Mechanisms of failure in nanoscale metallic glass

Molecular dynamics simulations of cold welding of nanoporous amorphous alloys: effects of welding conditions and microstructures

Cold welding behaviors of nanoporous amorphous alloys investigated by molecular dynamics.

Recreating the shear band evolution in nanoscale metallic glass by mimicking the atomistic rolling deformation: a molecular dynamics study

Rolling processes are extensively used to induce network of shear bands (SBs) in the bulk metallic glasses, which in turn enhances the overall plasticity of the specimen. However, the atomic-level understanding of shear band formation/propagation mechanism during mechanical processing is still limited. In this perspective, we have developed a molecular dynamics (MD) simulation model to recreate the rolling deformation process and investigate the SB formation in Cu-Zr metallic glass (MG) specimen. Results have shown that dense and concentrated primary SBs along with secondary branching are formed during cryo-rolling, whereas a scattered and thicker SBs are formed during hot rolling process. Meanwhile, Voronoi cluster analysis revealed that the high five-fold symmetry clusters tend to decrease, while the crystalline-like cluster increases during the hot rolling process. These findings from the study are in good agreement with previous experimental studies substantiated in literature, which shows that the model correctly predicts the shear-banding phenomenon.

Localization model description of the interfacial dynamics of crystalline Cu and Cu64Zr36 metallic glass films

Recent studies of structural relaxation in Cu-Zr metallic glass materials having a range of compositions and over a wide range of temperatures and in crystalline UO₂ under superionic conditions have indicated that the localization model (LM) can predict the structural relaxation time τ_α of these materials from the intermediate scattering function without any free parameters from the particle mean square displacement ⟨r²⟩ at a caging time on the order of ps, i.e., the "Debye-Waller factor" (DWF). In the present work, we test whether this remarkable relation between the "fast" picosecond dynamics and the rate of structural relaxation τ_α in these model amorphous and crystalline materials can be extended to the prediction of the local interfacial dynamics of model amorphous and crystalline films. Specifically, we simulate the free-standing amorphous Cu₆₄Zr₃₆ and crystalline Cu films and find that the LM provides an excellent parameter-free prediction for τ_α of the interfacial region. We also show that the Tammann temperature, defining the initial formation of a mobile interfacial layer, can be estimated precisely for both crystalline and glass-forming solid materials from the condition that the DWFs of the interfacial region and the material interior coincide.

Ductile Metallic Glass Nanoparticles via Colloidal Synthesis

The design of ductile metallic glasses has been a longstanding challenge. Here, we use colloidal synthesis to fabricate nickel-boron metallic glass nanoparticles that exhibit homogeneous deformation at room temperature and moderate strain rates. In situ compression testing is used to characterize the mechanical behavior of 90-260 nm diameter nanoparticles. The force-displacement curves consist of two regimes separated by a slowly propagating shear band in small, 90 nm particles. The propensity for shear banding decreases with increasing particle size, such that large particles are more likely to deform homogeneously through gradual shape change. We relate this behavior to differences in composition and atomic bonding between particles of different size using mass spectroscopy and XPS. We propose that the ductility of the nanoparticles is related to their internal structure, which consists of atomic clusters made of a metalloid core and a metallic shell that are connected to neighboring clusters by metal-metal bonds.

Chiral metallic glass nanolattices with combined lower density and improved auxeticity

Auxetic materials are promising structural and functional candidates due to their unique lateral expansion when stretched, however, bulk metallic glasses (MGs) could not show any auxeticity because of their intrinsic isotropic nature. Here we construct chiral Cu₅₀Zr₅₀ metallic glass nanolattices with cavities, and investigate their auxeticity and underlying mechanism with molecular dynamics simulations. It is found that, compared to monolithic MGs, all the chiral metallic glass nanolattices (CMGNs) exhibit improved auxeticity and lower density. For CMGNs with cavities, the negative Poisson's ratio and ultimate tensile strength (UTS) increase first and then decrease with increasing cavity radius, with the cavity radius of 2.5 nm being the most favorable for auxeticity and enhanced UTS. The auxetic mechanism is attributed to the competition between rotation behavior and non-affine deformation under tension. Our study not only reveals the mechanism of auxeticity in CMGNs having cavities but also provides a feasible method to optimize their auxetic performance and density by structure designing of MGs.

