Strength Of Metallic Glasses Research Articles

Formation and fracture of Mg88Al6Zn6 MGs analyzed by topologically close-packed cluster correlation

The microstructure of metallic glasses (MGs) significantly impacts glass transition and mechanical properties, but the current research on this topic is still not comprehensive enough. Therefore, the rapid solidification and tensile processes of Mg88Al6Zn6 MGs have been investigated in this paper by molecular dynamics simulations and their microstructure has been analyzed by the Largest Standard Cluster Analysis (LaSCA). The results show the transformation of other clusters to topological close-packed (TCP) and defective TCP (D-TCP) clusters during the rapid solidification process. The transformation rate slows down significantly near the glass transition temperature (Tg) 530 K, and the two finally reach 39.19%. These clusters can be divided into five groups (c1–c5) based on their correlation indexes, and their dynamic behavior and hereditary are discussed. Clusters with strong correlations tend to form larger-sized medium-range ordered nanoclusters, which serve as the “structural skeleton” of MGs. During the tensile process, TCP and D-TCP clusters with a lower average atomic potential energy (−0.57 eV) exhibit a higher packing density and structural stability, consequently enhancing the strength of MGs (3.29 GPa). Furthermore, the larger-sized nanoclusters formed by strongly correlated clusters also contribute to the strength of local regions, thereby improving the overall tensile performance of MGs. These findings offer valuable insights into the microstructural mechanisms involved in the formation and fracture of MGs.

Modulating mechanical performances of metallic amorphous materials through phase gradient

Increasing strength is usually at the cost of sacrificing ductility in structural materials. The tradeoff becomes even conspicuous in a category of metallic amorphous materials, the so-called metallic glasses (MGs) featured without any atomic-scale translational symmetry. Therefore, there remains little room for simultaneous optimization of strength and ductility in MGs through tailoring the morphology and kinetics of structural imperfections. Here we propose an alternative strategy for modulating the mechanical properties of MGs through introducing proper content of compositional or phase gradient inspired by the mechanistic strain gradient theory. We design two types of CuZr-based phase gradient metallic glasses (PGMGs) with different compositional concentration gradient directions in either continuous or stepped gradient form. Extensive molecular dynamics simulations demonstrate that phase gradient raises the concentration of mechanically stable icosahedral and icosahedron-like Voronoi polyhedra and, thus, increases the strength of MGs. In terms of plastic deformation, free volume mismatch between phases invalidates the autocatalytic activation mechanism of shear transformation zones, resulting in greater resistance to shear band propagation. The phase gradient also encourages branches of shear band and nucleation of multiple shear bands, which mechanism delocalizes deformation and postpones failure. These mechanisms lead to an improvement in the overall ductility of the PGMGs. The present strategy sheds light on evading the long-standing strength-ductility tradeoff in amorphous metals through extrinsic chemical and geometrical modulation that can be handled by appropriate thermal processing and fabrication technique.

Effect of spherical indenter radius and loading rate on the kinetic nanoindentation creep behavior of La-based metallic glasses

Although the effect of the deformation field size on the mechanical strength of metallic glasses (MGs) has been investigated thoroughly, a few studies have investigated the effect of the deformation field size on the kinetic creep deformation behavior for these materials. This issue is addressed in the current work by investigating the nanoindentation creep kinetics of La-based MGs under the application of spherical indenters with two different tip radii and three different loading rates. The experimental data are fit to a viscoelastic model to extract the activation volume and relaxation time characteristics of the kinetic nanoindentation creep deformation of the samples. The results demonstrate that the nanoindentation creep kinetics are not only strongly dependent on the loading rate, but also vary significantly with the indenter tip radius. The deformation dynamics is interpreted by the stretched exponent β, indicating its relatively small value at a large loading rate along with a large indenter tip radius. Furthermore, the observed effect of the deformation field size can be attributed to an interplay between the deformation field under the spherical indenter and flow defects associated with the excess free volume, shear transition zone, or shear band embryo with an evolution that is essentially dependent on the loading rate. The current findings contribute to a fundamental understanding of the mechanism of shear softening during creep deformation in MGs.

