The nanostructured origin of deformation twinning

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.

Visualization and validation of twin nucleation and early-stage growth in magnesium

The abrupt occurrence of twinning when Mg is deformed leads to a highly anisotropic response, making it too unreliable for structural use and too unpredictable for observation. Here, we describe an in-situ transmission electron microscopy experiment on Mg crystals with strategically designed geometries for visualization of a long-proposed but unverified twinning mechanism. Combining with atomistic simulations and topological analysis, we conclude that twin nucleation occurs through a pure-shuffle mechanism that requires prismatic-basal transformations. Also, we verified a crystal geometry dependent twin growth mechanism, that is the early-stage growth associated with instability of plasticity flow, which can be dominated either by slower movement of prismatic-basal boundary steps, or by faster glide-shuffle along the twinning plane. The fundamental understanding of twinning provides a pathway to understand deformation from a scientific standpoint and the microstructure design principles to engineer metals with enhanced behavior from a technological standpoint.

Disconnection-Mediated Transition in Segregation Structures at Twin Boundaries

Twin boundaries play an important role in the thermodynamics, stability, and mechanical properties of nanocrystalline metals. Understanding their structure and chemistry at the atomic scale is key to guide strategies for fabricating nanocrystalline materials with improved properties. We report an unusual segregation phenomenon at gold-doped platinum twin boundaries, which is arbitrated by the presence of disconnections, a type of interfacial line defect. By using atomistic simulations, we show that disconnections containing a stacking fault can induce an unexpected transition in the interfacial-segregation structure at the atomic scale, from a bilayer, alternating-segregation structure to a trilayer, segregation-only structure. This behavior is found for faulted disconnections of varying step heights and dislocation characters. Supported by a structural analysis and the classical Langmuir-McLean segregation model, we reveal that this phenomenon is driven by a structurally induced drop of the local pressure across the faulted disconnection accompanied by an increase in the segregation volume.

Exceptional Deformability of Wurtzite Zinc Oxide Nanowires with Growth Axial Stacking Faults

To ensure reliability and facilitate the strain engineering of zinc oxide (ZnO) nanowires (NWs), it is significant to understand their flexibility thoroughly. In this study, single-crystalline ZnO NWs with rich axial pyramidal I (π₁) and prismatic stacking faults (SFs) are synthesized by a metal oxidation method. Bending properties of the as-synthesized ZnO NWs are investigated at the atomic scale using an in situ high-resolution transmission electron microscopy (HRTEM) technique. It is revealed that the SF-rich structures can foster multiple inelastic deformation mechanisms near room temperature, including active axial SFs' migration, deformation twinning and detwinning process in the NWs with growth π₁ SFs, and prevalent nucleation and slip of perfect dislocations with a continuous increased bending strain, leading to tremendous bending strains up to 20% of the NWs. Our results record ultralarge bending deformations and provide insights into the deformation mechanisms of single-crystalline ZnO NWs with rich axial SFs.

Effect of Ion Irradiation Introduced by Focused Ion-Beam Milling on the Mechanical Behaviour of Sub-Micron-Sized Samples

The development of xenon plasma focused ion-beam (Xe+ PFIB) milling technique enables site-specific sample preparation with milling rates several times larger than the conventional gallium focused ion-beam (Ga+ FIB) technique. As such, the effect of higher beam currents and the heavier ions utilized in the Xe+ PFIB system is of particular importance when investigating material properties. To investigate potential artifacts resulting from these new parameters, a comparative study is performed on transmission electron microscopy (TEM) samples prepared via Xe+ PFIB and Ga+ FIB systems. Utilizing samples prepared with each system, the mechanical properties of CrMnFeCoNi high-entropy alloy (HEA) samples are evaluated with in situ tensile straining TEM studies. The results show that HEA samples prepared by Xe+ PFIB present better ductility but lower strength than those prepared by Ga+ FIB. This is due to the small ion-irradiated volumes and the insignificant alloying effect brought by Xe irradiation. Overall, these results demonstrate that Xe+ PFIB systems allow for a more efficient material removal rate while imparting less damage to HEAs than conventional Ga+ FIB systems.