Distinguishing failure modes in oligomeric polymer nanopillars

Brittle failure is ubiquitous in amorphous materials that are sufficiently cooled below their glass transition temperature, T_g. This catastrophic failure mode is limiting for amorphous materials in many applications, and many fundamental questions surrounding it remain poorly understood. Two challenges that prevent a more fundamental understanding of the transition between a ductile response at temperatures near T_g to brittle failure at lower temperatures are (i) a lack of computationally inexpensive molecular models that capture the failure modes observed in experiments and (ii) the lack of quantitative metrics that can distinguish various failure mechanisms. In this work, we use molecular dynamics simulations to capture ductile-to-brittle transition in glass-forming oligomeric polymer systems where we systematically vary both the temperature relative to T_g and the form of the interaction potential to induce a variety of failure modes. We characterized the effects of this new potential on macroscopic mechanical properties as well as microscopic structural and dynamical response during deformation. Finally, we develop several quantitative metrics to distinguish between different failure modes, and we find that the transition between catastrophic brittle failure, necking, and homogeneous plastic flow is gradual as the temperature is increased.

Lightweight, flaw-tolerant, and ultrastrong nanoarchitected carbon

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. The smallest characteristic size of the nanolattices approached the resolution limits of the available 3D lithography technologies. Due to the designable unit-cell geometries, reduced feature sizes, and high quality of pyrolytic carbon, the created nanoarchitected carbon structures are lightweight, can be made virtually insensitive to fabrication-induced defects, attain nearly theoretical strength of the constituent material, and achieve specific strength up to one to three orders of magnitude above that of all existing micro/nanoarchitected materials.

It has been a long-standing challenge in modern material design to create low-density, lightweight materials that are simultaneously robust against defects and can withstand extreme thermomechanical environments, as these properties are often mutually exclusive: The lower the density, the weaker and more fragile the material. Here, we develop a process to create nanoarchitected carbon that can attain specific strength (strength-to-density ratio) up to one to three orders of magnitude above that of existing micro- and nanoarchitected materials. We use two-photon lithography followed by pyrolysis in a vacuum at 900 °C to fabricate pyrolytic carbon in two topologies, octet- and iso-truss, with unit-cell dimensions of ∼2 μm, beam diameters between 261 nm and 679 nm, and densities of 0.24 to 1.0 g/cm³. Experiments and simulations demonstrate that for densities higher than 0.95 g/cm³ the nanolattices become insensitive to fabrication-induced defects, allowing them to attain nearly theoretical strength of the constituent material. The combination of high specific strength, low density, and extensive deformability before failure lends such nanoarchitected carbon to being a particularly promising candidate for applications under harsh thermomechanical environments.

Critical Analysis of an FeP Empirical Potential Employed to Study the Fracture of Metallic Glasses

An empirical potential that has been widely used to perform molecular dynamics studies on the fracture behavior of FeP metallic glasses is shown to exhibit spinodal decomposition in the composition range commonly studied. The phosphorous segregation induces a transition from ductility to brittleness. During brittle fracture the atomically sharp crack tip propagates along a percolating path with higher P concentration. This embrittlement is observed to occur over a wide range of chemical compositions, and toughness decreases linearly with the degree of compositional segregation over the entire regime studied. Stable glass forming alloys that can be quenched at low quench rates do not, as a rule, exhibit such thermodynamically unstable behavior near to or above their glass transition temperatures. Hence, the microstructures exhibited in these simulations are unlikely to reflect the actual microstructures or fracture behaviors of the glassy alloys they seek to elucidate.

Deformation Behavior of Bulk Metallic Glasses and High Entropy Alloys under Complex Stress Fields: A Review

The plastic deformation of bulk metallic glasses (BMGs) depends significantly on applied stress states, and more importantly, in practical applications of BMGs as structural materials, they always deform under complex stress fields. The understanding of deformation behavior of BMGs under complex stress fields is important not only for uncovering the plastic deformation mechanisms of BMGs, but also for developing BMG components with excellent mechanical performance. In this article, we briefly summarize the recent research progress on the deformation behavior of BMGs under complex stress fields, including the formation and propagation of shear bands, tunable macroscopic plasticity, and serrated plastic flows. The effect of complex stress fields on the plastic deformation mechanisms of BMGs is discussed from simple stress gradient to tailored complex stress fields. The deformation behavior of high entropy alloys (HEAs) under complex stress states has also been discussed. Challenges, potential implications and some unresolved issues are proposed.

Deformation mechanism of innovative 3D chiral metamaterials

Rational design of artificial microstructured metamaterials with advanced mechanical and physical properties that are not accessible in nature materials is very important. Making use of node rotation and ligament bending deformation features of chiral materials, two types of innovative 3D chiral metamaterials are proposed, namely chiral- chiral- antichiral and chiral- antichiral- antichiral metamaterials. In-situ compression and uniaxial tensile tests are performed for studying the mechanical properties and deformation mechanisms of these two types of 3D chiral metamaterials. Novel deformation mechanisms along different directions are explored and analyzed, such as: uniform spatial rotation deformation, tensile-shearing directed (compression-shearing directed), tensile-expansion directed (compression-shrinkage directed) deformation mechanisms of 3D chiral metamaterials, and competitions between different types of deformation mechanisms are discussed. The proposed 3D chiral metamaterials represents a series of metamaterials with robust microstructures design feasibilities.

