Atomic scale fluctuations govern brittle fracture and cavitation behavior in metallic glasses

Athermal quasistatic cavitation in amorphous solids: Effect of random pinning

Amorphous solids are known to fail catastrophically via fracture, and cavitation at nano-metric scales is known to play a significant role in such a failure process. Micro-alloying via inclusions is often used as a means to increase the fracture toughness of amorphous solids. Modeling such inclusions as randomly pinned particles that only move affinely and do not participate in plastic relaxations, we study how the pinning influences the process of cavitation-driven fracture in an amorphous solid. Using extensive numerical simulations and probing in the athermal quasistatic limit, we show that just by pinning a very small fraction of particles, the tensile strength is increased, and also the cavitation is delayed. Furthermore, the cavitation that is expected to be spatially heterogeneous becomes spatially homogeneous by forming a large number of small cavities instead of a dominant cavity. The observed behavior is rationalized in terms of screening of plastic activity via the pinning centers, characterized by a screening length extracted from the plastic-eigenmodes.

Influence of the Hydrogen Doping Method on the Atomic Structure, Mechanical Properties and Relaxation Behaviors of Metallic Glasses

The interaction of metallic glasses (MGs) with hydrogen can trigger many interesting physical, chemical and mechanical phenomena. However, atomic-scale understanding is still lacking. In this work, molecular dynamics (MD) simulations are employed to study the atomic structure, mechanical properties and relaxation behaviors of H-doped Ni50Al50 MGs doped by two methods. The properties of H-doped MGs are determined not only by the hydrogen content but also by the doping method. When H atoms are doped into the molten state of samples, H atoms can fully diffuse and interact with metallic atoms, resulting in loose local atomic structures, homogeneous deformation and enhanced β relaxation. In contrast, when H atoms are doped into as-cast MGs, the H content is crucial in affecting the atomic structure and mechanical properties. A small number of H atoms has little influence on the elastic matrix, while the percolation of shear transformation zones (STZs) is hindered by H atoms, resulting in the delay of shear band (SB) formation and an insignificant change in the strength. However, a large number of H atoms can make the elastic matrix loose, leading to the decrease in strength and the transition of the deformation mode from SB to homogeneous deformation. The H effects on the elastic matrix and flow units are also applied to the dynamic relaxation. The deformability of H-doped Ni50Al50 MGs is enhanced by both H-doping methods; however, our results reveal that the mechanisms are different.

Universal mechanical instabilities in the energy landscape of amorphous solids: Evidence from athermal quasistatic expansion

Using numerical simulations, we study the failure of an amorphous solid under athermal quasistatic expansion starting from a homogeneous high-density state. During the expansion process, plastic instabilities occur, manifested via sudden jumps in pressure and energy, with the largest event happening via cavitation leading to the material's yielding. We demonstrate that all these plastic events are characterized by saddle-node bifurcation, during which the smallest nonzero eigenvalue of the Hessian matrix vanishes via a square-root singularity. We find that after yielding and prior to complete fracture, the statistics of pressure or energy jumps corresponding to the plastic events show subextensive system-size scaling, similar to the case of simple shear but with different exponents. Thus, overall, our paper reveals universal features in the fundamental characteristics during mechanical failure in amorphous solids under any quasistatic deformation protocol.

Interfacial cavitation

Cavitation has long been recognized as a crucial predictor, or precursor, to the ultimate failure of various materials, ranging from ductile metals to soft and biological materials. Traditionally, cavitation in solids is defined as an unstable expansion of a void or a defect within a material. The critical applied load needed to trigger this instability -- the critical pressure -- is a lengthscale independent material property and has been predicted by numerous theoretical studies for a breadth of constitutive models. While these studies usually assume that cavitation initiates from defects in the bulk of an otherwise homogeneous medium, an alternative and potentially more ubiquitous scenario can occur if the defects are found at interfaces between two distinct media within the body. Such interfaces are becoming increasingly common in modern materials with the use of multimaterial composites and layer-by-layer additive manufacturing methods. However, a criterion to determine the threshold for interfacial failure, in analogy to the bulk cavitation limit, has yet to be reported. In this work, we fill this gap. Our theoretical model captures a lengthscale independent limit for interfacial cavitation, and is shown to agree with our observations at two distinct lengthscales, via two different experimental systems. To further understand the competition between the two cavitation modes (bulk versus interface), we expand our investigation beyond the elastic response to understand the ensuing unstable propagation of delamination at the interface. A phase diagram summarizes these results, showing regimes in which interfacial failure becomes the dominant mechanism.

