Coherent Twin Boundaries Research Articles

Bisphenol A Polyoxyethylene Ether Sodium Sulfate as a New Leveler for Electrodeposition of Copper.

The electrochemical behavior of three types of bisphenol A polyoxyethylene ether sodium sulfate (BPA) is investigated through cyclic voltammetry striping tests. The results indicate that BPA-25 exhibited the most significant inhibition of Cu deposition. The nanotwinned Cu (nt-Cu) films are prepared by direct current electrodeposition in an acidic Cu sulfate bath containing a combination of conventional bath additives. Transmission electron microscopy (TEM) and cross-sectional scanning electron microscopy (SEM) results revealed that nt-Cu consisted of microcolumnar grains with high-density nanoscale coherent twin boundaries oriented parallel to the growth surface. The interaction between BPA-25 and the poly2-sulfur-2-propane sodium sulfonate (SPS) was further analyzed through galvanostatic measurements (GMs) and electron backscatter diffraction (EBSD). The results demonstrate that BPA-25 and SPS underwent competitive adsorption on the cathode surface, leading to the formation of nt-Cu. This growth mechanism provides a new perspective for the preparation of nt-Cu by using direct current electrodeposition.

The deformation mechanisms responsible for strain localization in nanotwinned nickel alloys

Nanotwinned Ni-Mo-W alloys possess a combination of unique mechanical and thermal properties, such as ultrahigh strength and microstructural stability, which are correlated with the presence of densely packed growth twins. In a previous study, the ultrahigh compressive strength of Ni84Mo11W5 (atomic percent) micropillars was associated with the formation of highly localized shear bands, but the trigger for such localized plasticity was not identified. Here, Ni86Mo3W11 (atomic percent) micropillars were carefully compressed to various levels to uncover the nanoscale deformation mechanisms that trigger the strain localization. Post-mortem transmission electron microscopy investigations of pillars after the first measurable strain burst revealed ∼50 nm thick shear bands consisting of reoriented and twin-free grains, while the columnar grains adjacent to the shear bands were partly detwinned. More importantly, unlike the Mo-rich pillars, the W-rich pillars showed discernible plasticity before the first strain burst. Close inspection made before the formation of a mature shear band revealed a detwinning region of ∼30 nm thickness that aligned more parallel to the coherent twin boundaries, and multiple nanotwins truncated with incoherent twin boundaries were resolved between the detwinning band and the nanotwinned grains. These observations strongly suggest detwinning, facilitated by migration of incoherent twin boundaries, to be the precursor to strain localization and the intensive shear banding observed in nanotwinned Ni-Mo-W alloys. Comparing the present results with the literature further highlights the general role of detwinning in governing the plastic behavior of nanotwinned alloys with a wide range of stacking fault energy.

Effect of hot rolling and annealing on microstructure, crystallographic texture, and mechanical properties of Fe11.5 Co20.6 Ni40.7 Cr12.2 Al7.8 Ti7.2 high entropy alloy

The evolution of microstructure and texture during the thermomechanical processing of a Fe-Co-Ni-Cr-Al-Ti dual phase high entropy alloy consists of L12 (γ′) ordered precipitates in the disordered FCC (γ) matrix and its effect on the mechanical properties was investigated. The as-cast alloy was annealed at 1100 °C after 25 % and 50 % hot rolling reduction. The grain size becomes finer, and a considerable amount of annealing twin boundaries (Ʃ3 boundaries) developed in the hot rolling and annealed (HRA) samples. The α-fiber (ND||<110>) texture and twin-related texture components increase after HRA treatment. The mechanical response at room temperature is evaluated employing hardness and uniaxial tensile tests. A significant reduction in hardness, from 448 ± 6 HV to 381 ± 12 HV, is observed in the 50 % hot rolled annealed sample compared to the as-cast sample. Additionally, the yield strength reduced from 779 to 595 MPa with an increment in the ductility from 12.5 to 17.2 % in the as-cast and HR50A samples, respectively. Finally, it has been demonstrated that the increase in coherent twin boundaries and grain size excluding twin boundaries, led to the observed mechanical behavior.