Direct observation of dual-step twinning nucleation in hexagonal close-packed crystals

Design and processing of advanced lightweight structural alloys based on magnesium and titanium rely critically on a control over twinning that remains elusive to date and is dependent on an explicit understanding on the twinning nucleation mechanism in hexagonal close-packed (HCP) crystals. Here, by using in-situ high resolution transmission electron microscopy, we directly show a dual-step twinning nucleation mechanism in HCP rhenium nanocrystals. We find that nucleation of the predominant {1 0 −1 2} twinning is initiated by disconnections on the Prismatic│Basal interfaces which establish the lattice correspondence of the twin with a minor deviation from the ideal orientation. Subsequently, the minor deviation is corrected by the formation of coherent twin boundaries through rearrangement of the disconnections on the Prismatic│Basal interface; thereafter, the coherent twin boundaries propagate by twinning dislocations. The findings provide high-resolution direct evidence of the twinning nucleation mechanism in HCP crystals.

Aspects of twinning in hexagonal-close-packed crystals remain elusive. Here, the authors directly image twinning in rhenium nanocrystals and show the process is mediated by disconnections on Prismatic│Basal interfaces as the twin initially deviates from its ideal orientation before it is corrected.

Nanoscale stacking fault-assisted room temperature plasticity in flash-sintered TiO2

Ceramic materials have been widely used for structural applications. However, most ceramics have rather limited plasticity at low temperatures and fracture well before the onset of plastic yielding. The brittle nature of ceramics arises from the lack of dislocation activity and the need for high stress to nucleate dislocations. Here, we have investigated the deformability of TiO2 prepared by a flash-sintering technique. Our in situ studies show that the flash-sintered TiO2 can be compressed to ~10% strain under room temperature without noticeable crack formation. The room temperature plasticity in flash-sintered TiO2 is attributed to the formation of nanoscale stacking faults and nanotwins, which may be assisted by the high-density preexisting defects and oxygen vacancies introduced by the flash-sintering process. Distinct deformation behaviors have been observed in flash-sintered TiO2 deformed at different testing temperatures, ranging from room temperature to 600°C. Potential mechanisms that may render ductile ceramic materials are discussed.

Characterization of Nanolayer Intermetallics Formed in Cold Sprayed Al Powder on Mg Substrate

Supersonic impact of particles in their solid state with substrate at a low temperature creates a complex bonding mechanism and surface modification in cold spray coating. Here, we report the formation of a layer of 200 to 300 nm intermetallic at the interface of cold spray coated AZ31B-type Mg alloy with AA7075-type Al alloy powder. XRD, SAED, and FFT analysis confirmed the layer possessed BBC crystal structure of Mg₁₇Al₁₂ intermetallic. The HR-TEM image analysis at the interface identified the BBC crystal structure with interplanar spacing of 0.745 nm for (110) planes, suggesting the Mg₁₇Al₁₂ phase. The nanoindentation tests showed that the hardness at the interface was ~3 times higher than the substrate. It was also noticed that Young’s modulus at the interface was 117GPa. The combined action of impact energy and carrier gas temperature, along with the multiple passes during coating, caused the formation of intermetallic.

Plastic Deformation Behavior of Bi-Crystal Magnesium Nanopillars with a {1012} Twin Boundary under Compression: Molecular Dynamics Simulations

In this study, molecular dynamics simulations were performed to study the uniaxial compression deformation of bi-crystal magnesium nanopillars with a { 10 1 ¯ 2 } twin boundary (TB). The generation and evolution process of internal defects of magnesium nanopillars were analyzed in detail. Simulation results showed that the initial deformation mechanism was mainly caused by the migration of the twin boundary, and the transformation of TB into (basal/prismatic) B/P interface was observed. After that, basal slip as well as pyramidal slip nucleated during the plastic deformation process. Moreover, a competition mechanism between twin boundary migration and basal slip was found. Basal slip can inhibit the migration of the twin boundary, and { 10 1 ¯ 1 } ⟨ 10 1 ¯ 2 ⟩ twins appear at a certain high strain level ( ε = 0.104). In addition, Schmid factor (SF) analysis was conducted to understand the activations of deformation modes.

Deformation twinning mechanism in hexagonal-close-packed crystals

The atomic structure of {10 [Formula: see text] 2} twin boundary (TB) from a deformed Mg-3Al-1Zn (AZ31) magnesium alloy was examined by using high-resolution transmission electron microscopy (HRTEM). By comparing the lattice structure of TB with the previously established model, a kind of special atomic combinations, here named primitive cells (PCs), were discovered at the TB. The PCs reorientation induced mechanism of twinning in hexagonal-close-packed (HCP) crystals was hence verificated. Meanwhile, the relationship between the misorientation of adjacent layers of PCs and the width of TB was discussed. The verification of the mechanism clarifies the twinning mechanism in HCP crystals and opens up opportunities for further researches.