Beyond the Average: Spatial and Temporal Fluctuations in Oxide Glass-Forming Systems

Atomic structure dictates the performance of all materials systems; the characteristic of disordered materials is the significance of spatial and temporal fluctuations on composition-structure-property-performance relationships. Glass has a disordered atomic arrangement, which induces localized distributions in physical properties that are conventionally defined by average values. Quantifying these statistical distributions (including variances, fluctuations, and heterogeneities) is necessary to describe the complexity of glass-forming systems. Only recently have rigorous theories been developed to predict heterogeneities to manipulate and optimize glass properties. This article provides a comprehensive review of experimental, computational, and theoretical approaches to characterize and demonstrate the effects of short-, medium-, and long-range statistical fluctuations on physical properties (e.g., thermodynamic, kinetic, mechanical, and optical) and processes (e.g., relaxation, crystallization, and phase separation), focusing primarily on commercially relevant oxide glasses. Rigorous investigations of fluctuations enable researchers to improve the fundamental understanding of the chemistry and physics governing glass-forming systems and optimize structure-property-performance relationships for next-generation technological applications of glass, including damage-resistant electronic displays, safer pharmaceutical vials to store and transport vaccines, and lower-attenuation fiber optics. We invite the reader to join us in exploring what can be discovered by going beyond the average.

Characterization of the deformation behaviors under uniaxial stress for bicontinuous nanoporous amorphous alloys

In this paper, the deformation behaviors of Cu₅₀Zr₅₀ bicontinuous nanoporous amorphous alloys (BNAMs) under uniaxial tension/compression are explored by molecular dynamics simulations. Scaling laws between mechanical properties and relative density are investigated. The results demonstrate that the bending deformation of the ligament is the main elastic deformation mechanism under tension. Necking and subsequent fracture of ligaments are the primary failure mechanism under tension. Under tensile loading, shear bands emerge near the plastic hinges for the BNAMs with large porosities. The typical compressive behaviors of porous structure are observed in the BNAMs with large porosities. However, for small porosity, no distinguished plateau and densification are captured under compression. The tension-compression asymmetry of modulus increases with increasing porosity, whereas the BNAMs can be seen as tension-compression symmetry of yield strength. The modulus and yield strength are negatively correlated with temperature, but a positive relationship between the tensile ductility and temperature is shown. This work will help to provide a useful understanding of the mechanical behaviors of the BNAMs.

Substantially enhanced plasticity of bulk metallic glasses by densifying local atomic packing

Introducing regions of looser atomic packing in bulk metallic glasses (BMGs) was reported to facilitate plastic deformation, rendering BMGs more ductile at room temperature. Here, we present a different alloy design approach, namely, doping the nonmetallic elements to form densely packed motifs. The enhanced structural fluctuations in Ti-, Zr- and Cu-based BMG systems leads to improved strength and renders these solutes' atomic neighborhoods more prone to plastic deformation at an increased critical stress. As a result, we simultaneously increased the compressive plasticity (from ∼8% to unfractured), strength (from ∼1725 to 1925 MPa) and toughness (from 87 ± 10 to 165 ± 15 MPa√m), as exemplarily demonstrated for the Zr₂₀Cu₂₀Hf₂₀Ti₂₀Ni₂₀ BMG. Our study advances the understanding of the atomic-scale origin of structure-property relationships in amorphous solids and provides a new strategy for ductilizing BMG without sacrificing strength.