Unraveling nano-scale effects of topotactic reduction in LaNiO2 crystals

Infinite-layer nickelates stand as a promising frontier in the exploration of unconventional superconductivity. Their synthesis through topotactic oxygen reduction from the parent perovskite phase remains a complex and elusive process. This study delves into the nano-scale effects of the topotactic lattice transformation within LaNiO2 crystals. Leveraging high-resolution scanning transmission electron microscopy and spectroscopy, our investigations uncover a panorama of structural alterations, including grain boundaries and coherent twin boundaries, triggered by reduction-induced transformations. In addition, our analyses unveil the formation of an oxygen-rich disordered transition phase encircling impurities and pervading crystalline domains and the internal strain is accommodated by grain boundary formation. By unraveling these nano-scale effects, our findings provide insights into the microscopic intricacies of the topotactic reduction process elucidating the transition from the perovskite to the infinite-layer phase within nickelate bulk crystals.

Unconventional hcp/fcc Nickel Heteronanocrystal with Asymmetric Convex Sites Boosts Hydrogen Oxidation.

Developing non-platinum group metal catalysts for the sluggish hydrogen oxidation reaction (HOR) is critical for alkaline fuel cells. To date, Ni-based materials are the most promising candidates but still suffer from insufficient performance. Herein, we report an unconventional hcp/fcc Ni (u-hcp/fcc Ni) heteronanocrystal with multiple epitaxial hcp/fcc heterointerfaces and coherent twin boundaries, generating rugged surfaces with plenty of asymmetric convex sites. Systematic analyses discover that such convex sites enable the adsorption of *H in unusual bridge positions with weakened binding energy, circumventing the over-strong *H adsorption on traditional hollow positions, and simultaneously stabilizing interfacial *H2O. It thus synergistically optimizes the HOR thermodynamic process as well as reduces the kinetic barrier of the rate-determining Volmer step. Consequently, the developed u-hcp/fcc Ni exhibits the top-rank alkaline HOR activity with a mass activity of 40.6 mA mgNi -1 (6.3 times higher than fcc Ni control) together with superior stability and high CO-tolerance. These results provide a paradigm for designing high-performance catalysts by shifting the adsorption state of intermediates through configuring surface sites.

An alignment algorithm using coherent twin boundaries as internal reference in 3D-EBSD.

A three-dimensional (3D) microstructural volume is reconstructed from a stack of two-dimensional sections which was obtained by serial sectioning coupled with electron back scattering diffraction (EBSD) mapping of a 316L austenitic stainless steel. A new alignment algorithm named linear translation by minimising the indicator (LTMI) is proposed to reduce the translational misalignments between adjacent sections by referencing to coherent twin boundaries which are flat and lying on {111} planes. The angular difference between the measured orientation of a flat twin boundary and that of the {111} plane is used as an indicator of the accuracy of the alignment operations. This indicator is minimised through linear translations of the centroids of triangular facets, which constitute grain boundaries at a distance not restricted by the in-plane step size of the EBSD maps. And hence the systematic trend in the translational misalignments can be effectively reduced. The LTMI alignment procedure proposed herein effectively corrects the misalignments remained by other methods on a 3D-EBSD data prepared using serial sectioning methods. The accuracy in distinguishing between coherent and incoherent twin boundaries is significantly improved.

A new criterion for formation capability of annealing twin in face-centered cubic metals/alloys

Annealing twins (ATs) are prevalent in face-centered cubic (FCC) metals/alloys and significantly influence their mechanical properties. Therefore, understanding the formation capability of ATs is crucial for controlling the mechanical properties of FCC metals/alloys. Although the "Stacking Fault Energy" is a widely used factor to characterize AT formation capability, it doesn't comprehensively explain the observed experimental rankings of AT formation capability. In this study, through a combination of experiments and molecular dynamics simulations, we demonstrated that AT growth is primarily driven by the migration of the Σ3 incoherent twin boundary. This migration arises from the elastic strain energy imbalance between grains on either side of the twin boundary caused by the elastic anisotropy of crystal. Based on these findings, we propose a new criterion, defined as a ratio of the coherent twin boundary energy to the anisotropy factor, to represent the AT formation capability (inverse relationship). Such new criterion well explains the experimental observation, and can be generalized to the zero macroscopic strain deformation twin.