A Critical Review on Metallic Glasses as Structural Materials for Cardiovascular Stent Applications

Functional and mechanical properties of novel biomaterials must be carefully evaluated to guarantee long-term biocompatibility and structural integrity of implantable medical devices. Owing to the combination of metallic bonding and amorphous structure, metallic glasses (MGs) exhibit extraordinary properties superior to conventional crystalline metallic alloys, placing them at the frontier of biomaterials research. MGs have potential to improve corrosion resistance, biocompatibility, strength, and longevity of biomedical implants, and hence are promising materials for cardiovascular stent applications. Nevertheless, while functional properties and biocompatibility of MGs have been widely investigated and validated, a solid understanding of their mechanical performance during different stages in stent applications is still scarce. In this review, we provide a brief, yet comprehensive account on the general aspects of MGs regarding their formation, processing, structure, mechanical, and chemical properties. More specifically, we focus on the additive manufacturing (AM) of MGs, their outstanding high strength and resilience, and their fatigue properties. The interconnection between processing, structure and mechanical behaviour of MGs is highlighted. We further review the main categories of cardiovascular stents, the required mechanical properties of each category, and the conventional materials have been using to address these requirements. Then, we bridge between the mechanical requirements of stents, structural properties of MGs, and the corresponding stent design caveats. In particular, we discuss our recent findings on the feasibility of using MGs in self-expandable stents where our results show that a metallic glass based aortic stent can be crimped without mechanical failure. We further justify the safe deployment of this stent in human descending aorta. It is our intent with this review to inspire biodevice developers toward the realization of MG-based stents.

Size effect on atomic structure in low-dimensional Cu-Zr amorphous systems

The size effect on atomic structure of a Cu₆₄Zr₃₆ amorphous system, including zero-dimensional small-size amorphous particles (SSAPs) and two-dimensional small-size amorphous films (SSAFs) together with bulk sample was investigated by molecular dynamics simulations. We revealed that sample size strongly affects local atomic structure in both Cu₆₄Zr₃₆ SSAPs and SSAFs, which are composed of core and shell (surface) components. Compared with core component, the shell component of SSAPs has lower average coordination number and average bond length, higher degree of ordering, and lower packing density due to the segregation of Cu atoms on the shell of Cu₆₄Zr₃₆ SSAPs. These atomic structure differences in SSAPs with various sizes result in different glass transition temperatures, in which the glass transition temperature for the shell component is found to be 577 K, which is much lower than 910 K for the core component. We further extended the size effect on the structure and glasses transition temperature to Cu₆₄Zr₃₆ SSAFs, and revealed that the T_g decreases when SSAFs becomes thinner due to the following factors: different dynamic motion (mean square displacement), different density of core and surface and Cu segregation on the surface of SSAFs. The obtained results here are different from the results for the size effect on atomic structure of nanometer-sized crystalline metallic alloys.

Ultralarge Bending Strain and Fracture-Resistance Investigation of Tungsten Carbide Nanowires

Hard tungsten carbide (WC) with brittle behavior is frequently applied for mechanical purposes. Here, ultralarge elastic bending deformation is reported in defect-rare WC [0001] nanowires; the tested bending strain reaches a maximum of 20% ± 3.33%, which challenges the traditional understanding of this material. The lattice analysis indicates that the dislocations are confined to the inner part of the WC nanowires. First, the high Peierls-Nabarro barrier hinders the movement of the locally formed dislocations, which causes rapid dislocation aggregation and hinders long-range glide, resulting in a dense distribution of the dislocation network. In this case, the loading is dispersed along multiple points, which is then balanced by the complex internal mechanical field. In the compressive part, the possible dislocations predominantly emerge in the (0001) plane and mainly slip along the axial direction. The disordered shell first forms at the tensile side and prevents the generation of nanocracks at the surface. The novel lattice kinetics make WC nanowires capable of substantial bending strain resistance. Analytical results of the force-displacement (F-d) curves based on the double-clamped beam model exhibit an obvious nonlinear elastic characteristic, which originates fundamentally from the lattice anharmonicity under moderate stress.