Atypical Defect Motions in Brittle Layered Sodium Titanate Nanowires

In situ tensile tests show atypical defect motions in the brittle Na₂Ti₃O₇ (NTO) nanowire (NW) within the elastic deformation range. After brittle fracture, elastic recovery of the NTO NW is followed by reversible motion of the defects in a time-dependent manner. An in situ cyclic loading-unloading test shows that these mobile defects shift back and forth along the NW in accordance with the loading-unloading cycles and eventually restore their initial positions after the load is completely removed. The existence of the defects within the NTO NWs and their motions does not lead to plastic deformation of the NW. The atypical defect motion is speculated to be the result of the glidibility of the TiO₆ layers, where weakly bonded cation layers are in between. Exploration of the above novel observation can establish new understandings of the deformation behavior of superlattice nanostructures.

High-density deformation nanotwin induced significant improvement in the plasticity of polycrystalline γ-TiAl-based intermetallic alloys

Intermetallic alloys with high melting point can mostly serve as promising high-temperature structural materials, but their intrinsic brittleness limits their further application. Herein, we developed a strategy to realize high strength and high plasticity simultaneously in Cr-rich γ-TiAl-based intermetallic alloys via introducing high-density deformation nanotwins. Non-equilibrium continuous casting followed by annealing in the (α + γ) phase region generated numerous Shockley partial dislocations and stacking faults as well as a number of α2 nanoparticles in the γ-TiAl phase. The substantial Shockley partial dislocations and stacking faults acting as effective heterogeneous nucleation sites favored the generation of high-density nanotwins in the as-annealed alloys during deformation, especially within the γ lamellae. This strategy can also be applied to other brittle alloys with a favorable twinning deformation mechanism and paves the way for the development of high-strength and high-ductility materials.

Asymmetric twins in boron rich boron carbide

Twin boundaries (TBs) play an essential role in enhancing the mechanical, electronic and transport properties of polycrystalline materials. However, the mechanisms are not well understood. In particular, we considered that they may play an important role in boron rich boron carbide (BvrBC), which exhibits promising properties such as low density, super hardness, high abrasion resistance, and excellent neutron absorption. Here, we apply first-principles-based simulations to identify the atomic structures of TBs in BvrBC and their roles for the inelastic response to applied stresses. In addition to symmetric TBs in BvrBC, we identified a new type of asymmetric twin that constitutes the phase boundaries between boron rich boron carbide (B13C2) and BvrBC (B14C). The predicted mechanical response of these asymmetric twins indicates a significant reduction of the ideal shear strength compared to single crystals B13C2 and B14C, suggesting that the asymmetric twins facilitate the disintegration of icosahedral clusters under applied stress, which in turn leads to amorphous band formation and brittle failure. These results provide a mechanistic basis towards understating the roles of TBs in BvrBC and related superhard ceramics.

Achieving High Strength and Ductility in Magnesium Alloys via Densely Hierarchical Double Contraction Nanotwins

Light-weight magnesium alloys with high strength are especially desirable for the applications in transportation, aerospace, electronic components, and implants owing to their high stiffness, abundant raw materials, and environmental friendliness. Unfortunately, conventional strengthening methods mainly involve the formation of internal defects, in which particles and grain boundaries prohibit dislocation motion as well as compromise ductility invariably. Herein, we report a novel strategy for simultaneously achieving high specific yield strength (∼160 kN m kg^-1) and good elongation (∼23.6%) in a duplex magnesium alloy containing 8 wt % lithium at room temperature, based on the introduction of densely hierarchical {101̅1}-{101̅1} double contraction nanotwins (DCTWs) and full-coherent hexagonal close-packed (hcp) particles in twin boundaries by ultrahigh pressure technique. These hierarchical nanoscaled DCTWs with stable interface characteristics not only bestow a large fraction of twin interface but also form interlaced continuous grids, hindering possible dislocation motions. Meanwhile, orderly aggregated particles offer supplemental pinning effect for overcoming latent softening roles of twin interface movement and detwinning process. The processes lead to a concomitant but unusual situation where double contraction twinning strengthens rather than weakens magnesium alloys. Those cutting-edge results provide underlying insights toward designing alternative and more innovative hcp-type structural materials with superior mechanical properties.