Tensile and Impact Toughness Properties of a Zr-Based Bulk Metallic Glass Fabricated via Laser Powder-Bed Fusion

In the past few years, laser powder-bed fusion (LPBF) of bulk metallic glasses (BMGs) has gained significant interest because of the high heating and cooling rates inherent to the process, providing the means to bypass the crystallization threshold. In this study, (for the first time) the tensile and Charpy impact toughness properties of a Zr-based BMG fabricated via LPBF were investigated. The presence of defects and lack of fusion (LoF) in the near-surface region of the samples resulted in low properties. Increasing the laser power at the borders mitigated LoF formation in the near-surface region, leading to an almost 27% increase in tensile yield strength and impact toughness. Comparatively, increasing the core laser power did not have a significant influence. It was therefore confirmed that, for BMGs like for crystalline alloys, near-surface LoFs are more detrimental than core LoFs. Although increasing the border and core laser power resulted in a higher crystallized fraction, detrimental to the mechanical properties, reducing the formation of LoF defects (confirmed using micro-computed tomography, Micro-CT) was comparatively more important.

Toughening and Crack Healing Mechanisms in Nanotwinned Diamond Composites with Various Polytypes

As an emerging ceramic material, recently synthesized nanotwinned diamond composites with various polytypes embedded in nanoscale twins exhibit unprecedented fracture toughness without sacrificing hardness. However, the toughening and crack healing mechanisms at the atomic scale and the associated crack propagation process of nanotwinned diamond composites remain mysterious. Here, we perform large-scale atomistic simulations of crack propagation in nanotwinned diamond composites to explore the underlying toughening and crack healing mechanisms in nanotwinned diamond composites. Our simulation results show that nanotwinned diamond composites have a higher fracture energy than single-crystalline and nanotwinned diamonds, which originates from multiple toughening mechanisms, including twin boundary and phase boundary impeding crack propagation, crack deflection and zigzag paths in nanotwins and sinuous paths in polytypes, and the formation of disordered atom clusters. More remarkably, our simulations reproduce more detailed crack propagation processes at the atomic scale, which is inaccessible by experiments. Moreover, our simulations reveal that crack healing occurs due to the rebonding of atoms on fracture surfaces during unloading and that the extent of crack healing is associated with whether the crack surfaces are clean. Our current study provides mechanistic insights into a fundamental understanding of toughening and crack healing mechanisms in nanotwinned diamond composites.

Tension-Tension Fatigue Behavior of High-Toughness Zr61Ti2Cu25Al12 Bulk Metallic Glass

Bulk metallic glasses have application potential in engineering structures due to their exceptional strength and fracture toughness. Their fatigue resistance is very important for the application as well. We report the tension-tension fatigue damage behavior of a Zr₆₁Ti₂Cu₂₅Al₁₂ bulk metallic glass, which has the highest fracture toughness among BMGs. The Zr₆₁Ti₂Cu₂₅Al₁₂ glass exhibits a tension-tension fatigue endurance limit of 195 MPa, which is higher than that of high-toughness steels. The fracture morphology of the specimens depends on the applied stress amplitude. We found flocks of shear bands, which were perpendicular to the loading direction, on the surface of the fatigue test specimens with stress amplitude higher than the fatigue limit of the glass. The fatigue cracking of the glass initiated from a shear band in a shear band flock. Our work demonstrated that the Zr₆₁Ti₂Cu₂₅Al₁₂ glass is a competitive structural material and shed light on improving the fatigue resistance of bulk metallic glasses.

Observation of cavitation governing fracture in glasses

Crack propagation is the major vehicle for material failure, but the mechanisms by which cracks propagate remain longstanding riddles, especially for glassy materials with a long-range disordered atomic structure. Recently, cavitation was proposed as an underlying mechanism governing the fracture of glasses, but experimental determination of the cavitation behavior of fracture is still lacking. Here, we present unambiguous experimental evidence to firmly establish the cavitation mechanism in the fracture of glasses. We show that crack propagation in various glasses is dominated by the self-organized nucleation, growth, and coalescence of nanocavities, eventually resulting in the nanopatterns on the fracture surfaces. The revealed cavitation-induced nanostructured fracture morphologies thus confirm the presence of nanoscale ductility in the fracture of nominally brittle glasses, which has been debated for decades. Our observations would aid a fundamental understanding of the failure of disordered systems and have implications for designing tougher glasses with excellent ductility.

