Ductile Metallic Glasses Research Articles

Deformation-Induced Crystal Growth or Redissolution, and Crystal-Induced Strengthening or Ductilization in Metallic Glasses Containing Nanocrystals.

It is generally known that the incorporation of crystals in the glass matrix can enhance the ductility of metallic glasses (MGs), at the expense of reduced strength, and that the deformation of MGs, particularly during shear banding, can induce crystal formation/growth. Here, we show that these known trends for the interplay between crystals and deformation of MGs may hold true or become inverted depending on the size of the crystals relative to the shear bands. We performed molecular dynamics simulations of tensile tests on nanocrystal-bearing MGs. When the crystals are relatively small, they bolster the strength rather than the ductility of MGs, and the crystals within a shear band undergo redissolution as the shear band propagates. In contrast, larger crystals tend to enhance ductility at the cost of strength, and the crystal volume fraction increases during deformation. These insights offer a more comprehensive understanding of the intricate relationship between deformation and crystals/crystallization in MGs, useful for fine-tuning the structure and mechanical properties of both MGs and MG-crystal composites.

Fracture toughness and destructive mechanism of ductile nanoporous metallic glass and its crystal-impregnated nanocomposite

Fracture toughness and destructive mechanism of ductile nanoporous metallic glass and its crystal-impregnated nanocomposite

Strengthening mechanism and plasticity deformation of crystalline/amorphous Cu3Fe/Fe3Cu nanolayered composite

Metallic glasses possess high strength and excellent elastic properties due to their amorphous atomic structure. However, they often display brittleness and suffer catastrophic failure because of shear localization, without exhibiting significant macroscopic plastic deformation under tension. In this paper, an effective method to enhance the strength and ductility of metallic glasses is presented by molecular dynamics simulation. The approach involves controlling the thickness of the crystalline Cu3Fe and amorphous Fe3Cu layers. The results indicate that increasing the thickness of the crystal layer improves the strength of the nanolayered composite. The crystalline/amorphous interface impedes stress propagation in the shear transformation zone, thereby enhancing the yield stress of the composite. Additionally, the presence of Hirth dislocations in the crystal layer strengthens the composite. The generation and annihilation of fixed dislocations in the crystal layer cause stress fluctuations in the composite, which further enhances its ductility. However, a small content of crystalline layer can cause stress concentration towards the shear band, leading to rapid fracture failure and reduced ductility. These findings shed light on the plastic deformation behavior of crystalline/amorphous composites at the nanoscale, providing valuable theoretical guidance for designing high-strength and ductile metallic materials.

A group of ductile metallic glasses prepared by modifying local structure of icosahedral quasicrystals

Inspired by research into the association between icosahedral local orders and the plasticity of metallic glasses (MGs), beryllium (Be) is added to the icosahedral quasi-crystal forming alloy Zr40Ti40Ni20. In this way, bulk metallic glasses (BMGs) with favorable compressive plasticity are fabricated. Therein, the icosahedral quasi-crystalline phase is the main competing phase of amorphous phases and icosahedral local orders are the main local atomic motifs in amorphous phases. The alloys of (Zr40Ti40Ni20)76Be24 and (Zr40Ti40Ni20)72Be28 with their greater plastic strain capacity show similar characteristics to highly plastic amorphous systems: The serrated flow of compression curves always follows a near-exponential distribution. The primary and secondary shear bands intersect each other, bifurcate, and bend. Typical vein patterns are densely distributed on the fracture surfaces. The relaxation enthalpy of four MGs is linearly correlated with the plastic strain, that is, the greater the relaxation enthalpy, the larger the plastic strain.

Super ductile metallic glasses for energy-saving solid-state processing

Energy-efficient materials are key to combating the high energy costs and climate change. The manufacturing temperatures of industrially important Zr-based bulk metallic glasses (BMGs) relative to steels are low, and exist between the liquidus temperature Tl (∼850 °C) and glass transition temperature Tg (∼400 °C). However, these materials show limited plastic deformability (ductility) at room temperature (strains typically less than 3%); moreover they soften but exhibit limited ductility at high processing temperatures. Their low ductility should be improved because it impedes fatigue resistance and machinability, such as via cold (plastic) forming. In this study, chemical composition changes, which reduced Tg, resulted in remarkably ductile BMGs with extreme deformations of over 70% under compression, thereby enabling their energy-efficient processing at low temperatures. In contrast to previously reported conclusions on the high GFA and deformation-induced nanocrystallization being the precursors to ductility, formation of a low amount of meso-crystallites within the glassy material during cooling efficiently hindered the propagation of shear bands and microcracks under loading, thus increasing significantly ductility. This characteristic, in addition to optimal chemical composition, played an important role in improving the ability of BMGs to undergo solid-state processing at low temperatures and increased deformation rates.

