Twin Structure Research Articles

Gradient Nanotwins and Enhanced Weighted Mobility Synergistically Upgrade Bi0.5Sb1.5Te3 Thermoelectric and Mechanical Performance

AbstractBi2Te3‐based alloys have historically dominated the commercial realm of near room‐temperature thermoelectric (TE) materials. However, the more widespread application is currently constrained by its mediocre TE performance and inferior mechanical properties resulting from intrinsic hierarchical structure. Herein, microstructure modulation and carrier transport optimization strategies are employed to efficiently balance the electro‐thermal transport performance. Specifically, the weighted mobility increases by 24%, while the lattice thermal conductivity decreases by 31% at 350 K compared to the matrix. Consequently, the Bi0.5Sb1.496Cu0.004Te2.98 sample attains a peak ZT of 1.45 at 350 K and an average ZT of 1.20 (300–500 K). Moreover, intricated microstructure design, exemplified by the gradient twin structure, significantly enhances the mechanical performance metrics, including Vickers hardness, compressive strength, and bending strength, to notable levels of 0.94 GPa, 224 MPa, and 58 MPa, respectively. Consequently, the constructed 17‐pair TE modules demonstrate a maximum conversion efficiency of 6.5% at ΔT = 200 K, surpassing the majority of reported Bi2Te3‐based modules. This study provides novel insights into the synergistic enhancement of TE and mechanical properties in Bi2Te3‐based materials, with potential applicability to other TE systems.

Uneven Strain Distribution Induces Consecutive Dislocation Slipping, Plane Gliding, and Subsequent Detwinning of Penta-Twinned Nanoparticles.

Twin structures possess distinct physical and chemical properties by virtue of their specific twin configuration. However, twinning and detwinning processes are not fully understood on the atomic scale. Integrating in situ high resolution transmission electron microscopy and molecular dynamic simulations, we find tensile strain in the asymmetrical 5-fold twins of Au nanoparticles leads to twin boundary migration through dislocation sliding (slipping of an atomic layer) along twin boundaries and dislocation reactions at the 5-fold axis under an electron beam. Migration of one or two layers of twin planes is governed by energy barriers, but overall, the total energy, including surface, lattice strain, and twin boundary energy, is relaxed after consecutive twin boundary migration, leading to a detwinning process. In addition, surface rearrangement of 5-fold twinned nanoparticles can aid in the detwinning process.

Atomic-scale observation of nucleation- and growth-controlled deformation twinning in body-centered cubic nanocrystals

Twinning is an essential mode of plastic deformation for achieving superior strength and ductility in metallic nanostructures. It has been generally believed that twinning-induced plasticity in body-centered cubic (BCC) metals is controlled by twin nucleation, but facilitated by rapid twin growth once the nucleation energy barrier is overcome. By performing in situ atomic-scale transmission electron microscopy straining experiments and atomistic simulations, we find that deformation twinning in BCC Ta nanocrystals larger than 15 nm in diameter proceeds by reluctant twin growth, resulting from slow advancement of twinning partials along the boundaries of finite-sized twin structures. In contrast, reluctant twin growth can be obviated by reducing the nanocrystal diameter to below 15 nm. As a result, the nucleated twin structure penetrates quickly through the cross section of nanocrystals, enabling fast twin growth via facile migration of twin boundaries leading to large uniform plastic deformation. The present work reveals a size-dependent transition in the nucleation- and growth-controlled twinning mechanism in BCC metals, and provides insights for exploiting twinning-induced plasticity and breaking strength-ductility limits in nanostructured BCC metals.

Visualization of structural domains in a single crystal of iron pnictide EuFe<sub>2</sub>As<sub>2</sub>

It is known that during the synthesis of superconducting EuRbFe4As4 single crystals, inclusions of its parent EuFe2As2 phase are encountered. The kinetics of the formation of the orthorhombic phase in EuRbFe4As4 crystals containing a large fraction of the EuFe2As2 phase has been studied using polarization optical microscopy. It is shown that the orthorhombic phase grows into the tetragonal one in stripes with the same crystallographic orientation, forming macro-domains. Then these domains abruptly penetrate parallel stripes of the orthorhombic phase of the second orientation, and a twin system of orthorhombic domains is formed. The process is accompanied by the appearance of macrostresses: stretching and compression waves with a characteristic period of 100–300 μm along and across the twin system, leading to spatial modulation of magnetic permeability. It has been found that even weak magnetic fields (up to 100 Oe) significantly affect the spatial distribution of the twin structure and have an effect similar to external stresses.