Simultaneous improvement of plasticity and strength of metallic glasses by tailoring residual stress: Role of stress gradient on shear banding

Inducing residual stress in metallic glasses (MGs) is recognized to be beneficial for plasticity but the mechanisms on how it affects shear band nucleation, propagation and multiplication remain poorly understood. With the aid of experimental and computational approaches, we address this issue by comparatively studying the deformation behavior of two types of MG samples, which were individually prepared by surface imprinting and photo-chemical etching but having similar surface patterns. Results showed that the imprinted MGs exhibit simultaneously enhanced plasticity and compressive strength, while the etched ones show limited plasticity improvement and reduced strength. The enhanced mechanical properties of the imprinted MGs are attributed to the compressive residual stresses generated near the surfaces, rather than the resultant geometrical pits. Finite element analysis revealed that the residual stress induces obvious stress gradient and inhomogeneous plastic deformation, which facilitate heterogeneous nucleation of multiple shear bands near the surfaces. Complementary atomistic simulations further revealed that the stress gradient resulting from the residual stress slows down the shear banding dynamics and causes deflection and branching, which consequently promotes shear band multiplication during propagation. This work uncovers the interactions between the residual stresses and shear bands, which are useful for processing MGs with desirable mechanical properties.

Effects of minor alloying on the mechanical properties of Al based metallic glasses

Minor alloying is widely used to control mechanical properties of metallic glasses (MGs). The present understanding of how a small amount of alloying element changes strength is that the additions lead to more efficient packing of atoms and increased local topological order, which then increases the barrier for shear transformations and the resistance to plastic deformation. Here, we discover that minor alloying can improve the strength of MGs by increasing the chemical bond strength alone and show that this strengthening is distinct from changes in topological order. The results were obtained using Al–Sm based MGs minor alloyed with transition metals (TMs). The addition of TMs led to an increase in the hardness of the MGs which, however, could not be explained based on changes in the topological ordering in the structure. Instead we found that it was the strong bonding between TM and Al atoms which led to a higher resistance to shear transformation that resulted in higher strength and hardness, while the topology around the TM atoms had no influence on their mechanical response. This finding demonstrates that the effects of topology and chemistry on mechanical properties of MGs are independent of each other and that they should be understood as separate, sometimes competing mechanisms of strengthening. This understanding lays a foundation for design of MGs with improved mechanical properties.

Improving fatigue property of metallic glass by tailoring the microstructure to suppress shear band formation

Metallic glass (MG) usually exhibits poor fatigue property, limiting its application as structural materials. Although fatigue failure happens at stress level much lower than the yield strength of MGs, shear band has been found to be one of main origins of fatigue crack initiation under high stress level. In this work, to enhance the fatigue property of MG, we proposed and verified a new strategy through tailoring the microstructure to suppress the shear band formation and then to impede the fatigue crack initiation, in contrast to the previous way of proliferating shear bands. Heat treatment, which is controllable, nondestructive and easy to conduct in practice, was utilized to adjust the MG microstructure. In order to make shear band difficult to initiate, annealing before glass transition temperature was performed. To determine better microstructure of MG with enhanced fatigue property, we employed the results of simple mechanical tests including uniaxial tension, compression and notch tension as feedbacks. The treated MG materials with improved elastic limit but only slightly decreased notch toughness than the as-cast samples were expected to show enhanced fatigue property, which has been examined by subsequent experiments. The enhanced elastic limit implies the improved resistances for shear band formation and thus fatigue crack initiation, while the still high notch toughness guarantees the large tolerance for fatigue failure. The present strategy provides a promising way for enhancing the fatigue endurance property of MGs without the requirement of intrinsically large plasticity, which may thus be beneficial for the structural application of most MGs with high glass forming ability.

Enhancing the plasticity of noncrystalline Cu[sbnd]Zr multilayer: Insights from molecular dynamics simulations

Amorphous alloys with pre-fabricated defective structure have been proved to have splendid ductility, making it a prospective second phase for metallic glass (MG) matrix composites. Here, the plastic deformation behavior of amorphous-amorphous (A/A) dual-phase nano-multilayers is successfully simulated by performing molecular dynamics method. The results reveal that the soft phase with structural defects in A/A nano-multilayers plays a key role in improving plasticity of MG. Specifically, when the density of structural defects is small, most of plastic deformation dominated by a single shear band is mainly confined to the flabby soft phase. A suitable density makes the clusters with large atomic shear strain more likely to be activated at the A/A interfaces, thereby linking the shear deformation between the two amorphous phases and promoting a relatively uniform global plasticity. Nevertheless, the free volume gradient caused by excessive density difference between two amorphous phases will create the rapid release of large stress at the A/A interfaces. The results indicate that the introduction of A/A interfaces and soft MG phase is an effective approach for significantly improving the plasticity without damage and expense the strength of MG.

