Xiaoyan Li

Tsinghua University | TH · Department of Engineering Mechanics

Nanoscale small-volume metallic materials typically exhibit high strengths but often suffer from a lack of tensile ductility due to undesirable premature failure. Here, we report unusual room-temperature uniform elongation up to ~110% at a high flow stress of 0.6–1.0 GPa in single-crystalline <110>-oriented CoCrFeNi high-entropy alloy nanopillars w...

Understanding the competing modes of brittle versus ductile fracture is critical for preventing the failure of body-centered cubic (BCC) refractory metals. Despite decades of intensive investigations, the nanoscale fracture processes and associated atomistic mechanisms in BCC metals remain elusive due to insufficient atomic-scale experimental evide...

Solitary waves are unique in nonlinear systems, but their formation and propagation in the nonlinear fluid-structure interactions have yet to be further explored. As a typical nonlinear system, the buckling of solid thin films is fundamentally related to the film-substrate interface that is further vulnerable to environments, especially when fluids...

Prelithiation can boost the performance of lithium-ion batteries (LIBs). A cost-effective prelithiation strategy with high quality and high industrial compatibility is urgently required. Herein we developed a roll-to-roll electrodeposition and transfer-printing system for continuous prelithiation of LIB anodes. By roll-to-roll calendering, pre-manu...

Dynamic engineering of buckling deformation is of vital importance as it provides multiphase modulation of thin film devices. In particular, dynamic switch of buckles between one-dimensional (1D) and two-dimensional (2D) configurations in a single film system on rigid substrates is intriguing but very challenging. The current approach to changing b...

The rise of twistronics has increased the attention of the community to the twist-angle-dependent properties of two-dimensional van der Waals integrated architectures. Clarification of the relationship between twist angles and interlayer mechanical interactions is important in benefiting the design of two-dimensional twisted structures. However, cu...

As emerging and revolutionary alloy materials, high entropy alloys (HEAs) have been extensively studied because of their unique composition, microstructures and excellent mechanical properties and performances. However, there have been limited studies on the deformation behaviors and mechanisms of HEA single crystals at the micro/nanoscale. Here, w...

Model composites consisting of SiC fiber and Yb2SiO5 were processed by the spark plasma sintering (SPS) method. The mechanical compatibility and chemical stability between Yb2SiO5 and SiC fiber were studied to evaluate the potential application of Yb monosilicate as the interphase of silicon carbide fiber reinforced silicon carbide ceramic matrix c...

Ceramics are considered intrinsically brittle at room temperature, which is mainly attributed to the limited availability of crystallographic slips and pre-existing geometrical flaws. Moreover, the lack of flexibility has severely hindered many high-end applications of ceramic materials. Here, we produce ceramic sponges that are simultaneously ultr...

If a bulk material can withstand a high load without any irreversible damage (such as plastic deformation), it is usually brittle and can fail catastrophically1,2. This trade-off between strength and fracture toughness also extends into two-dimensional materials space3,4,5. For example, graphene has ultrahigh intrinsic strength (about 130 gigapasca...

Strengthening of magnesium (Mg) is known to occur through dislocation accumulation, grain refinement, deformation twinning, and texture control or dislocation pinning by solute atoms or nano-sized precipitates. These modes generate yield strengths comparable to other engineering alloys such as certain grades of aluminum but below that of high-stren...

Due to having a single atom layer, two-dimensional (2D) materials represented by graphene monolayers exhibit unique and excellent mechanical properties, such as ultrahigh moduli and strengths. A large number of experiments and atomistic simulations have demonstrated nonlinear stress-strain responses. However, there is no theoretical model that anal...

Carbon nanoparticles (CNPs) have been considered as essential components for various applications including sensors, quantum dots, electrocatalysts, energy storages, lubrication, and functional coatings. Uniform and functional CNP materials can be obtained from candle soot. However, the production of CNPs from candle soot is not a continuous proces...

There have been very limited studies on plastic deformation mechanisms in single-crystalline high-entropy alloys (HEAs) with body-centered cubic (BCC) phases. We performed in situ uniaxial compression on single-crystalline BCC AlCrFeCoNi micropillars/nanopillars with three orientations (including [100], [110], and [111]) and diameters of 270-1583 n...