In Situ TEM Study of Interaction between Dislocations and a Single Nanotwin under Nanoindentation

Nanotwinned (nt) materials exhibit excellent mechanical properties, and have been attracting much more attention of late. Nevertheless, the fundamental mechanism of interaction between dislocations and a single nanotwin is not understood. In this study, in situ transmission electron microscopy (TEM) nanoindentation is performed, on a specimen of a nickel (Ni) alloy containing a single nanotwin of 89 nm in thickness. The specimen is prepared using focused ion beam (FIB) technique from an nt surface, which is formed by a novel approach under indentation using a developed diamond panel with tips array. The stiffness of the specimen is ten times that of the pristine counterparts during loading. The ultrahigh stiffness is attributed to the generation of nanotwins and the impediment of the single twin to the dislocations. Two peak loads are induced by the activation of a new slip system and the penetration of dislocations over the single nanotwin, respectively. One slip band is parallel to the single nanotwin, indicating the slip of dislocations along the nanotwin. In situ TEM observation of nanoindentation reveals a new insight for the interaction between dislocations and a single nanotwin. This paves the way for design and preparation of high-performance nt surfaces of Ni alloys used for aircraft engines, gas turbines, turbocharger components, ducts, and absorbers.

Hard-sphere displacive model of deformation twinning in hexagonal close-packed metals. Revisiting the case of the (56°,a) contraction twins in magnesium

Contraction twinning in magnesium alloys leads to new grains that are misoriented from the parent grain by a rotation (56°,a). The classical shear theory of deformation twinning does not specify the atomic displacements and does not explain why contraction twinning is less frequent than extension twinning. The paper proposes a new displacive model in line with our previous work on martensitic transformations and extension twinning. A continuous angular distortion matrix that transforms the initial hexagonal close-packed (h.c.p.) crystal into a final h.c.p. crystal is determined such that the atoms move as hard spheres and reach the final positions expected by the orientation relationship. The calculations prove that the distortion is not a simple shear when it is considered in its continuity. The ({0{\overline 1}1}) plane is untilted and restored, but it is not fully invariant because some interatomic distances in this plane evolve during the distortion process; the unit volume also increases up to 5% before coming back to its initial value when the twinning distortion is complete. Then, the distortion takes the form of a simple shear on the ({0{\overline 1}1}) plane with a shear along the direction [{18,{\overline 5},{\overline 5}}] of amplitude 0.358. Experiments are proposed to validate or disprove the model.

Slip-activated surface creep with room-temperature super-elongation in metallic nanocrystals

Nanoscale metallic crystals have been shown to follow a 'smaller is stronger' trend. However, they usually suffer from low ductility due to premature plastic instability by source-limited crystal slip. Here, by performing in situ atomic-scale transmission electron microscopy, we report unusual room-temperature super-elongation without softening in face-centred-cubic silver nanocrystals, where crystal slip serves as a stimulus to surface diffusional creep. This interplay mechanism is shown experimentally and theoretically to govern the plastic deformation of nanocrystals over a material-dependent sample diameter range between the lower and upper limits for nanocrystal stability by surface diffusional creep and dislocation plasticity, respectively, which extends far beyond the maximum size for pure diffusion-mediated deformation (for example, Coble-type creep). This work provides insight into the atomic-scale coupled diffusive-displacive deformation mechanisms, maximizing ductility and strength simultaneously in nanoscale materials.

Deformation mechanisms to ameliorate the mechanical properties of novel TRIP/TWIP Co-Cr-Mo-(Cu) ultrafine eutectic alloys

In the present study, the microstructural evolution and the modulation of the mechanical properties have been investigated for a Co-Cr-Mo (CCM) ternary eutectic alloy by addition of a small amount of copper (0.5 and 1 at.%). The microstructural observations reveal a distinct dissimilarity in the eutectic structure such as a broken lamellar structure and a well-aligned lamellar structure and an increasing volume fraction of Co lamellae as increasing amount of copper addition. This microstructural evolution leads to improved plasticity from 1% to 10% without the typical tradeoff between the overall strength and compressive plasticity. Moreover, investigation of the fractured samples indicates that the CCMCu alloy exhibits higher plastic deformability and combinatorial mechanisms for improved plastic behavior. The improved plasticity of CCMCu alloys originates from several deformation mechanisms; i) slip, ii) deformation twinning, iii) strain-induced transformation and iv) shear banding. These results reveal that the mechanical properties of eutectic alloys in the Co-Cr-Mo system can be ameliorated by micro-alloying such as Cu addition.