Design strategy for high plasticity and strength in metallic glasses: A molecular dynamics simulation study

The poor plasticity of metallic glasses (MGs) limits their application as structural materials. How to enhance dramatically the plasticity of the MGs without compromising the strength is a highly attractive topic. Here, the effect of layer thicknesses and analogous flow defects concentrations (AFDCs) on the mechanical behavior of amorphous/amorphous (A1/A2) nano-multilayers is investigated by molecular dynamics method. The results indicate, when layer thickness is small (from 1.8 to 8.5 nm), for any given layer thickness, the plastic deformation of A1/A2 nano-multilayer evolves from a “shear localization” fashion to a “uniform deformation” mode, and eventually to a “shear localization” deformation again with the increase of AFDC. Comparing with the monolithic MGs, the A1/A2 nano-multilayers with optimum AFDC manifesting uniform deformation achieve a perfect combination of high strength and superior plasticity. What is more, the optimum AFDC could always be found for the sample with arbitrary layer thickness, and shows an upward tendency with increasing layer thickness. When layer thickness is large (13.0 and 25.0 nm), no matter how the AFDC changes, the optimum AFDC disappears for determined layer thickness. In this case, the plastic deformation behavior is arrested in A1 soft layer, resulting in intense shear localization and the reduced plasticity.

The stochastic transition from size dependent to size independent yield strength in metallic glasses

It has been an enduring and heated debate whether the yield strength of metallic glasses (MGs) is size dependent or size independent. In this work, we first develop a micromechanical model by taking into account the stochasticity for shear band initiation in microcompression. Our modeling is subsequently verified through the extensive in-situ and ex-situ microcompression experiments. Through the efforts of combined experiments and modeling, we show a size-controlled stochastic transition from the size dependent to the size independent yield strength in the MG micropillars. Such a stochastic transition is featured with a strong fluctuation in the measured yield strengths when the micropillar size is near an intrinsic length scale which varies with the chemical composition of MGs. In contrast, such a size-controlled transition appear deterministic with little data scattering in tension. At the fundamental level, our results unfold a size dependent shear band initiation process in MGs, which may be applicable to other amorphous materials of technological importance.

The breakdown of strength size scaling in spherical nanoindentation and microcompression of metallic glasses

It was previously reported that the strength of metallic glasses (MGs) would scale inversely with the size of a sample or a deformation field, commonly known as “smaller-being-stronger”. However, based on the extensive spherical nanoindentation experiments conducted across a variety of MGs, we demonstrate that such strength-size scaling breaks down at a critical indenter tip radius, which is caused by the transition of the yielding mechanism from bulk- to surface-controlled shear band initiation. Our experimental findings also provide an explanation for the unusual strength scattering observed in the micro-compression of MGs.

Rate dependent of strength in metallic glasses at different temperatures

The correlation between the strength at the macroscale and the elastic deformation as well as shear cracking behavior at the microscale of bulk metallic glasses (BMGs) is investigated. The temperatures of 298 K and 77 K as well as the strain rate ranging from 10−6 s−1 to 10−2 s−1 are applied to the BMGs, in which the mechanical responses of the BMGs are profiled through the compression tests. The yield strength is associated with the activation of the elementary deformation unit, which is insensitive to the strain rate. The maximum compressive strength is linked to the crack propagation during shear fracture process, which is influenced by the strain rate. The cryogenic temperature of 77 K significantly improves the yield strength and the maximum compressive strength of the BMGs.

Metallic Glass as a Mechanical Material for Microscanners

Microelectromechanical system (MEMS) actuators essentially have movable silicon structures where the mechanical motion can be activated electronically. The microscanner is one of the most successfully commercialized MEMS devices which are widely used for collecting optical information, manipulating light, and displaying images. While silicon is abundant, it is also brittle and stiff and when microprocessed, defects are not uncommon. These defects result in weakness under torsional stress and this has been the key factor limiting the scanning performance of the microscanner. Here a metallic glass (MG)‐based microscanner is reported with MG as the material for the moving torsion bars. The low elastic modulus, high fracture toughness, and high strength of MG offers, for the first time, an ultralarge rotating angle of 146° with power consumption lowered to the microwatt range, and a smaller driving force and better actuation performance, than conventional single crystal silicon and polycrystalline silicon. The high spatial resolution and large scanning field of the MG‐based microscanner are demonstrated in the tomographic imaging of a human finger. This development of an MG‐based MEMS possibly opens a new field of low‐powered MEMS devices with extreme actuation and enhanced sensing.