Since the success of monolayer graphene exfoliation, two-dimensional (2D) materials have been extensively studied due to their unique structures and unprecedented properties. Among these fascinating studies, the most predominant focus has been on their atomic structures, defects, and mechanical behaviors and properties, which serve as the basis for...

TiAl alloys exhibit high specific strength and stiffness and especially excellent mechanical properties at elevated temperatures, making them appealing for high-temperature applications. Understanding the underlying creep mechanisms of TiAl alloys is essential for their design, fabrication and high-temperature applications. Here, we performed a ser...

The orientation between twin boundary (TB) and loading direction may play an intriguing role in the deformation behaviors of twinned metallic materials. In this aspect, its essential effect on the high-entropy alloy (HEA) nanocrystals is elusive. Attention herein is focused on the atomic-scaled deformation mechanisms and fracture behaviors of HEA n...

Current synthetic approaches to metal nanoparticles are mostly batch processes that use a large quantity of reagents and surfactants, producing enormous amount of solid and liquid waste. Here, we developed a rotating electrodeposition and separation (REDS) technique, which entails electrochemically depositing nanoparticles onto a continuously rotat...

Heterogeneous-structured materials are a new class of metallic materials that have recently emerged due to development of advanced processing and structural/architectural design techniques. These materials are made of heterogeneous domains having different constitutive behaviors and achieve superior mechanical properties, such as extra strengthenin...

The thermal conductivity of two-dimensional materials, such as graphene, typically decreases when tensile strain is applied, which softens their phonon modes. Here, we report an anomalous strain effect on the thermal conductivity of monolayer silicene, a representative low-buckled two-dimensional (LB-2D) material. ReaxFF-based molecular dynamics si...

High-entropy alloys, a new class of metallic materials, exhibit excellent mechanical properties at high temperatures. In spite of the worldwide interest, the underlying mechanisms for temperature dependence of mechanical properties of these alloys remain poorly understood. Here, we systemically investigate the mechanical behaviors and properties of...

Inspired by the gradient structures of biological materials, researchers have explored compositional and structural gradients for about 40 years as an approach to enhance the properties of engineering materials, including metals and metallic alloys. The synthesis of various gradient nanostructured materials, such as gradient nanograined, nanolamina...

Model composites consisting of SiC fiber embedded in β-Yb2Si2O7 matrix were processed by Spark Plasma Sintering method and the feasibility of tunable SiCf/Yb2Si2O7 interface in SiC-based CMCs were estimated. Weak and strengthened SiCf/Yb2Si2O7 interfaces were achieved by adjusting sintering temperatures. The indentation crack test and fiber push ou...

Nanolayered metals are shown to have significantly improved radiation resistance, owing to the copious interplay between their densely populated internal boundaries with irradiation-induced defects. Therefore, they have become promising candidates for structural applications in advanced nuclear reactors. Here we fabricated bulk nanolaminated graphe...

Phase transformation is an important deformation and blunting mechanism at the crack tip of body-centered cubic (BCC) metals. The BCC-to-face-centered orthogonal (FCO) transformation was only predicted by ab initio calculations. In this paper, the deformation behavior of the nano-sized single-crystalline Mo is in situ investigated in a high-resolut...

In recent years, a large number of experimental studies have shown that graphene fibers, a new type of carbon fiber consisting of many monolayers of wrinkled and curved graphene sheets aligned in the axial direction of the fiber, exhibit high tensile strength and many functionalities. Although much effort has been devoted to improving their mechani...

In article number 1902842, Xiaoyan Li and co‐workers summarize the recent advances in design, fabrication and mechanics of 3D micro‐/nano‐lattices, which is one new type of architected material with minimum characteristic sizes ranging from several micrometers to hundreds of nanometers. Taking advantage of the topological design, structural optimiz...

The umpolung reaction catalyzed by N‐heterocyclic carbenes (NHCs) has been widely studied and well recognized, but their role in the nonpolar inversion reaction under oxidative condition has been rarely reported. In this paper, the mechanism of the oxidative nonpolar inversion reaction catalyzed by NHCs to produce benzoxazole is investigated in det...