Synthesis and Mechanical Characterisation of an Ultra-Fine Grained Ti-Mg Composite

The importance of lightweight materials such as titanium and magnesium in various technical applications, for example aerospace, medical implants and lightweight construction is well appreciated. The present study is an attempt to combine and improve the mechanical properties of these two materials by forming an ultra-fine grained composite. The material, with a composition of 75 vol% (88.4 wt%) Ti and 25 vol% (11.4 wt%) Mg , was synthesized by powder compression and subsequently deformed by high-pressure torsion. Using focused ion beam machining, miniaturised compression samples were prepared and tested in-situ in a scanning electron microscope to gain insights into local deformation behaviour and mechanical properties of the nanocomposite. Results show outstanding yield strength of around 1250 MPa, which is roughly 200 to 500 MPa higher than literature reports of similar materials. The failure mode of the samples is accounted for by cracking along the phase boundaries.

Low-temperature plasticity of olivine revisited with in situ TEM nanomechanical testing

The rheology of the lithospheric mantle is fundamental to understanding how mantle convection couples with plate tectonics. However, olivine rheology at lithospheric conditions is still poorly understood because experiments are difficult in this temperature range where rocks and mineral become very brittle. We combine techniques of quantitative in situ tensile testing in a transmission electron microscope and numerical modeling of dislocation dynamics to constrain the low-temperature rheology of olivine. We find that the intrinsic ductility of olivine at low temperature is significantly lower than previously reported values, which were obtained under strain-hardened conditions. Using this method, we can anchor rheological laws determined at higher temperature and can provide a better constraint on intermediate temperatures relevant for the lithosphere. More generally, we demonstrate the possibility of characterizing the mechanical properties of specimens, which can be available in the form of submillimeter-sized particles only.

Breakdown of Shape Memory Effect in Bent Cu-Al-Ni Nanopillars: When Twin Boundaries Become Stacking Faults

Bent Cu-Al-Ni nanopillars (diameters 90-750 nm) show a shape memory effect, SME, for diameters D > 300 nm. The SME and the associated twinning are located in a small deformed section of the nanopillar. Thick nanopillars (D > 300 nm) transform to austenite under heating, including the deformed region. Thin nanopillars (D < 130 nm) do not twin but generate highly disordered sequences of stacking faults in the deformed region. No SME occurs and heating converts only the undeformed regions into austenite. The defect-rich, deformed region remains in the martensite phase even after prolonged heating in the stability field of austenite. A complex mixture of twins and stacking faults was found for diameters 130 nm < D < 300 nm. The size effect of the SME in Cu-Al-Ni nanopillars consists of an approximately linear reduction of the SME between 300 and 130 nm when the SME completely vanishes for smaller diameters.

Dislocation-Governed Plastic Deformation and Fracture Toughness of Nanotwinned Magnesium

In this work, the plastic deformation mechanisms responsible for mechanical properties and fracture toughness in {101¯2}<101¯1¯>nanotwinned (NT). magnesium is studied by molecular dynamics (MD) simulation. The influence of twin boundary (TBs) spacing and crack position on deformation behaviors are investigated. The microstructure evolution at the crack tip are not exactly the same for the left edge crack (LEC) and the right edge crack (REC) models according to calculations of the energy release rate for dislocation nucleation at the crack tip. The LEC growth initiates in a ductile pattern and then turns into a brittle cleavage. In the REC model, the atomic decohesion occurs at the crack tip to create a new free surface which directly induces a brittle cleavage. A ductile to brittle transition is observed which mainly depends on the competition between dislocation motion and crack growth. This competition mechanism is found to be correlated with the TB spacing. The critical values are 10 nm and 13.5 nm for this transition in LEC and REC models, respectively. Essentially, the dislocation densities affected by the TB spacing play a crucial role in the ductile to brittle transition.