Coherent Twin Boundary Induced Phonon Softening in Boron Arsenide

Coherent Twin Boundary Induced Phonon Softening in Boron Arsenide

Role of slip in hydrogen-assisted crack initiation in Ni-based alloy 725.

We conduct in situ tensile straining experiments to investigate the role of hydrogen and slip in crack initiation in nickel-based alloy 725. Our experiments reveal no tendency for hydrogen to enhance localized slip and no necessity of slip for crack initiation. We use electrochemical charging to introduce hydrogen into samples, melt extraction to measure hydrogen content, and digital image correlation to analyze localized plastic strains during in situ tensile tests in a scanning electron microscope. Cracks initiate both in regions with and without nearby localized slip. Moreover, the fraction of cracks initiating with no nearby slip is greater at higher hydrogen content. Slip-assisted crack initiation generally occurs at locations where intergranular slip is arrested, especially at intersections of slipping coherent twin boundaries with thin twin lamellae. Cracks that initiate without nearby slip occur at a wider variety of microstructural features, including inclusions, triple junctions, and surface flaws.

Unraveling Mechanism for Microstructure Engineering toward High-Capacity Nickel-Rich Cathode Materials.

Microstructural engineering on nickel-rich layered oxide (NRLO) cathode materials is considered a promising approach to increase both the capacity and lifespan of lithium-ion batteries by introducing high valence-state elements. However, rational regulation on NRLO microstructures based on a deep understanding of its capacity enhancement mechanism remains challenging. Herein for the first time, it is demonstrated that an increase of 14mAhg-1 in reversible capacity at the first cycle can be achieved via tailoring the micro and nano structure of NRLO through introducing tungsten. Aberration-corrected scanning transmission electron microscopy (STEM) characterization reveals that the formation of a modified microstructure featured as coherent spinel twin boundaries. Theoretical modeling and electrochemical investigations further demonstrate that the capacity increase mechanism is related to such coherent spinel twin boundaries, which can lower the Li+ diffusion barrier and thus allow more Li+ to participate in deeper phase transitions. Meanwhile, the surface and grain boundaries of NRLOs are found to be modified by generating a dense and uniform LiWxOy phase, which further extends its cycle life by reducing side reactions with electrolytes. This work enables a comprehensive understanding of the capacity-increased mechanism and endows the remarkable potential of microstructural engineering for capacity- and lifespan-increased NRLOs.

Introducing high-density growth twins in aluminum alloys by laser surface remelting via templated nucleation of grains

It is difficult to generate coherent twin boundaries in bulk Al alloys due to their high intrinsic stacking fault energy. Here, we report a strategy to induce high-density growth twins in aluminum alloys through the heterogeneous nucleation of twinned Al grains on twin-structured TiC nucleants and the preferred growth of twinned dendrites by laser surface remelting of bulk metals. The solidification structure at the surface shows a mixture of lamellar twinned dendrites with ultra-fine twin boundary spacing (∼2 μm), isolated twinned dendrites, and regular dendrites. EBSD analysis and finite element method (FEM) simulations have been used to understand the competitive growth between twinned and regular dendrites, and the solidification conditions for the preferred growth of twinned dendrites during laser remelting and subsequent rapid solidification are established. It is shown that the reduction in the ratio of temperature gradient G to solidification rate V promotes the formation of lamellar twinned dendrites. The primary trunk spacing of lamellar twinned dendrites is refined by the high thermal gradient and solidification rate. The present work paves a new way to generate high-density growth twins in additive-manufactured Al alloys.

Inhibition of the intergranular brittleness of HR3C heat-resistant steel by strain-aging induced nano-M23C6 dispersion precipitation

M23C6 chromium-rich carbides are common grain-boundary precipitations in Cr-containing steel. The presence of grain-boundary carbides often leads to intergranular brittleness and decreases mechanical properties. This study proposes a deformation and aging technique to obtain a high-volume-fraction dispersion distribution of the hard nano-M23C6 phase by changing the nucleation sites from grain boundaries to deformation coherent twin boundaries produced during cold deformation. The M23C6 precipitation-strengthened austenitic stainless steel has a strength of up to 1.4 GPa but maintains favorable plasticity (> 11 %). This study provides a novel approach for the control of intergranular brittleness in metallic materials.