Grand herringbone architecture securing the high thermoelectric performance of GeTe

This work demonstrates a unique two-stage cooling procedure for synthesizing high-performance thermoelectric (TE) GeTe alloys with a maximum zT value of 2.0 (720 K) without assistance of chemical doping. The efficiency breakthrough stems from the grand herringbone structure resulted from the annealing step intentionally inserted near the phase transition temperature Tc of GeTe when cooling. Compared with the samples prepared by conventional methods, the two-stage cooled GeTe processes not only lower carrier concentrations p (4.7–5.3 × 1020 cm−3) but also higher mobility (∼125 cm2 V−1 s−1) that prevent electrical conductivity and TE power factor from declining as p decreases. The twinned domains in the two-stage cooled samples are further analyzed and classified by the compatibility theory. Our calculations show that the twinning structures in the two-stage cooled samples fit well with the compatibility criterion for twin variant selections and are thermodynamically in a stable state. These results not only attest to the efficacy of using the two-stage cooling procedure to manipulate the microstructures of GeTe but also provides a new paradigm for optimizing the performance of GeTe-based TE materials.

Understanding Twinning and Hierarchical Structure of Synthetic Guanine with 4D-Scanning Transmission Electron Microscopy

Understanding Twinning and Hierarchical Structure of Synthetic Guanine with 4D-Scanning Transmission Electron Microscopy

Research Progress of Weld Tracking Image Processing Technology Based on Deep Learning Theory

Abstract In this paper, a convolutional neural network is used to localize the weld seam feature points with noise interference in complex welding environments. A priori frames are introduced into the feature point extraction network, combined with position prediction and confidence prediction, to improve the accuracy and anti-interference ability of the weld tracking system. To improve welding efficiency by utilizing the continuity of weld tracking, the weld tracking network is designed based on the twin structure. The weld detection network designs the first frame to locate the key position of the bevel and inputs into the weld tracking network as a template, and the weld tracking network completes the automatic tracking of the subsequent welds. At the same time, the network introduces a hybrid domain attention mechanism, which makes full use of the weld feature channel dependence and spatial location relationship and puts more attention near the inflection point of the weld laser line to ensure the accuracy of weld tracking. The research results show that the extraction error of weld seam feature points based on the convolutional neural network is within 17, which is much lower than that of the grayscale center of gravity method and Steger's algorithm. In the weld tracking experiments under the workpiece tilting state, the average value of the absolute error of the tracking trajectory in the X-axis direction is not more than 0.7 mm, and the maximum value is less than 1.15 mm. The absolute tracking error in the Z-axis direction is relatively low, with an average of 0.638 mm and a maximum of 1.573 mm. Therefore, the weld-tracking image processing technique proposed in this paper has strong anti-noise interference capabilities and high localization accuracy. And high accuracy in localization.

The structure of deformation twins in BCC transition metals during nucleation and growth

Nucleation and growth of deformation twins in BCC transition metals have been investigated in many computational and experimental studies without conclusively describing the critical twin nuclei thickness and its subsequent growth. While most studies conclude that the thickness is three layers by assuming conservation of Burgers vector and nucleation from a screw dislocation, recent experimental studies have contradicted this, suggesting a minimum of six layers. Here we demonstrate, under nonzero finite pure shear stresses, that the minimum thickness during twin nucleation is two layers in Group VB and VIB metals whereas it is three layers in iron. The twinning dislocations that constitute the nuclei have been determined which are dependent on metal's electronic structure i.e, position in periodic table. Growth of twins occurs in a layer-by-fashion maintaining isosceles twin boundary structures in Group VB metals and Fe whereas the more commonly acknowledged reflection twin boundaries are favored in Group VIB.

Tri-functional carbon nanocages coated CoNi electrocatalyst with micro twinning structure for high-performance electrochemical devices

The twinning structure could lead to unexpected effects on electrocatalytic performance; however, research on relevant mechanisms is still rare and there is still a lack of an effective and convenient strategy to introduce twinning structures into nanoparticles. In this work, inspired by the in-situ fcc-hcp phase transition in metals and benefiting from the features of Prussian blue analogues (PBAs), N-doped carbon cages-coated CoNi nanoparticles, which contain multi-active sites including alloyed nanoparticles, micro twinning structures and single atom sites could be effectively fabricated by pyrolyzing of CoNi-based PBAs. The catalyst exhibits an obvious enhancement in oxygen evolution/reduction reaction and hydrogen evolution reaction under alkaline conditions. Theoretical calculations verify that the micro-twinning structures originating from the fcc-hcp phase transition promote electrocatalytic activities significantly due to the optimized adsorption energy towards different intermediates. Moreover, the synergic effect between the multi-active sites may further optimize the electronic structures and lead to the deviation from the Scaling Effect. The water-splitting device and Zinc-Air battery are also assembled based on this micro- twinning electrocatalyst, which yields a maximum power density of 95 mW cm−2 and a low voltage of 1.76 V to reach 10 mA cm−2 current density, respectively. In all, this work contributes a simple and universal strategy to introduce both the multi-active sites and the twinning structure into transition metal-based catalysts, which improves the multi-catalytic activity towards different reactions significantly.