Smaller is Plastic: Polymorphic Structures and Mechanism of Deformation in Nanoscale hcp Metals

Using first-principles calculations, we establish the existence of highly stable polymorphs of hcp metals (Ti, Mg, Be, La and Y) with nanoscale structural periodicity. They arise from heterogeneous deformation of the hcp structure occurring in response to large shear stresses localized at the basal planes separated by a few nanometers. Through Landau theoretical analysis, we show that their stability derives from nonlinear coupling between strains at different length scales. Such multiscale hyperelasticity and long-period structures constitute a new mechanism of size-dependent plasticity and its enhancement in nanoscale hcp metals.

Converting 2D inorganic-organic ZnSe-DETA hybrid nanosheets into 3D hierarchical nanosheet-based ZnSe microspheres with enhanced visible-light-driven photocatalytic performances

Engineering two-dimensional (2D) nanosheets into three-dimensional (3D) hierarchical structures is one of the great challenges in nanochemistry and materials science. We report a facile and simple chemical conversion route to fabricate 3D hierarchical nanosheet-based ZnSe microspheres by using 2D inorganic-organic hybrid ZnSe-DETA (DETA = diethylenetriamine) nanosheets as the starting precursors. The conversion mechanism involves the controlled depletion of the organic-component (DETA) from the hybrid precursors and the subsequent self-assembly of the remnant inorganic-component (ZnSe). The transformation reaction of ZnSe-DETA nanosheets is mainly influenced by the concentration of DETA in the reaction solution. We demonstrated that this organic-component depletion method could be extended to the synthesis of other hierarchical structures of metal sulfides. In addition, the obtained hierarchical nanosheet-based ZnSe microspheres exhibited outstanding performance in visible light photocatalytic degradation of methyl orange and were highly active for photocatalytic H2 production.

In situ atomic-scale observation of twinning-dominated deformation in nanoscale body-centred cubic tungsten

Twinning is a fundamental deformation mode that competes against dislocation slip in crystalline solids. In metallic nanostructures, plastic deformation requires higher stresses than those needed in their bulk counterparts, resulting in the 'smaller is stronger' phenomenon. Such high stresses are thought to favour twinning over dislocation slip. Deformation twinning has been well documented in face-centred cubic (FCC) nanoscale crystals. However, it remains unexplored in body-centred cubic (BCC) nanoscale crystals. Here, by using in situ high-resolution transmission electron microscopy and atomistic simulations, we show that twinning is the dominant deformation mechanism in nanoscale crystals of BCC tungsten. Such deformation twinning is pseudoelastic, manifested through reversible detwinning during unloading. We find that the competition between twinning and dislocation slip can be mediated by loading orientation, which is attributed to the competing nucleation mechanism of defects in nanoscale BCC crystals. Our work provides direct observations of deformation twinning as well as new insights into the deformation mechanism in BCC nanostructures.

Bimodal sintered silver nanoparticle paste with ultrahigh thermal conductivity and shear strength for high temperature thermal interface material applications

A bimodal silver nanoparticle (AgNP) paste has been synthesized via the simple ultrasonic mixing of two types of unimodal AgNPs (10 and 50 nm in diameter). By sintering this paste at 250 °C for 30 min, we obtained an ultrahigh thermal conductivity of 278.5 W m(-1) K(-1), approximately 65% of the theoretical value for bulk Ag. The shear strength before and after thermal cycling at 50-200 °C for 1000 cycles was approximately 41.80 and 28.75 MPa, respectively. The results show that this excellent performance is attributable to the unique sintered structures inside the bimodal AgNP paste, including its low but stable porosity and the high density coherent twins. In addition, we systematically discuss the sintering behavior of this paste, including the decomposition of the organic layers and the formation of the coherent twins. On the basis of these results, we confirm that our bimodal AgNP paste has excellent potential as a thermal interface material for high temperature power device applications.

Twinning-like lattice reorientation without a crystallographic twinning plane

Twinning on the plane is a common mode of plastic deformation for hexagonal-close-packed metals. Here we report, by monitoring the deformation of submicron-sized single-crystal magnesium compressed normal to its prismatic plane with transmission electron microscopy, the reorientation of the parent lattice to a ‘twin’ lattice, producing an orientational relationship akin to that of the conventional twinning, but without a crystallographic mirror plane, and giving plastic strain that is not simple shear. Aberration-corrected transmission electron microscopy observations reveal that the boundary between the parent lattice and the ‘twin’ lattice is composed predominantly of semicoherent basal/prismatic interfaces instead of the twinning plane. The migration of this boundary is dominated by the movement of these interfaces undergoing basal/prismatic transformation via local rearrangements of atoms. This newly discovered deformation mode by boundary motion mimics conventional deformation twinning but is distinct from the latter and, as such, broadens the known mechanisms of plasticity.