Study on fracture behavior of individual InAs nanowires using an electron-beam-drilled notch

The mechanical properties and fracture behavior of individual InAs nanowires (NWs) were investigated under uniaxial tensile loading in a transmission electron microscope.

Flaw-induced plastic-flow dynamics in bulk metallic glasses under tension

Inheriting amorphous atomic structures without crystalline lattices, bulk metallic glasses (BMGs) are known to have superior mechanical properties, such as high strength approaching the ideal value, but are susceptible to catastrophic failures. Understanding the plastic-flow dynamics of BMGs is important for achieving stable plastic flow in order to avoid catastrophic failures, especially under tension, where almost all BMGs demonstrate limited plastic flow with catastrophic failure. Previous findings have shown that the plastic flow of BMGs displays critical dynamics under compression tests, however, the plastic-flow dynamics under tension are still unknown. Here we report that power-law critical dynamics can also be achieved in the plastic flow of tensile BMGs by introducing flaws. Differing from the plastic flow under compression, the flaw-induced plastic flow under tension shows an upward trend in the amplitudes of the load drops with time, resulting in a stable plastic-flow stage with a power-law distribution of the load drop. We found that the flaw-induced plastic flow resulted from the stress gradients around the notch roots, and the stable plastic-flow stage increased with the increase of the stress concentration factor ahead of the notch root. The findings are potentially useful for predicting and avoiding the catastrophic failures in tensile BMGs by tailoring the complex stress fields in practical structural-applications.

Brittle-to-Ductile Transition in Metallic Glass Nanowires

When reducing the size of metallic glass samples down to the nanoscale regime, experimental studies on the plasticity under uniaxial tension show a wide range of failure modes ranging from brittle to ductile ones. Simulations on the deformation behavior of nanoscaled metallic glasses report an unusual extended strain softening and are not able to reproduce the brittle-like fracture deformation as found in experiments. Using large-scale molecular dynamics simulations we provide an atomistic understanding of the deformation mechanisms of metallic glass nanowires and differentiate the extrinsic size effects and aspect ratio contribution to plasticity. A model for predicting the critical nanowire aspect ratio for the ductile-to-brittle transition is developed. Furthermore, the structure of brittle nanowires can be tuned to a softer phase characterized by a defective short-range order and an excess free volume upon systematic structural rejuvenation, leading to enhanced tensile ductility. The presented results shed light on the fundamental deformation mechanisms of nanoscaled metallic glasses and demarcate ductile and catastrophic failure.

Feasibility of using bulk metallic glass for self-expandable stent applications

Self-expandable stents are widely used to restore blood flow in a diseased artery segment by keeping the artery open after angioplasty. Despite the prevalent use of conventional crystalline metallic alloys, for example, nitinol, to construct self-expandable stents, new biomaterials such as bulk metallic glasses (BMGs) are being actively pursued to improve stent performance. Here, we conducted a series of analyses including finite element analysis and molecular dynamics simulations to investigate the feasibility of using a prototypical Zr-based BMG for self-expandable stent applications. We model stent crimping of several designs for different percutaneous applications. Our results indicate that BMG-based stents with diamond-shaped crowns suffer from severe localization of plastic deformation and abrupt failure during crimping. As a possible solution, we further illustrate that such abrupt failure could be avoided in BMG-based stents without diamond shape crowns. This work would open a new horizon for a quest toward exploiting superior mechanical and functional properties of metallic glasses to design future stents. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 1874-1882, 2017.

Insensitivity to Flaws Leads to Damage Tolerance in Brittle Architected Meta-Materials

Cellular solids are instrumental in creating lightweight, strong, and damage-tolerant engineering materials. By extending feature size down to the nanoscale, we simultaneously exploit the architecture and material size effects to substantially enhance structural integrity of architected meta-materials. We discovered that hollow-tube alumina nanolattices with 3D kagome geometry that contained pre-fabricated flaws always failed at the same load as the pristine specimens when the ratio of notch length (a) to sample width (w) is no greater than 1/3, with no correlation between failure occurring at or away from the notch. Samples with (a/w) > 0.3, and notch length-to-unit cell size ratios of (a/l) > 5.2, failed at a lower peak loads because of the higher sample compliance when fewer unit cells span the intact region. Finite element simulations show that the failure is governed by purely tensile loading for (a/w) < 0.3 for the same (a/l); bending begins to play a significant role in failure as (a/w) increases. This experimental and computational work demonstrates that the discrete-continuum duality of architected structural meta-materials may give rise to their damage tolerance and insensitivity of failure to the presence of flaws even when made entirely of intrinsically brittle materials.