An independent derivation and verification of the voids nucleation failure mechanism: significance for materials failure

Independent derivations are given for the failure criteria of the purely dilatational stress state involving voids nucleation failure as well as for the purely distortional stress state involving shear bands failure. The results are consistent with those from a recently derived failure theory and they further substantiate the failure theory. The voids nucleation mechanism is compared with the ideal theoretical strength of isotropic materials as derived by density functional theory and two other atomic-scale methods. It is found that a cross-over occurs from the voids nucleation failure mechanism to the ideal strength limitation as the tensile to compressive strengths ratio, T / C , increases toward a value of unity. All the results are consistent with the failure modes transition results from the general failure theory.

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.

3D surface condensation of large atomic shear strain in nanoscale metallic glasses under low uniaxial stress

Nanoscale metallic glasses (MGs) are frequently used in experimental and computational studies to probe the deformation mechanisms in amorphous metals. Potential consequences of the significant surface to volume ratio in these extremely small materials, nevertheless, are not well understood. Here, using molecular dynamics simulations and novel selective 3D visualization, we show that significant irreversible atomic shear strain condenses on the 3D surface of these materials under low uniaxial stress, while the interior atoms are bearing much lower, mostly reversible shear strain. This is observed for various sample geometries, dimensions, strain rates and temperatures, and attributable to the correlations of atomic shear strain with atomic potential energy and coordination number. The results reveal the profound influence of the surface on the strain partitioning in nanoscale MGs across the 3D volume, critical to the initiation and continuation of plasticity.

An evaluation of the failure modes transition and the Christensen ductile/brittle failure theory using molecular dynamics

The Christensen ductile/brittle failure theory can be interpreted in terms of the associated failure modes, those of shear bands and voids nucleation. Their conjunction is then termed as the failure modes transition and it is studied here using molecular dynamics. The test material is taken as a particular metallic glass, CuZr. First the theoretical failure criteria are evaluated and then the theoretical failure modes transition is evaluated. Both are found to perform extremely well. The overall failure theory contains three modes of failure, the two already mentioned plus a fracture criterion. A general conclusion from the work is that the voids nucleation criterion is of unusually broad relevance. Voids nucleation leads to voids growth and then further deteriorating mechanisms and ultimately failure. But the voids nucleation is the precipitating event of all that subsequently occurs in this process. Access to these capabilities is gained through the failure theory for all homogeneous, full density, isotropic materials. Only two standard testing measurements are needed to calibrate the entire failure theory, including the transitions.

Dilatancy induced ductile-brittle transition of shear band in metallic glasses

Dilatancy-generated structural disordering, an inherent feature of metallic glasses (MGs), has been widely accepted as the physical mechanism for the primary origin and structural evolution of shear banding, as well as the resultant shear failure. However, it remains a great challenge to determine, to what degree of dilatation, a shear banding will evolve into a runaway shear failure. In this work, using in situ acoustic emission monitoring, we probe the dilatancy evolution at the different stages of individual shear band in MGs that underwent severely plastic deformation by the controlled cutting technology. A scaling law is revealed that the dilatancy in a shear band is linearly related to its evolution degree. A transition from ductile-to-brittle shear bands is observed, where the formers dominate stable serrated flow, and the latter lead to a runaway instability (catastrophe failure) of serrated flow. To uncover the underlying mechanics, we develop a theoretical model of shear-band evolution dynamics taking into account an atomic-scale deformation process. Our theoretical results agree with the experimental observations, and demonstrate that the atomic-scale volume expansion arises from an intrinsic shear-band evolution dynamics. Importantly, the onset of the ductile-brittle transition of shear banding is controlled by a critical dilatation.

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.