In article number 1903645, Qiang Zhang and co‐workers describe the failure mechanism of a solid electrolyte interphase (SEI) on a Li metal anode based on a quantitative electrochemical‐mechanical model. The impacts of structural uniformity and mechanical strength on the stability of the SEI under different working conditions are revealed. Theoretic...

Practical application of lithium (Li) metal anodes has been hindered by Li dendrite growth, which renders a low Coulombic efficiency and short lifespan of working Li metal batteries. A stable solid electrolyte interphase (SEI) is crucial in suppressing the formation of Li dendrites. Herein the local stress and deformation evolvement status of a SEI...

Metal-based photosensitizers are of great interest in photodynamic therapy (PDT) due to their tunable photophysicochemical characteristics and structure flexibility. Herein, an iridium-based photosensitizer (1) with a long-lived intraligand (³IL) excited state has been designed and synthesized, which shows significantly enhanced singlet oxygen (¹O2...

Over the past several decades, lattice materials have been developed and used as engineering materials for lightweight and stiff industrial structures. Recent advances in additive manufacturing techniques have prompted the emergence of architected materials with minimum characteristic sizes ranging from several micrometers to hundreds of nanometers...

In this study, density functional theory (DFT) calculations were performed to gain insight into the possible reaction mechanism and substituent effects of benzene arylation at the molecular level. The results showed that both [Me3Si]⁺[WCA]⁻ and [Et3Si]⁺[WCA]⁻ promoted the reaction. The whole catalytic cycle generally involved three processes, namel...

The creation of materials with a combination of high strength, substantial deformability and ductility, large elastic limit and low density represents a long-standing challenge, because these properties are, in general, mutually exclusive. Using a combination of two-photon lithography and high-temperature pyrolysis, we have created micro-sized pyro...

Recent experiments have demonstrated the fabrication of nickel nanolaminated structures with low-angle grain boundaries and that these nanolaminated structures exhibit an ultrahigh hardness of 6.4 GPa and excellent thermal stability. However, the detailed microstructure of the low-angle grain boundary in these nanolaminated structures and plastic d...

Significance A long-standing challenge in modern materials manufacturing and design has been to create porous materials that are simultaneously lightweight, strong, stiff, and flaw-tolerant. Here, we fabricated pyrolytic carbon nanolattices with designable topologies by a two-step procedure: direct laser writing and pyrolysis at high temperature. T...

Thin films with large compressive residual stress and low interface adhesion can buckle and delaminate from relatively rigid substrates, which is a common failure mode of film/substrate interfaces. Current studies mainly focused on the geometry of various buckling patterns and related physical origins based on a static point of view. However, funda...

Precipitation-hardening high-entropy alloys (PH-HEAs) with good strength-ductility balances are a promising candidate for advanced structural applications. However, current HEAs emphasize near-equiatomic initial compositions, which limit the increase of intermetallic precipitates that are closely related to the alloy strength. Here we present a str...

Diffusion of lithium atoms in the silicon anode is a key process for the lithiation and de-lithiation steps in lithium-ion batteries. The relationship between atomic structures of silicon, in forms of crystals and glasses, and the diffusivity of lithium atoms are thus of critical importance to assess the performance of batteries using silicon as th...

Grain refinement to the nano-/ultrafine-grained regime can make metals several times stronger, but this process is usually accompanied by a dramatic loss of ductility. Such strength-ductility trade-off originates from a lack of strain-hardening capacity in tiny grains. Here, we present a strategy to regain the strain-hardening ability of high-stren...

Topological defects (e.g. pentagons, heptagons and pentagon-heptagon pairs) have been widely observed in large scale graphene and have been recognized to play important roles in tailoring the mechanical and physical properties of two-dimensional materials in general. Thanks to intensive studies over the past few years, optimizing properties of grap...

In this study, we developed a novel blow spinning technology to construct large-scale porous sponges composed by carbon microfibers with strong absorption of oil. The microfiber sponge had a weight expansion of 184 times after oil absorption. Considering the outstanding mechanical properties and good thermal stability of the 3D carbon sponge, the c...