Stretchable, Transparent Electrodes as Wearable Heaters Using Nanotrough Networks of Metallic Glasses with Superior Mechanical Properties and Thermal Stability

Mechanical robustness, electrical and chemical reliabilities of devices against large deformations such as bending and stretching have become the key metrics for rapidly emerging wearable electronics. Metallic glasses (MGs) have high elastic limit, electrical conductivity, and corrosion resistance, which can be promising for applications in wearable electronics. However, their applications in wearable electronics or transparent electrodes have not been extensively explored so far. Here, we demonstrate stretchable and transparent electrodes using CuZr MGs in the form of nanotrough networks. MG nanotroughs are prepared by electrospinning and cosputtering process, and they can be transferred to various desired substrates, including stretchable elastomeric substrates. The resulting MG nanotrough network is first utilized as a stretchable transparent electrode, presenting outstanding optoelectronic (sheet resistance of 3.8 Ω/sq at transmittance of 90%) and mechanical robustness (resistance change less than 30% up to a tensile strain of 70%) as well as excellent chemical stability against hot and humid environments (negligible degradation in performance for 240 h in 85% relative humidity and 85 °C). A stretchable and transparent heater based on the MG nanotrough network is also demonstrated with a wide operating temperature range (up to 180 °C) and excellent stretchability (up to 70% in the strain). The excellent mechanical robustness of these stretchable transparent electrode and heater is ascribed to the structural configuration (i.e., a nanotrough network) and inherent high elastic limit of MGs, as supported by experimental results and numerical analysis. We demonstrate their real-time operations on human skin as a wearable, transparent thermotherapy patch controlled wirelessly using a smartphone as well as a transparent defroster for an automobile side-view mirror, suggesting a promising strategy toward next-generation wearable electronics or automobile applications.

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.

In situ fabrication and investigation of nanostructures and nanodevices with a microscope

The widespread availability of nanostructures and nanodevices has placed strict requirements on their comprehensive characterization.

Deformation and failure mechanisms of nanoscale cellular structures of metallic glasses

Cellular metallic glasses (MGs) can be good candidates for structural and functional applications due to their light weight, enhanced ductility and excellent energy absorption performance.

Size Effect Suppresses Brittle Failure in Hollow Cu60Zr40 Metallic Glass Nanolattices Deformed at Cryogenic Temperatures

To harness "smaller is more ductile" behavior emergent at nanoscale and to proliferate it onto materials with macroscale dimensions, we produced hollow-tube Cu60Zr40 metallic glass nanolattices with the layer thicknesses of 120, 60, and 20 nm. They exhibit unique transitions in deformation mode with tube-wall thickness and temperature. Molecular dynamics simulations and analytical models were used to interpret these unique transitions in terms of size effects on the plasticity of metallic glasses and elastic instability.

Necking and notch strengthening in metallic glass with symmetric sharp-and-deep notches

Notched metallic glasses (MGs) have received much attention recently due to their intriguing mechanical properties compared to their unnotched counterparts, but so far no fundamental understanding of the correlation between failure behavior and notch depth/sharpness exists. Using molecular dynamics simulations, we report necking and large notch strengthening in MGs with symmetric sharp-and-deep notches. Our work reveals that the failure mode and strength of notched MGs are strongly dependent on the notch depth and notch sharpness. By increasing the notch depth and the notch sharpness, we observe a failure mode transition from shear banding to necking, and also a large notch strengthening. This necking is found to be caused by the combined effects of large stress gradient at the notch roots and the impingement and subsequent arrest of shear bands emanating from the notch roots. The present study not only shows the failure mode transition and the large notch strengthening in notched MGs, but also provides significant insights into the deformation and failure mechanisms of notched MGs that may offer new strategies for the design and engineering of MGs.

General nanomoulding with bulk metallic glasses

Bulk metallic glasses (BMGs) are ideal for nanomoulding as they possess desirable strength for molds as well as for moldable materials and furthermore lack intrinsic size limitations. Despite their attractiveness, only recently Pt-based BMGs have been successfully molded into pores ranging 10-100 nm (Kumar et al 2009 Nature 457 868-72). Here, we introduce a quantitative theory, which reveals previous challenges in filling nanosized pores. This theory considers, in addition to a viscous and a capillary term, also oxidation, which becomes increasingly more important on smaller length scales. Based on this theory we construct a nanomoulding processing map for BMG, which reveals the limiting factors for BMG nanomoulding. Based on the quantitative prediction of the processing map, we introduce a strategy to reduce the capillary effect through a wetting layer, which allows us to mold non-noble BMGs below 1 μm in air. An additional benefit of this strategy is that it drastically facilitates demoulding, one of the main challenges of nanomoulding in general.