysis of twin-GB interactions is performed for { 10 1 ̅ 2 } tensile twins in hexagonal close-packed (HCP) metals Mg, Zr, and Ti at different strain levels. In addition, a full-field crystal plasticity model is employed to quantify the role of local stresses and the applicability of m’ as a criterion for the TT process. This combined study addresses the following three main questions: (i) What is the fidelity of m’ in describing experimentally observed TTs? (ii) Can m’ be used as a metric to predict/anticipate TT? (iii) Does m’ naturally capture local stress effects? As a descriptor, m’ cannot rationalize ~25% of TT events observed in Mg or more than 50% of TT events in Zr and Ti. As a predictor, the m’-measure does not predict TT events in over ~50% of twin-GB interactions analyzed. Further, the applicability of m’ to describe and predict TT events decreases with an increase in elastic anisotropy, plastic anisotropy, and macroscopic strain levels. Finally, the twinning simulations reveal that m’ does not capture the key effects of local stresses on variant selection upon twin transmission. The local stress induced by the twinning shear transformation plays a dominant role in driving the TT process compared to the geometric alignment of the constituting twins, i.e., m’ .

Deformation mechanism in the mechanical response of nano polycrystalline HCP Zr and Zr-2.5 Nb: An atomistic study

Deformation-induced dislocations in polycrystalline hexagonal close-packed (HCP) metals have a great influence on the mechanical properties of materials. We use atomistic simulations to study the deformation behavior of Zr and Zr-2.5 Nb. We investigate the effects of grain orientation and loading directions on the nucleation and interactions of dislocations. We find that different deformation modes are activated under different loading directions, which may explain experimentally-observed superior mechanical strength under transverse loading than under radial loading. In addition, our simulations show that alloying leads to a more deformation-resistant material, due to the segregation of solute atoms to grain boundaries (GBs) as well as dislocation pinning, which makes emission from GBs more difficult. This study lays the foundation for investigating deformation through atomistic structural analysis.

A new constitutive model to describe evolving elastoplastic behaviours of hexagonal close-packed sheet metals

This study develops a new phenomenological constitutive model to capture the unique evolving cyclic elastoplastic behaviours of hexagonal close-packed (HCP) sheet metals. This new constitutive model is developed by adopting the concepts of multiple-yield surface approaches. Four phenomenological deformation modes, including Monotonic Compression (MC), Monotonic Tension (MT), Reverse Compression (RC), and Reverse Tension (RT), are considered to represent the hardening evolution of the materials, including the twining/untwining behaviours. Reference flow stress equations are introduced, and a Cazacu-Barlat 2004 (CB2004) type yield surface is employed to each deformation mode. In addition, the RT hardening parameters are defined as functions of plastic pre-strains to mitigate the interpolation error caused by parameter determination processes of existing models. For validation, the calculated stress–strain curves of AZ31B magnesium alloy are compared with experimental curves available from literature. Moreover, to show the accuracy of the proposed analytical model, the reproduced stress–strain curves are compared with those of an existing model—the modified homogeneous anisotropic hardening (HAH) model. The obtained results show that the new constitutive model can successfully reproduce experimental Tension–Compression-Tension (TCT) and Compression-Tension–Compression (CTC) stress–strain curves of HCP sheet metals with considerably less percentage errors.

Helical Substructures of Close-Packed Metals Determined by a Universal Building Unit (Tetrablock)

Helical Substructures of Close-Packed Metals Determined by a Universal Building Unit (Tetrablock)

Faceting and Twin–Twin Interactions in {1121} and {1122} Twins in Titanium

Twin–twin interactions are an important component of the microstructural evolution of hexagonal close-packed metals undergoing plasticity. These interactions are prevalent because of the predominance of twinning due to limited easy slip modes. Despite their importance, the complexities of the atomic-scale behavior of interacting twins has limited robust characterization. Using interfacial defect theory, we developed a three-dimensional model of twin–twin interactions, double twinning and other complex interfacial reactions that occur between twins acting on different interface planes. Using molecular dynamics, {1122} and {1121} twins in titanium were activated and produced facets, twin–twin interactions and double twins that we characterized with our model. The results showed excellent agreement between the molecular dynamics results and the model. Surprisingly, some highly ordered and mobile boundaries can be produced by these complex reactions, which could provide important insights for higher scale models of plasticity.

Anomalously Isotropic Electron Transport and Weak Electron-Phonon Interactions in Hexagonal Noble Metals.

The electrical transport properties of typical hexagonal metals are anisotropic because of their anisotropic lattice structures. Unexpectedly, we show that the electron transport properties in hexagonal close-packed (hcp) noble metals are almost isotropic. Although the electron transport properties of an individual electronic band are highly anisotropic, the total contributions are almost equal in different crystalline orientations because of the complementary contributions of different bands. The electron transport is severely limited by phonons for metals with multisheet Fermi surfaces and optical phonon polarizations. However, it is found the electron-phonon interactions are weak in hcp noble metals, although their Fermi surfaces and phonon spectra are complicated. The electronic structure acts as a phonon filter, resulting in small electron-phonon scattering rates. The weak electron-phonon interactions are beneficial to electron and thermal transport, indicating hcp noble metals have great potential to be used in electronics and solar cells.

Effect of stacking faults on magnetic properties and magnetization reversal in Co nanowires

Ferromagnetic 3d transition metal nanowires based on the assembly of shape anisotropy seem apt candidates to replace materials based on rare-earth elements in current permanent magnets. Recently, great efforts have been devoted into enhancing the coercivity of Co nanowires, through accurate control of shape and morphology. Nevertheless, stacking faults can be formed relatively easily in close-packed metals like Co, and its effect on the magnetic properties was usually obscured by the geometrical effect and being ignored. In this work, the Co nanowires with different aspect ratios were synthesized by solvothermal chemistry processes. The effect of aspect ratio on the magnetic properties of Co nanowires was investigated, and a high magnetic property with a coercivity of 9 kOe was obtained in the Co nanowires with an aspect ratio of 10. The stacking faults were observed in the Co nanowires with an aspect ratio beyond 10, which may be a contributing factor to the lower coercivity than expected. The micromagnetic simulation was employed to reveal the effect of stacking faults on the magnetization reversal mechanism, based on which the coercivity degradation mechanism was discussed. This work is instructive for developing high-performance magnetic nanowires.