This report describes a novel method to produce fluffy and resilient nanotube aerogels by combining solution blow spinning and Atom Layer Deposition (ALD). Polyvinylpyrrolidone (PVP) sponges obtained by blow spinning are used as templates and are deposited in ALD. After removal of template, semitransparent aerogels whose density can be as low as 0....

2D Si nanomaterials have attracted tremendous attention due to their novel properties and a wide range of potential applications from electronic devices to energy storage and conversion. However, high-quality and large-scale fabrication of 2D Si remains challenging. This study reports a room-temperature and one-step synthesis technique that leads t...

Penta-twinned Ag nanowires (pt-AgNWs) have recently attracted much attention due to their interesting mechanical and physical properties. Here we perform large-scale atomistic simulations to investigate the influence of sample size and strain rate on the tensile strength of pt-AgNWs. The simulation results show an apparent size effect in that the n...

We proposed an eccentric ellipse criterion to describe the failure of amorphous materials under a combination of normal stress r and shear stress s. This criterion can reflect a tension-compression strength asymmetry, and unify four previous failure criteria in the ν-τ stress space, including von Mises criterion, Drucker-Prager criterion, Christens...

Ultralight and resilient porous nanostructures have been fabricated in various material forms, including carbon, polymers, and metals. However, the development of ultralight and high-temperature resilient structures still remains extremely challenging. Ceramics exhibit good mechanical and chemical stability at high temperatures, but their brittlene...

Recent experiments have revealed that external bending breaks the symmetry of lithiation in germanium nanowires. However, the effects of external stress on lithiation in silicon and the associated underlying mechanisms remain unclear. Here, we have performed a series of large-scale atomistic simulations based on a newly developed reactive force fie...

The application of silicon as ultrahigh capacity electrodes in lithium-ion batteries has been limited by a number of mechanical degradation mechanisms including fracture, delamination and plastic ratcheting, as a result of its large volumetric change during lithiation and delithiation. Graphene coating is one feasible technique to mitigate the mech...

Recent experiments have reported that nanotwinned ceramics with covalent or ionic bonds exhibit unprecedented hardness, enhanced fracture toughness and improved thermal stability. In this viewpoint paper, we highlight some preliminary studies on mechanical properties of nanotwinned ceramics either synthesized in the laboratory or extracted from bio...

Lithium-ion batteries with a Si anode can drive large mechanical actuation by utilizing the dramatic volume changes of the electrode during the charge/discharge cycles. A large loading of more than 10 MPa can be actuated by a LiFePO4 ||Si full battery with a rapid response while the driving voltage is lower than 4 V.

There is growing interest in promoting deformation twinning for plasticity in advanced materials, as highly organized twin boundaries are beneficial to better strength-ductility combination in contrast to disordered grain boundaries. Twinning deformation typically involves the kinetics of stacking faults, its interaction with dislocations, and disl...

As a relatively new class of hierarchically structured materials, nanotwinned (NT) metals exhibit an exceptional combination of high strength, good ductility, large fracture toughness, remarkable fatigue resistance, and creep stability. This article reviews current studies on fracture, fatigue, and creep of NT metals, with an emphasis on the fundam...

Fabrication of an ultra-strong glassy carbon nanolattice with a strut diameter of around 200 nm could stimulate the realization of advanced nanoscale architected materials.

As a natural biocomposite, Strombus gigas, commonly known as the giant pink queen conch shell, exhibits outstanding mechanical properties, especially a high fracture toughness. It is known that the basic building block of conch shell contains a high density of growth twins with average thickness of several nanometres, but their effects on the mecha...

Supplementary Figures 1-21, Supplementary Table 1, Supplementary Notes 1-8, Supplementary Methods and Supplementary References

Formation of nanocracks and their interaction with main crack in the conch shell. The advancing main crack was blocked frequently by the slanted TBs. While the main crack is trapped at the TBs for a while, nanocracks (marked by yellow arrows in the movie) were nucleated ahead of the main crack. The main crack propagated through repeated deflection...