Deformation twinning and feature size mediated strain hardening behavior in a medium-entropy alloy at the mesoscopic scale

MP35N medium-entropy alloy with low stacking fault energy (SFE) is widely used in the biomedical field due to its excellent biocompatibility and mechanical property matching. With the miniaturization of products in the biomedical field, the deformation behavior of MP35N alloy has changed, which is different from the traditional plastic deformation at the macroscopic scale. In the present paper, tensile experiments of MP35N alloy at the deformation of 293 K and 93 K at the mesoscopic scale were carried out to analyze its mechanical properties and deformation mechanism. The results show that the reduction of the deformation temperature leads to a synergistic increase in the strength and plasticity of MP35N alloy. The feature size (ratio of sample thickness to grain size, t/d) and deformation twins are decisive in the mesoscopic deformation process. At the same deformation temperature, the reduction of the feature size makes the deformation twin occupy a dominant position in the competition with dislocations. At the deformation temperature of 93 K, the strain corresponding to the first inflection point decreases from 0.18 to 0.12 as t/d decreases from 8.21 to 1.52. However, the alloy with t/d of 8.21 preferentially produces the second inflection point. The decrease in deformation temperature drives the inflection point down to a larger strain for the same feature size. The reduction of deformation temperature gives dislocations an advantage in competition with deformation twins in the initial deformation stage. The decrease in deformation temperature promotes the emergence of multi-level deformation twin structures in the later deformation stage, causing the second inflection point to move toward larger strains.

Multiple morphologies and twin structure generated by a definite growth behavior of grain boundary M23C6 in tempered Fe–15Mn–3Al-0.7C steel

M23C6 carbide can precipitate simultaneously on both sides of austenite grain boundary in Fe–15Mn–3Al-0.7C steel tempered at 600 °C. In this paper, the morphology and growth behavior of M23C6 are studied by means of OM, SEM and TEM. The results show that the morphology of M23C6 located on both sides of grain boundary is different. On one side of the grain boundary, the carbides appear to be dispersed and disconnected, each with an irregular particle-like morphology, while on the opposite side, they are dispersed along the grain boundary in a short-rod morphology, and the adjacent ones tend to join together to form a network structure. In fact, regardless of which side of the grain boundary the carbides are on, they exhibit a definite growth behavior, growing along the close-packed plane and grain boundary. Also, the morphology of M23C6 depends on the angle between the close-packed plane and the grain boundary. In addition, twin structures with the same orientation can be found in both austenite and M23C6. A notable distinction between them is that there is a flat twin plane in the austenite twin structure but not in the carbide twin structure. Therefore, this paper highlights that the formation of these two twin structures is different. The austenite twin structure is caused by quenching treatment, while the carbide twin structure is formed by a new method called “growing-contacting”.

Separation of Nanoparticle Seed Pseudoisomers via Amplification of Their Crystallographic Differences.

Seed-mediated syntheses rely on small nanoparticle (NP) precursors that act as templates for growth but are often inhomogeneous with respect to their internal twinning structures (e.g., single crystalline, multiply twinned), leading to nonuniform product morphologies. To address this, we developed a method for separating seed NPs of the same approximate size (∼ 10 nm) but with different interior twinning (i.e., NP "pseudoisomers") by exaggerating their crystallographic differences through heteroexpitaxial metal overgrowth. Specifically, single crystalline and pentatwinned Au seeds that are natively inseparable via traditional methods exhibit drastically different Ag shell morphologies that allow for their selective precipitation through colloidal depletion forces. Oxidation of the Ag shell from separated particles results in seeds that are both size uniform and crystallographically pure (>99%), allowing for the controlled synthesis of a library of Oh- and D5h-symmetric gold NPs bearing {111}, {110}, {730}, {310}, {720}, and {100} facets, several of which have no precedent in the literature. These results lay the foundation for precision nanosynthesis by establishing a new paradigm for the purification of NP precursors.