Deformation twinning and dislocations are known to govern the plastic behaviour of metals at room temperature. Here the authors demonstrate a new deformation mechanism in single-crystal magnesium characterized by twin-like crystal reorientation and special interfaces.

Origin of dramatic oxygen solute strengthening effect in titanium

Structural alloys are often strengthened through the addition of solute atoms. However, given that solute atoms interact weakly with the elastic fields of screw dislocations, it has long been accepted that solution hardening is only marginally effective in materials with mobile screw dislocations. By using transmission electron microscopy and nanomechanical characterization, we report that the intense hardening effect of dilute oxygen solutes in pure α-Ti is due to the interaction between oxygen and the core of screw dislocations that mainly glide on prismatic planes. First-principles calculations reveal that distortion of the interstitial sites at the screw dislocation core creates a very strong but short-range repulsion for oxygen that is consistent with experimental observations. These results establish a highly effective mechanism for strengthening by interstitial solutes.

A first-principles study on the mechanical and thermodynamic properties of (Nb1−xTix)C complex carbides based on virtual crystal approximation

Tailoring the properties of complex carbides was achieved by component control, which enables it as a better candidate for specific application.

Direct observation of hierarchical nucleation of martensite and size-dependent superelasticity in shape memory alloys

Martensitic transformation usually creates hierarchical internal structures beyond mere change of the atomic crystal structure. Multi-stage nucleation is thus required, where nucleation (level-1) of the underlying atomic crystal lattice does not have to be immediately followed by the nucleation of higher-order superstructures (level-2 and above), such as polysynthetic laths. Using in situ transmission electron microscopy (TEM), we directly observe the nucleation of the level-2 superstructure in a Cu-Al-Ni single crystal under compression, with critical super-nuclei size L2c around 500 nm. When the sample size D decreases below L2c, the superelasticity behavior changes from a flat stress plateau to a continuously rising stress-strain curve. Such size dependence definitely would impact the application of shape memory alloys in miniaturized MEMS/NEMS devices.

Defective twin boundaries in nanotwinned metals

Coherent twin boundaries (CTBs) are widely described, both theoretically and experimentally, as perfect interfaces that play a significant role in a variety of materials. Although the ability of CTBs in strengthening, maintaining the ductility and minimizing the electron scattering is well documented, most of our understanding of the origin of these properties relies on perfect-interface assumptions. Here we report experiments and simulations demonstrating that as-grown CTBs in nanotwinned copper are inherently defective with kink-like steps and curvature, and that these imperfections consist of incoherent segments and partial dislocations. We further show that these defects play a crucial role in the deformation mechanisms and mechanical behaviour of nanotwinned copper. Our findings offer a view of the structure of CTBs that is largely different from that in the literature, and underscore the significance of imperfections in nanotwin-strengthened materials.

Reducing deformation anisotropy to achieve ultrahigh strength and ductility in Mg at the nanoscale

In mechanical deformation of crystalline materials, the critical resolved shear stress (CRSS; τCRSS) is the stress required to initiate movement of dislocations on a specific plane. In plastically anisotropic materials, such as Mg, τCRSS for different slip systems differs greatly, leading to relatively poor ductility and formability. However, τCRSS for all slip systems increases as the physical dimension of the sample decreases to approach eventually the ideal shear stresses of a material, which are much less anisotropic. Therefore, as the size of a sample gets smaller, the yield stress increases and τCRSS anisotropy decreases. Here, we use in situ transmission electron microscopy mechanical testing and atomistic simulations to demonstrate that τCRSS anisotropy can be significantly reduced in nanoscale Mg single crystals, where extremely high stresses (∼2 GPa) activate multiple deformation modes, resulting in a change from basal slip-dominated plasticity to a more homogeneous plasticity. Consequently, an abrupt and dramatic size-induced "brittle-to-ductile" transition occurs around 100 nm. This nanoscale change in the CRSS anisotropy demonstrates the powerful effect of size-related deformation mechanisms and should be a general feature in plastically anisotropic materials.