MD simulation of crack propagation in nanotwinned aragonite (with twin boundary spacing of 20nm). The results are qualitatively similar to those in Supplementary Movie 8.

Phase transformation in conch shell during In-situ TEM nanoindentation. The stress fields under the indenter induced a type of phase transformation (or amorphization) of nanotwinned aragonite with no indication of dislocation slip or deformation twinning. The loading axis was normal to the TBs of nanotwinned aragonite sampled from the conch shell....

MD simulation of crack propagation in single-crystalline aragonite in T-orientation. Structural transformation occurs ahead of the crack tip while the crack propagates in a straight path.

MD simulation of crack propagation in single-crystalline aragonite in M-orientation. The crack is slightly deflected along a cleavage plane during the propagation and the transformed region driven by the crack-tip stress field is smaller than that in single-crystalline aragonite samples in T-orientation.

Crack propagation paths in conch shell, nacre, and single crystal aragonite. The plot of crack length versus loading time is displayed in sync with the corresponding TEM movie. In the conch shell, the main crack propagated through crack deflection and coalescence with nanocracks (indicated by yellow allows). In the case of nacre, a single crack pro...

MD simulation of crack propagation in nanotwinned aragonite (with twin boundary spacing of 10nm). Under increasing loading, an edge crack starts to propagate toward an array of TBs. When the crack reaches a TB, it is trapped by the TB over a time period. The crack tip becomes blunted due to sliding on the TB. Plastic deformation via structural tran...

In-situ TEM nanoscale CTOD measurements of conch shell. The results of two separate tests are displayed in series. The TEM movies were recorded at 25 fps and played at 37.5 fps (1.5 times faster playback speed) for the first movie (Conch A, analyzed and presented as part of Fig. 2) and at 25 fps for the second movie (Conch B). The mechanical testin...

In-situ TEM nanoscale CTOD measurements of single crystal aragonite. The results of two separate tests are displayed in series. The TEM movies were recorded at 25 fps and played at the same playback speed for the first movie (Aragonite A), and at 37.5 fps (1.5 faster playback speed) for the second movie (aragonite B, analyzed and presented as part...

Deformation twinning during crack propagation in single crystal aragonite. Deformation twinning occurs ahead of crack tip as a major plastic deformation induced by the crack tip stress fields. The deformation twins form and propagate along the and planes with width of a few nm.

Deformation twinning in aragonite during In-situ TEM nanoindentation. Deformation twinning occurred under the indenter as a major plastic deformation driven by the stress fields. The loading axis was parallel to the direction of aragonite. The deformation twins formed and propagated along the and (110) planes with width of a few nm. In addition to...

Abstract Nanotwinned Cu exhibits an unusual combination of ultra-high strength and high tensile ductility. However, its fracture behavior and associated microscopic mechanisms remain largely unexplored. Here we study the fracture in a free-standing thin film of nanotwinned Cu using molecular dynamics (MD) simulations. For a pre-crack inclined to th...

We investigate the fatigue behavior of nanotwinned Cu using a combination of molecular statics and molecular dynamics simulations. The presence of nanoscale twins is found to enhance fatigue crack growth resistance. For the twin-free nanocrystalline samples, the fatigue crack propagates by linking the nanovoids that are formed ahead of the crack ti...

Stretchable and transparent inorganic semiconductors play a key role for the next generation of wearable optoelectronics. Achieving stretchability in intrinsically rigid inorganic materials is far more challenging than in polymers and metals. Here, we present a low-cost and scalable strategy to engineer inorganic semiconductors into a buckling open...

Fracture is one of the most prominent concerns for large scale applications of graphene. In this paper, we review some of the recent progresses in experimental and theoretical studies on the fracture behaviors of graphene, with discussions touching theoretical strength, mode I fracture toughness, mixed mode fracture, chemical fracture, irradiation...

How to maintain sustained deformation in one-dimensional nanostructures without localized failure is an important question for many applications of nanotechnology. Here we report a phenomenon of torsional detwinning domino which leads to giant rotational deformation without localized failure in nanotwinned one-dimensional metallic nanostructures. T...