Balancing strength, hardness and ductility of Cu64Zr36 nanoglasses via embedded nanocrystals

Superplasticity can be achieved in nanoglasses but at the expense of strength, and such a loss can be mitigated via embedding stronger nanocrystals, i.e., forming nanoglass/nanocrystal composites. As an illustrative case, we investigate plastic deformation of Cu₆₄Zr₃₆ nanoglass/nanocrystalline Cu composites during uniaxial tension and nanoindentation tests with molecular dynamics simulations. With an increasing fraction of nanocrystalline grains, the tensile strength of the composite is enhanced, while its ductility decreases. The dominant interface type changes from a glass-glass interface to glass-crystal interface to grain boundary, corresponding to a failure mode transition from superplastic flow to shear banding to brittle intercrystal fracture, respectively. Accordingly, the indentation hardness increases continuously and strain localization beneath the indenter is more and more severe. For an appropriate fraction of nanocrystalline grains, a good balance among strength, hardness and ductility can be realized, which is useful for the synthesis of novel nanograined glass/crystalline composites with high strength, high hardness and superior ductility.

Fracture behaviors under pure shear loading in bulk metallic glasses

Pure shear fracture test, as a special mechanical means, had been carried out extensively to obtain the critical information for traditional metallic crystalline materials and rocks, such as the intrinsic deformation behavior and fracture mechanism. However, for bulk metallic glasses (BMGs), the pure shear fracture behaviors have not been investigated systematically due to the lack of a suitable test method. Here, we specially introduce a unique antisymmetrical four-point bend shear test method to realize a uniform pure shear stress field and study the pure shear fracture behaviors of two kinds of BMGs, Zr-based and La-based BMGs. All kinds of fracture behaviors, the pure shear fracture strength, fracture angle and fracture surface morphology, are systematically analyzed and compared with those of the conventional compressive and tensile fracture. Our results indicate that both the Zr-based and La-based BMGs follow the same fracture mechanism under pure shear loading, which is significantly different from the situation of some previous research results. Our results might offer new enlightenment on the intrinsic deformation and fracture mechanism of BMGs and other amorphous materials.

Unraveling the Anomalous Grain Size Dependence of Cavitation

Experimental studies have identified an anomalous grain size dependence associated with the critical tensile pressure that a metal may sustain before catastrophic failure by cavitation processes. Here we derive the first quantitative theory (and its associated closed-form solution) capable of explaining this phenomena. The theory agrees well with experimental measurements and atomistic calculations over a very wide range of conditions. Utilizing this theory, we are able to map out three distinct regimes in which the critical tensile pressure for cavitation failure (i) increases with decreasing grain size in accordance with conventional wisdom, (ii) nonintuitively decreases with decreasing grain size, and (iii) is independent of grain size. The theory also predicts microscopic signatures of the cavitation process which agree with available data.

Cavitation-Induced Fracture Causes Nanocorrugations in Brittle Metallic Glasses

Brittle metallic glasses exhibit a unique and intriguing fracture morphology of periodic nanocorrugations whose spacing and amplitude are of the order of tens of nanometers. We show through continuum simulations that they fail by spontaneous and simultaneous cavitation within multiple weak zones arising due to intrinsic atomic density fluctuations ahead of a notch tip. Dynamic crack growth would then occur along curved but narrowly confined shear bands that link the growing cavities. This mechanism involves little dissipation and also explains the formation of nanocorrugations.

How the toughness in metallic glasses depends on topological and chemical heterogeneity

To gain insight into the large toughness variability observed between metallic glasses (MGs), we examine the origin of fracture toughness through bending experiments and molecular dynamics (MD) simulations for two binary MGs: Pd82Si18 and Cu46Zr54 The bending experiments show that Pd82Si18 is considerably tougher than Cu46Zr54, and the higher toughness of Pd82Si18 is attributed to an ability to deform plastically in the absence of crack nucleation through cavitation. The MD simulations study the initial stages of cavitation in both materials and extract the critical factors controlling cavitation. We find that for the tougher Pd82Si18, cavitation is governed by chemical inhomogeneity in addition to topological structures. In contrast, no such chemical correlations are observed in the more brittle Cu46Zr54, where topological low coordination number polyhedra are still observed around the critical cavity. As such, chemical inhomogeneity leads to more difficult cavitation initiation in Pd82Si18 than in Cu46Zr54, leading to a higher toughness. The absence of chemical separation during cavitation initiation in Cu46Zr54 decreases the energy barrier for a cavitation event, leading to lower toughness.