Unconventional hcp/fcc Nickel Heteronanocrystal with Asymmetric Convex Sites Boosts Hydrogen Oxidation

Developing non‐platinum group metal catalysts for the sluggish hydrogen oxidation reaction (HOR) is critical for alkaline fuel cells. To date, Ni‐based materials are the most promising candidates but still suffer from insufficient performance. Herein, we report an unconventional hcp/fcc Ni (u‐hcp/fcc Ni) heteronanocrystal with multiple epitaxial hcp/fcc heterointerfaces and coherent twin boundaries, generating rugged surfaces with plenty of asymmetric convex sites. Systematic analyses discover that such convex sites enable the adsorption of *H in unusual bridge positions with weakened binding energy, circumventing the over‐strong *H adsorption on traditional hollow positions, and simultaneously stabilizing interfacial *H2O. It thus synergistically optimizes the HOR thermodynamic process as well as reduces the kinetic barrier of the rate‐determining Volmer step. Consequently, the developed u‐hcp/fcc Ni exhibits the top‐rank alkaline HOR activity with a mass activity of 40.6 mA mgNi‐1 (6.3 times higher than fcc Ni control) together with superior stability and high CO‐tolerance. These results provide a paradigm for designing high‐performance catalysts by shifting the adsorption state of intermediates through configuring surface sites.

Effect of Coherent Twin Boundary on the Low-Cycle Fatigue Property of 321 Austenitic Stainless Steel

Effect of Coherent Twin Boundary on the Low-Cycle Fatigue Property of 321 Austenitic Stainless Steel

Multi-scale annealing twins generate superior ductility in an additively manufactured high-strength medium entropy alloy

Coherent twin boundaries (CTBs) are internal planar defects that offer a promising pathway for designing advanced metallic materials with superior strength-ductility synergy. However, incorporating nanoscale CTBs into additive manufacturing (AM) microstructures is highly challenging without severe plastic deformation. Here, by utilizing the intrinsic cellular structures in AM alloys, we for the first time achieved a high density of multi-scale annealing twins in a laser powder bed fusion (LPBF) Ni35Co35Cr25Ti3Al2 medium-entropy alloy. These multi-scale annealing twins, together with nanoprecipitates and dislocations, resulted in gigapascal strength (∼1.4 GPa) and substantial tensile ductility (∼25 %). We reveal that the AM-induced cellular structures, decorated with entangled dislocations and Ti segregation at the cellular boundaries, facilitate the abundant nucleation of multi-scale annealing twins through interactions with migrating recrystallization boundaries. Additionally, the cellular precipitation networks enhance the thermal stability of nanoscale annealing twins. Frequent dislocation-TB interactions during deformation contribute to superior strain hardenability and thus good ductility. Synergized multiple strengthening mechanisms, i.e., boundary strengthening, precipitation strengthening, and dislocation strengthening, are responsible for the excellent strength. Our present findings advance the design of AM microstructures by harnessing the beneficial effects of cellular structures and provide valuable guidance for developing alloys with exceptional mechanical properties.

New insights into ductility improvement of a nickel-based superalloy through grain boundary engineering

Grain boundary engineering (GBE) provides an effective approach for tailoring the properties of metallic materials by optimizing the grain boundary character distribution (GBCD). Using electron backscatter diffraction analysis and uniaxial tensile testing at room temperature, this study systematically investigated the impact of GBE-type thermomechanical processing (TMP) on the ductility of an Incoloy 925 nickel-based superalloy (Alloy 925). The results showed that at 5 % strain, increasing the annealing temperature results in larger twin-related domains through strain-induced boundary migration. The GBE-type TMP can significantly enhance the ductility of Alloy 925, namely, the elongation increases from 64.22 % to 95.86 % as the annealing temperature increases from 1050 °C to 1100 °C. The evolved GBCD introduces more coherent twin boundaries with lower average residual Burgers vectors, enabling efficient dislocation transmission. In addition, extensive twinning can widely generate some soft orientations and simultaneously reduce the average Taylor factor within the twin clusters, leading to easy dislocation activation during plastic deformation. The synergistic effect of the optimized grain boundary network and twin cluster anisotropy promotes uniform plastic flow and effective strain accommodation, thereby enhancing the ductility of Alloy 925. Overall, this work provides novel and valuable insights into ductility optimization through GBE in a nickel-based superalloy.