Shear localization induced brittleness is the main drawback of metallic glasses which restricts their practical applications. Previous experiments have provided insights on how to suppress shear localization by reducing the sample size of metallic glasses to the order of 100 nm. In order to reveal the size effects and associated deformation mechani...

There has been relatively little study on time-dependent mechanical properties of nanowires, in spite of their importance for the design, fabrication and operation of nanoscale devices. Here we report a dislocation-mediated, time-dependent and fully reversible plastic behaviour in penta-twinned silver nanowires. In situ tensile experiments inside s...

A novel design methodology combining phase field crystal method and atomistic simulations is proposed to solve the inverse problem of finding the optimized distribution and type of topological defects that make a graphene sheet conform to a targeted arbitrary three dimensional (3D) surface. To demonstrate potential applications of the proposed meth...

Atomistic simulations reveal a new and unique strengthening mechanism in nanotwinned metals governed by the collective motion of multiple necklace-like extended jogged dislocations. This mechanism prevails in a columnar-grained nanotwinned metal subject to an external stress parallel to the twin planes, provided the twin boundary spacing falls belo...

Due to its atomic scale thickness, the deformation energy in a free standing graphene sheet can be easily released through out-of-plane wrinkles which, if controllable, may be used to tune the electrical and mechanical properties of graphene. Here we adopt a generalized von Karman equation for a flexible solid membrane to describe graphene wrinklin...

Over the past two decades, there has been a whirlwind of worldwide research activities on nanostructured materials, with the long term promise to tailor-design material properties from the nanoscale and up. In this grand effort, computation is playing an increasingly important role in complementing experiments to unravel fundamental principles that...

The early Bauschinger effect in nanocrystalline Al thin films with different thicknesses and microstructural orientations was investigated using large-scale atomistic simulations. The simulation results indicate that the microstructural orientation heterogeneity has a significant influence on the early Bauschinger effect and the associated plastic...

Anisotropic plastic deformation in columnar-grained copper in which preferentially oriented nanoscale twins are embedded is studied by experimental testing, crystal plasticity modeling and molecular dynamics simulations. The dominant deformation mechanism can be effectively switched among three dislocation modes, namely dislocation glide in between...

Nanoporous metals are a class of novel nanomaterials with potential applications in many fields such as sensing, catalysis, and fuel cells. The present paper is aimed to investigate atomic mechanisms associated with the uniaxial tensile deformation behavior of nanoporous gold. A phase field method is adopted to generate the bicontinuous open-cell p...

We show from a series of molecular dynamics simulations that the tensile fracture behavior of a nanocrystalline graphene (nc-graphene) nanostrip can become insensitive to a pre-existing flaw (e.g., a hole or a notch) below a critical length scale in the sense that there exists no stress concentration near the flaw, the ultimate failure does not nec...

Nanotwinned metals are attractive in many applications because they simultaneously demonstrate high strength and high ductility, characteristics that are usually thought to be mutually exclusive. However, most nanotwinned metals are produced in polycrystalline forms and therefore contain randomly oriented twin and grain boundaries making it difficu...

In situ tensile experiments in a transmission electron microscope revealed that micro-cracks in ultrafine grained, free-standing, thin copper foils containing nanoscale twins initiated in matrix domains separated by the twins and then arrested at twin boundaries as twin boundary sliding proceeded. The adjacent microcracks eventually coalesced throu...

We have developed a mechanism-based plasticity model of nanotwinned metals to investigate the effect of twin spacing on strength, ductility and work hardening rate of such materials. In particular, the unique roles of dislocation pile-up zones near twin and grain boundaries, as well as twinning partial dislocations, in strengthening and work harden...

Classical fracture mechanics as well as modern strain gradient plasticity theories assert the existence of stress concentration (or strain gradient) ahead of a notch tip, albeit somewhat relaxed in ductile materials. In this study, we present experimental evidence of extreme stress homogenization in nanocrystalline metals that result in immeasurabl...

In conventional metals, there is plenty of space for dislocations-line defects whose motion results in permanent material deformation-to multiply, so that the metal strengths are controlled by dislocation interactions with grain boundaries and other obstacles. For nanostructured materials, in contrast, dislocation multiplication is severely confine...