Intrinsic correlation between β-relaxation and spatial heterogeneity in a metallic glass

β-relaxation has long been attributed to localized motion of constituent molecules or atoms confined to isolated regions in glasses. However, direct experimental evidence to support this spatially heterogeneous scenario is still missing. Here we report the evolution of nanoscale structural heterogeneity in a metallic glass during β-relaxation by utilizing amplitude-modulation dynamic atomic force microscopy. The successive degeneration of heterogeneity during β-relaxation can be well described by the Kohlrausch–Williams–Watts equation. The characteristic relaxation time and activation energy of the heterogeneity evolution are in accord with those of excess enthalpy release by β-relaxation. Our study correlates β-relaxation with nanoscale spatial heterogeneity and provides direct evidence on the structural origins of β-relaxation in metallic glasses.

Beta-relaxation in glasses is commonly attributed to the confined motions of constituent atoms in nanosized domains, but there is no direct evidence so far. Here, Zhu et al. show the correlation between the evolution of spatial heterogeneity at nanoscale and beta-relaxation below glass transition point.

Improved ductility of Cu64Zr36 metallic glass/Cu nanocomposites via phase and grain boundaries

We investigate tensile deformation of metallic glass/crystalline interpenetrating phase nanocomposites as regards the effects of specific area of amorphous/crystalline phase interfaces, and grain boundaries. As an illustrative case, large-scale molecular dynamics simulations are performed on Cu64Zr36 metallic glass/Cu nanocomposites with different specific interface areas and grain boundary characteristics. Plastic deformation is achieved via shear bands, shear transformation zones, and crystal plasticity. Three-dimensional amorphous/crystalline interfaces serve as effective barriers to the propagation of shear transformation zones and shear bands if formed, diffuse strain localizations, and give rise to improved ductility. Ductility increases with increasing specific interface area. In addition, introducing grain boundaries into the second phase facilitates crystal plasticity, which helps reduce or eliminate mature shear bands in the glass matrix.

The Critical Criterion on Runaway Shear Banding in Metallic Glasses

The plastic flow of metallic glasses (MGs) in bulk is mediated by nanoscale shear bands, which is known to proceed in a stick-slip manner until reaching a transition state causing catastrophic failures. Such a slip-to-failure transition controls the plasticity of MGs and resembles many important phenomena in natural science and engineering, such as friction, lubrication and earthquake, therefore has attracted tremendous research interest over past decades. However, despite the fundamental and practical importance, the physical origin of this slip-to-failure transition is still poorly understood. By tracking the behavior of a single shear band, here we discover that the final fracture of various MGs during compression is triggered as the velocity of the dominant shear band rises to a critical value, the magnitude of which is independent of alloy composition, sample size, strain rate and testing frame stiffness. The critical shear band velocity is rationalized with the continuum theory of liquid instability, physically originating from a shear-induced cavitation process inside the shear band. Our current finding sheds a quantitative insight into deformation and fracture in disordered solids and, more importantly, is useful to the design of plastic/tough MG-based materials and structures.

Suppression of Shear Banding and Transition to Necking and Homogeneous Flow in Nanoglass Nanopillars

In order to improve the properties of metallic glasses (MG) a new type of MG structure, composed of nanoscale grains, referred to as nanoglass (NG), has been recently proposed. Here, we use large-scale molecular dynamics (MD) simulations of tensile loading to investigate the deformation and failure mechanisms of Cu64Zr36 NG nanopillars with large, experimentally accessible, 50 nm diameter. Our results reveal NG ductility and failure by necking below the average glassy grain size of 20 nm, in contrast to brittle failure by shear band propagation in MG nanopillars. Moreover, the results predict substantially larger ductility in NG nanopillars compared with previous predictions of MD simulations of bulk NG models with columnar grains. The results, in excellent agreement with experimental data, highlight the substantial enhancement of plasticity induced in experimentally relevant MG samples by the use of nanoglass architectures and point out to exciting novel applications of these materials.