A deformation twin mediated sliding-opening zig-zag fracture mechanism in multi-principal element alloys

Multi-principal element alloys (MPEA) have emerged as a class of high-performance materials. Of particular interest is the excellent fracture toughness in face-centered cubic MPEAs with low stacking fault energy, where abundant deformation twins are commonly observed. To understand the fracture mechanism in such MPEAs, here we perform in situ nanomechanical testing inside transmission electron microscope, atomistic simulation, and theoretical analysis to investigate the mechanism of interaction between a propagating crack and a coherent twin boundary (TB) in CoCrFeNi MPEA. We unveil a hitherto unknown microscopic mechanism of crack-TB interaction in MPEAs, where sliding along secondary nanotwins (NTs) nucleated from the primary TB followed by sub-crack nucleation some distance ahead of the main crack constitute a consecutive sliding-opening crack propagation mechanism, leading to zig-zag fracture around the primary TB. The origin of this mechanism lies in the frequent nucleation of NTs from steps on the primary TB, which can effectively blunt and deflect a propagating crack. Quantitative analysis indicates nearly doubled fracture resistance associated with this unique mechanism of crack-TB interaction in MPEAs. These findings bring about a missing aspect in TB-modulated fracture in MPEAs and enrich our understanding of the enhanced damage tolerance of alloys with low stacking fault energy.

Transformation of Coherent Twin Boundary into Basal-Prismatic Boundary in HCP-Ti: A Molecular Dynamics Study.

The manufacturing process for wrought Ti alloys with the hexagonal close-packed (HCP) structure introduces a complicated microstructure with abundant intra- and inter-grain boundaries, which greatly influence performance. In the hexagonal close-packed (HCP) structure, two types of grain boundaries are commonly observed between grains with ~90° misorientation: the basal/prismatic boundary (BPB) and the coherent twin boundary (CTB). The mechanical response of the BPB and CTB under external loading was studied through molecular dynamic simulations of HCP-Ti. The results revealed that CTB undergoes transformation into BPB through the accumulation of twin boundary (TB) steps and subsequent emission of Shockley partial dislocations. When the total mismatch vector is close to the Burgers vector of a Shockley partial dislocation, BPB emits partial dislocations and further grows along the stacking faults. When a pair of CTBs are close to each other, severe boundary distortion occurs, facilitating the emission and absorption of partial dislocations, which further assists the CTB-BPB transformation. The present results thus help to explain the frequent observation of coexisting CTB and BPB in HCP alloys and further contribute to the understanding of their microstructure and property regulation.

Role of geometry and coherent twin boundaries in mechanical response of Cu〈110〉 nanopillars under tensile loading: insights from molecular dynamics simulations

Role of geometry and coherent twin boundaries in mechanical response of Cu〈110〉 nanopillars under tensile loading: insights from molecular dynamics simulations

Characteristic fatigue damage near the Σ3(111) coherent twin boundary in micron-sized copper specimen

To investigate the effect of a Σ3(111) coherent twin boundary on the fatigue damage of micron-sized Cu, a tension–compression cyclic-deformation test was performed on a bicrystal specimen. The fatigue damage (extrusion/intrusion) occurred preferentially around the twin boundary, induced by stress concentration on one side of the specimen due to the deformation constraint between the crystals. Even though stress relaxation occurred on the opposite side, the fatigue damage along the twin boundary penetrated the specimen. The shapes of the extrusion and intrusion of the opposing sides were coincident, and the dislocation density in the damaged region was lower than that in the far region. These results indicate that the dislocations generated at the stress-concentration sites overcame the stress-relaxation effects and traveled along the twin boundary to escape from the opposite side because of the close distance between the surfaces in the specimen. The ease of dislocation ejection promoted the formation of extrusion/intrusion and decreased the fatigue strength of the specimen. The findings will facilitate the development of materials with improved resistance to fatigue damage.