Strength-ductility synergy in twinned titanium fabricated via dynamic equal channel angular pressing and heat treatment

Dynamic equal channel angular pressing (DECAP) and heat treatment are applied to pure Ti to achieve a strength-ductility synergy. DECAP induces a complex multiscale twin structure in Ti including four primary twin types, secondary twins and tertiary twins with multiple variants, and an increased yield strength (58% higher than the as-received Ti). Subsequent annealing at 200 ℃ and 400 ℃ leads to improved ductility and strength. The yield strength (σy) and total elongation (ɛt) for the Ti sample processed with DECAP and 400 ℃ annealing are 555 MPa and 30%, respectively, and its σyɛt product is ∼45% higher than the as-received Ti. DECAP and subsequent annealing lead to a superior strength-ductility synergy in twinned Ti, providing a novel approach to produce high strength and ductility materials via dynamic deformation and subsequent heat treatment.

Unlocking the Potential of Li-Rich Mn-Based Oxides for High-Rate Rechargeable Lithium-Ion Batteries.

Lithium-rich Mn-based oxides have gained significant attention worldwide as potential cathode materials for the next generation of high-energy density lithium-ion batteries. Nonetheless, the inferior rate capability and voltage decay issues present formidable challenges. Here, a Li-rich material equipped with quasi-three-dimensional (quasi-3D)Li-ion diffusion channels is initially synthesized by introducing twin structures with high Li-ion diffusion coefficients into the crystal and constructing a "bridge" between different Li-ion diffusion tunnels. The as-prepared material exhibits monodispersed micron-sized primary particles (MP), delivering a specific capacity of 303mAhg-1 at 0.1 C and an impressive capacity of 253mAhg-1 at 1 C. More importantly, the twin structure also serves as a "breakwater" to inhibit the migration of Mn ions and improve the overall structural stability, leading to cycling stability with 85% capacity retention at 1 Cafter 200 cycles. The proposed strategy of constructing quasi-3Dchannels in the layered Li-rich cathodes will open up new avenues for the research and development of other layered oxide cathodes, with potential applications in industry.

Five-fold twin structures in sputter-deposited nickel alloy films

The deposition of a Ni-1at. % P film resulted in the facilitation of several quintuple twin junctions that give rise to a five-fold twin structure along the film's growth direction. This type of grain growth promoted a pentagonal pyramid surface topology. Upon increasing the deposition rate or the addition of cobalt to reduce the stacking fault energy, an increase in the areal density of such features was observed. Correlated atomic contrast imaging by electron microscopy with atom probe tomography revealed the preference of phosphorus and residual oxygen Cottrell atmospheres around a dislocation network that would be required to accommodate the misorientation strain from a five-fold twin structure filling space. The ability to tune these structures, over a large area of material, offers grain boundary engineering opportunities to readily create novel surface topologies that can act, for example, as an array of field emitters or textured surfaces.

Axial and bending very high cycle fatigue of completion string

Preventing or mitigating very high cycle fatigue (VHCF) damage and failure of completion string is essential for ensuring their long-term stable operation. Therefore, a comprehensive understanding of the string's fatigue performance in tension, compression, and bending over an extended service life is necessary. This study investigated the evolution, microscopic mechanisms, fracture behavior, and life prediction of VHCF damage in completion string under axial and bending conditions. It was found that the intragranular twin structures on the critical interface of VHCF source directly participate in lattice reconstruction, enhancing the grain's strain coordination ability, and increasing the proportion of facet area on the fatigue fracture surface. Based on the fracture characteristics of VHCF, the Mayer empirical formula was improved, and a novel convolutional neural network (CNN) based VHCF life prediction model was proposed. Utilizing the CNN's powerful fracture feature extraction and nonlinear modeling capabilities, the VHCF fatigue life prediction accuracy was significantly enhanced, with prediction errors all within 2 times the experimental results' error range. Based on the axial and bending VHCF fracture mechanisms of completion string and CNN, a more advanced method for predicting axial and bending VHCF life is provided, offering theoretical supports for the engineering safety service and life assessment of completion string.

Theoretical design of superhard twinned BC2N

BC2N is an ideal combination of diamond and boron nitride and is expected to exhibit better mechanical properties and thermal stability than boron nitride and diamond, respectively. Recent studies raise exciting prospects of building coherent nanotwinned boundaries to improve performance of ceramics. Herein, we developed a systematic method to construct twin structures of BC2N. The newly identified nanotwinned BC2N variant (τ-BC2N) exhibits better energetic stability and higher mechanical strength than single-crystal BC2N, attributed to the optimized bonding behavior and crystal orientations induced via twinning. The simulated X-ray diffraction patterns of τ-BC2N match those reported in previous experimental data. The nanotwinned BC2N displays superior Vickers hardness of ∼140 GPa, surpassing that of diamond. The computationally tailored approach and results obtained by twin-strengthening strategies offer powerful insights for the rational design and functional optimization of nanotwinned boron–carbon–nitrogen compounds containing intricate multiatomic constituents and structures.