Cyclic Deformation in Metallic Glasses

Despite the utmost importance and decades of experimental studies on fatigue in metallic glasses (MGs), there has been so far little or no atomic-level understanding of the mechanisms involved. Here we perform molecular dynamics simulations of tension-compression fatigue in Cu50Zr50 MGs under strain-controlled cyclic loading. It is shown that the shear band (SB) initiation under cyclic loading is distinctly different from that under monotonic loading. Under cyclic loading, SB initiation takes place when aggregates of shear transformation zones (STZs) accumulating at the MG surface reach a critical size comparable to the SB width, and the accumulation of STZs follows a power law with rate depending on the applied strain. It is further shown that almost the entire fatigue life of nanoscale MGs under low cycle fatigue is spent in the SB-initiation stage, similar to that of crystalline materials. Furthermore, a qualitative investigation of the effect of cycling frequency on the fatigue behavior of MGs suggests that higher cycling frequency leads to more cycles to failure. The present study sheds light on the fundamental fatigue mechanisms of MGs that could be useful in developing strategies for their engineering applications.

On the correlation between microscopic structural heterogeneity and embrittlement behavior in metallic glasses

In order to establish a relationship between microstructure and mechanical properties, we systematically annealed a Zr-based bulk metallic glass (BMG) at 100 ~ 300 °C and measured their mechanical and thermal properties. The as-cast BMG exhibits some ductility, while the increase of annealing temperature and time leads to the transition to a brittle behavior that can reach nearly-zero fracture energy. The differential scanning calorimetry did not find any significant changes in crystallization temperature and enthalpy, indicating that the materials still remained fully amorphous. Elastic constants measured by ultrasonic technique vary only slightly with respect to annealing temperature and time, which does obey the empirical relationship between Poisson’s ratio and fracture behavior. Nanoindentation pop-in tests were conducted, from which the pop-in strength mapping provides a “mechanical probe” of the microscopic structural heterogeneities in these metallic glasses. Based on stochastically statistic defect model, we found that the defect density decreases with increasing annealing temperature and annealing time and is exponentially related to the fracture energy. A ductile-versus-brittle behavior (DBB) model based on the structural heterogeneity is developed to identify the physical origins of the embrittlement behavior through the interactions between these defects and crack tip.

Reformation Capability of Short-Range Order and Their Medium-Range Connections Regulates Deformability of Bulk Metallic Glasses

Metallic glasses (MGs) typically have high yield strength while low ductility, and the latter is commonly considered as the Achilles’ heel of MGs. Elucidate the mechanism for such low ductility becomes the research focus of this field. With molecular level simulations, we show the degree of short-range order (SRO) of atomic structure for brittle Fe-based glass decreases dramatically during the stretch, while mild change occurs in ductile Zr-based glass. The reformation capability for SRO and their medium-range connections is found to be the primary characteristics to differentiate the deformability between the two metallic glasses. We suspect that, in addition to the strength of networks formed by SRO structure, the reformation capability to reform SRO networks also plays the key role in regulating the ductility in metallic glasses. Our study provides important insights into the understanding about the mechanisms accounting for ductility or brittleness of bulk metallic glasses.

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.