Size effects on tensile and compressive strengths in metallic glass nanowires

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 100nm. In order to reveal the size effects and associated deformation mechanisms of metallic glasses in an even finer scale, we perform large-scale atomistic simulations for the uniaxial compression and tension of metallic glass nanowires. The simulation results show that, as the diameter of metallic glass samples decreases from 45nm to 8nm, the tensile yield strength increases while the compressive yield strength decreases. Homogeneous flow is observed as the governing deformation mechanism in all simulated metallic glass samples, where plastic shearing tends to initiate on the sample surface and propagate into the interior. To rationalize the size dependence of yield strengths, we propose a theoretical model based on the concept of surface stress and Mohr–Coulomb criterion. The theoretical predictions agree well with the simulation results, implying the important role of surface stress on the yielding of MGs below 100nm. Finally, a discussion about the size effects of strength in metallic glasses at different length scales is provided. Our results suggest that the shear band energy and surface stress might be the two crucial parameters in determining the critical size required for the transition from shear localization to homogeneous deformation in MGs.

Effects of Aspect Ratio on the Shear Band Arrangements of Zr-Based Metallic Glasses

Effects of aspect ratios of Zr-based metallic glasses on the shear band arrangements are investigated through molecular dynamics simulations and experiments. It is found that as the aspect ratio decreases, the dense multiple shear bands form, effectively depressing the formation of the penetrating shear bands, which improves plasticity and strength of metallic glass. Simulation reproduces the images of the evolution of the shear bands in metallic glass, explaining the experimental observation. It is found that as the aspect ratio decreases, shear transformation zones disperse evenly in the entire model, restraining connecting into penetrating shear bands.

Estimate of the Maximum Strength of Metallic Glasses from Finite Deformation Theory

Maximum strength sets the limit of a material's intrinsic resistance to permanent deformation. Its significance, however, lies not in the highest strength value that a solid can possibly achieve, but rather in how this quantity is degraded, from which one could decipher the underlying mechanisms of yielding in a real material. A wide range of maximum strength values have been measured experimentally for metallic glasses. However, the true maximum strength remains unknown to date. Here, using finite deformation theory, we give the first theoretical estimate of the ultimate strength of metallic glasses. Our theoretical results, along with those from experiment and simulation, lead us to several mechanisms of degradation of the theoretical strength that are closely connected to correlated atomic motion with varying characteristic length in real metallic glasses.

Effect of the ultrasound action on the acoustic emission and mechanical properties of zirconium-based bulk metallic glasses

The effect of preliminary ultrasound action on the mechanical properties and features of the structure of zirconium-based bulk metallic glasses has been studied by the method of acoustic emission under uniaxial compression. Results of studies have been interpreted using the polycluster model of the structure of amorphous metallic alloys. Analysis of the obtained data has allowed us to substantiate the mechanism of the change in the structure and strength of metallic glasses as a result of alternating-sign mechanical loading with an ultrasound frequency of 20 kHz.

How does the initial free volume distribution affect shear band formation in metallic glass?

Introducing heterogeneities into the structure is an effective way to enhance the plasticity in metallic glasses (MGs). As natural heterogeneity, the original randomly distributed free volume in MGs has been found to be in favor of plasticity. However, the exact correlation between the free volume distribution and mechanical response is still unclear. In this paper, we investigate the shear banding in MGs with different structural disorders, characterized by both the free volume concentration (FVC) and the free volume dispersion (FVD). It is found that, either high FVC or wide FVD leads to low activation stress of shear band; wide FVD promotes the multiplication of shear bands but high FVC restricts it. It reveals that the yield strength in MGs is dependent on the amount of free volume while the plasticity mainly relies on the distribution. An optimum combination of the two aspects probably helps to design a MG of both good plasticity and high strength.

Low temperature mechanical properties of metallic glasses connection with structure

Available data on plasticity and strength of metallic glasses below the room temperature (down to 0.5 K) are considered and explained on the basis of the polycluster model of amorphous solids especially with taking into consideration possible atomic structure of clusters and defects of intercluster boundaries.