Morphological control of CaCO3 superstructures in seawater: Insights into Ca-source anion influence and formation mechanism

This work provides crucial insights into the crystallization and morphogenesis of CaCO3 polymorphs in seawater at elevated temperatures. Using Ca source anions as a controlling factor, we modify the crystallization of CaCO3 and demonstrate that stable and intricate particle morphologies of CaCO3, such as hexagonal bipyramidal, disks, and twin structures, can be generated in seawater without surfactants. The synthesis of CaCO3 was accomplished through a direct hydrothermal method utilizing water-soluble Ca salts (calcium chloride, calcium nitrate, and calcium acetate) as Ca source and urea as a carbonate source. Depending on the specific source anions used, various morphological changes were observed. The CaCO3 prepared with the nitrate source yields vaterite as the major polymorph with hexagonal bipyramidal morphology. Pseudohexagonal prisms of aragonite were the primary product obtained with calcium acetate as the source. In contrast, chloride ions yielded vaterite and aragonite depending on the reaction temperature and time. The purity of CaCO3 prepared with seawater was > 94 % regardless of the Ca source. We observe that the phase formation follows the order of gypsum-vaterite-aragonite with an increase in hydrothermal reaction time. Further, we propose the formation mechanism of vaterite hexagonal bipyramidal and aragonite pseudohexagonal prism superstructures where directional aggregation of particles occurs in the presence of ammonium ions and seawater solvent. These findings, obtained through the exploration of seawater and different Ca sources, offer a comprehensive understanding of how source anions influence the selective formation of CaCO3 polymorph and its morphological tuning. This information can be applied to tailor the synthesis of CaCO3 for specific applications.

Microstructure Evolution and Deformation Mechanism of Ti‐55531 Alloy During Hot‐Rolling Process

Ti‐55531 alloy is a novel near‐β‐Ti alloy, which has promising application potential as critical aircraft components due to its outstanding mechanical properties. In this work, it is aimed to tailor the microstructure and mechanical properties of Ti‐55531 alloy by hot‐rolling in α + β‐phase field. In experimental results, it is indicated that an increasing number of α lamellae becomes globular or ellipsoidal shape with the increase of rolling reduction. As a result, the strength and elongation of the rolled samples continually increase. Lath‐like substructures are observed inside some residual lamellar α after 20% rolling reduction. Nearly all lath‐like substructures have their habit plane corresponding to (100)α and exhibit the orientation relationship (OR) of [0001]HCP //[001]FCC, [20]HCP //[]FCC, and (100)HCP //(FCC with α phase. At high rolling reductions, the face‐centered cubic (FCC) substructures disappeared. Instead, {101} twin structures are frequently observed in lamellar α grains, indicating that both {101} twinning and martensitic transformation can be activated in Ti‐55531 alloy to accommodate the applied rolling deformation. To the authors’ knowledge, such stress‐induced martensitic transformation has not been reported before in Ti‐55531 alloy. Therefore, the findings in this work should advance the understandings on the deformation behavior and mechanism in this alloy.

Molecular dynamics analysis on the indentation hardness of nano-twinned nickel

Molecular dynamics simulations were employed to perform the nanoindentation analysis of nanocrystalline nickel with a variety of crystal structures (single-crystal, polycrystalline, and nano-twinned) and twin layer thicknesses under an indenter of spherical shape. To understand the effects of crystal structure and twin spacing on nanoindentation responses, the indentation force, strain, atomic rearrangement, dislocation, temperature, hardness, Young's modulus and elastic recovery rate of the indentation region were analyzed. It was noted that the three types of nanocrystalline nickel's mechanical properties and elastic recovery rate were greatly affected by their crystal structures. The results show that the hardness of the nano-twinned nickel is negatively correlated with the thickness of the twin layer, and the maximum hardness is 18.1 GPa when the thickness of twin layer is 1.2 nm. For the three structures, in the width direction, the elastic recovery ratio of the nano-twin structure is the largest, and the elastic recovery ratio of the nanotwin structure in the depth direction is the smallest. In the single crystal structure, the compression area is concentrated under the indenter and distributed in a ' V ' shape, and in the polycrystalline and twin structures, the compression area is scattered. In the polycrystalline structure, the compression area were distributed below the indenter and at crystal boundaries. In the nano-twinned structure, the compression area were mainly distributed below the indenter.