Effects of partitioned enthalpy of mixing on glass-forming ability

We explore the inherent reason at atomic level for the glass-forming ability of alloys by molecular simulation, in which the effect of partitioned enthalpy of mixing is studied. Based on Morse potential, we divide the enthalpy of mixing into three parts: the chemical part (ΔEnn), strain part (ΔEstrain), and non-bond part (ΔEnnn). We find that a large negative ΔEnn value represents strong AB chemical bonding in AB alloy and is the driving force to form a local ordered structure, meanwhile the transformed local ordered structure needs to satisfy the condition (ΔEnn/2 + ΔEstrain) < 0 to be stabilized. Understanding the chemical and strain parts of enthalpy of mixing is helpful to design a new metallic glass with a good glass forming ability. Moreover, two types of metallic glasses (i.e., "strain dominant" and "chemical dominant") are classified according to the relative importance between chemical effect and strain effect, which enriches our knowledge of the forming mechanism of metallic glass. Finally, a soft sphere model is established, different from the common hard sphere model.

Mechanisms of failure in nanoscale metallic glass

The emergence of size-dependent mechanical strength in nanosized materials is now well-established, but no fundamental understanding of fracture toughness or flaw sensitivity in nanostructures exists. We report the fabrication and in situ fracture testing of ∼70 nm diameter Ni-P metallic glass samples with a structural flaw. Failure occurs at the structural flaw in all cases, and the failure strength of flawed samples was reduced by 40% compared to unflawed samples. We explore deformation and failure mechanisms in a similar nanometallic glass via molecular dynamics simulations, which corroborate sensitivity to flaws and reveal that the structural flaw shifts the failure mechanism from shear banding to cavitation. We find that failure strength and deformation in amorphous nanosolids depend critically on the presence of flaws.

Cavitation in amorphous solids

Molecular dynamics simulations of cavitation in a Zr(50)Cu(50) metallic glass exhibit a waiting time dependent cavitation rate. On short time scales nucleation rates and critical cavity sizes are commensurate with a classical theory of nucleation that accounts for both the plastic dissipation during cavitation and the cavity size dependence of the surface energy. All but one parameter, the Tolman length, can be extracted directly from independent calculations or estimated from physical principles. On longer time scales strain aging in the form of shear relaxations results in a systematic decrease of cavitation rate. The high cavitation rates that arise due to the suppression of the surface energy in small cavities provide a possible explanation for the quasibrittle fracture observed in metallic glasses.

A universal fracture criterion for high-strength materials

Recently developed advanced high-strength materials like metallic glasses, nanocrystalline metallic materials, and advanced ceramics usually fracture in a catastrophic brittle manner, which makes it quite essential to find a reasonable fracture criterion to predict their brittle failure behaviors. Based on the analysis of substantial experimental observations of fracture behaviors of metallic glasses and other high-strength materials, here we developed a new fracture criterion and proved it effective in predicting the critical fracture conditions under complex stress states. The new criterion is not only a unified one which unifies the three classical failure criteria, i.e., the maximum normal stress criterion, the Tresca criterion and the Mohr-Coulomb criterion, but also a universal criterion which has the ability to describe the fracture mechanisms of a variety of different high-strength materials under various external loading conditions.

Fracture toughness of metallic glasses: annealing-induced embrittlement

Quantitative understanding of the fracture toughness of metallic glasses, including the associated ductile-to-brittle (embrittlement) transitions, is not yet available. Here, we use a simple model of plastic deformation in glasses, coupled to an advanced Eulerian level set formulation for solving complex free-boundary problems, to calculate the fracture toughness of metallic glasses as a function of the degree of structural relaxation corresponding to different annealing times near the glass temperature. Our main result indicates the existence of an elastoplastic crack tip instability for sufficiently relaxed glasses, resulting in a marked drop in the toughness, which we interpret as annealing-induced embrittlement transition similar to experimental observations.

Medium range order of bulk metallic glasses determined by variable resolution fluctuation electron microscopy

Variable resolution fluctuation electron microscopy (FEM) experiments are implemented with hollow-cone dark-field transmission electron microscopy. Medium range order lengths of zirconium and iron based bulk metallic glasses and amorphous silicon nitride are determined from the FEM results. It shows that maximum normalized intensity variances of FEM images occur when their nominal resolution approaches the correlation length Λ of the amorphous materials. Additionally, differences in the length and magnitude of medium range order are compared between metallic and covalent bond amorphous materials.