Shear Band Control for Improved Strength-Ductility Synergy in Metallic Glasses

Abstract Aside from ultrahigh strength and elasticity, metallic glasses (MGs) possess a number of favorable properties. However, their lack of dislocation based plastic deformation mechanisms in crystalline metals and the resulting loss of ductility have restricted the engineering applications of MGs over the last 60 years. This review aims to provide an overview of deformation and failure mechanisms of MGs via formation and propagation of shear bands (SBs), with an emphasis on the control of SBs to promote strength-ductility synergy. With this goal in mind, we highlight some of the emerging strategies to improve the ductility of MGs. Topics covered include postprocessing techniques such as precompression, heterogeneity tuning, and rejuvenation, with a primary focus on recent progresses in structural design based methods including nanoglasses, notched MGs, and MG nanolattices, as future innovations toward strength-ductility synergy beyond the current benchmark ranges.

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.

Fracture energy of high-Poisson's ratio oxide glasses

The apparent relationship between Poisson's ratio and fracture energy has been used to guide the discovery of ductile glasses with a brittle-to-ductile (BTD) transition at Poisson's ratio around 0.32. Most organic and metallic glasses possess Poisson's ratio above 0.32, and thus, feature fracture energy that is around three orders of magnitude higher than that of oxide glasses, which feature Poisson's ratio typically below 0.30. However, whether the BTD transition can also be observed in oxide glasses remains unknown due to the lack of fracture energy measurements on oxide glasses with high Poisson's ratio. In this work, we measure the fracture energy of six oxide glasses with high Poisson's ratio between 0.30 and 0.34. We find no clear relationship between the two parameters even in those that possess the same Poisson's ratio as ductile metallic glasses. This suggests that Poisson's ratio is not the main property to enhance the fracture energy of oxide glasses. To this end, we instead find a positive relation between fracture energy and Young's modulus of oxide glasses, and even for some metallic glasses, which could explain their absence of ductility.

Effect of Al composition tuning on properties and structure in Zr-Co-Al metallic glass

The variation of glass forming ability, mechanical properties and atomic structure in Zr72−xCo28Alx (x = 14–19 at%) and Zr56Co44−xAlx (x = 13–20 at%) bulk metallic glasses derived from composition tuning has been studied by using copper-mold suction, high-energy X-ray diffraction, extended X-ray absorption fine structure experiments and ab initio molecular dynamics simulations, respectively. Both experimental and theoretical results indicate that glass forming ability was enhanced when composition approached Zr56Co28Al16, attributed to the densification in the first shell around Co atoms with the increase of 11-coordinated polyhedra. In addition, a ductile-to-brittleness transition induced by minor substitution of larger-sized atoms by smaller-sized atoms has been found. As good glass forming ability is usually accompanied by dense atomic arrangement, the increasing larger-sized atoms may tend to offer more free volume which is beneficial to plastic flow. Compositional tuning, along with structural tuning to influence both glass forming ability and plastic flow, offers a reliable method to develop ductile metallic glasses.

Tetrachiral nanostructured metallic glasses with mechanically tunable performance

Making use of the size effect of mechanical properties at the nanoscale and rational structural design, topological design of Cu50Zr50 tetrachiral nanostructured metallic glasses (CNMGs) is proposed. Molecular dynamics simulations of uniaxial tension are performed to explore the robust mechanical performances of the CNMGs, focusing on the effects of geometric characteristics such as ligament thickness, ligament length, and cavity radius on their mechanical behaviors. Our results show that tetrachiral structure can be employed for designing nanostructured metallic glasses (MGs) with remarkably improved elastic limit, excellent auxeticity, and enhanced ductility. A superior combination of high strength, remarkable ductility, and lightweight is observed for CNMGs with large cavity radii, which is different from traditional nanoporous materials. The coupling effects of the rotation and deformation of nodes, bending and stretching of ligaments lead to these superior mechanical properties. Our work not only offers a practical pathway to improve the ductility of MGs, but also reveals that the combination of lightweight, superior auxeticity, elevated ductility, and high strength can be concurrently obtained via nanostructural design.

Microstructure and performance of novel ductile FeNi-based metallic glasses

Fe-based bulk metallic glasses (BMGs) are of significant interest because of their superior magnetic and mechanical properties, but the plastic strain deficiency at room temperature restricts their application. In the present study, the effect of Ni content on thermal stability, soft magnetic properties and compressive plastic deformation was investigated in Fe80−xNixP12B4.9Si2.5Cu0.6 (x = 0, 3, 6, 9, 12, 15) BMGs. We discovered that the Ni-bearing BMGs exhibit strong GFA, good soft magnetic and mechanical properties, and high Tc, while the excessive Ni addition causes performance deterioration. The Fe71Ni9P12B4.9Si2.5Cu0.6 bulk glass exhibits high strength of about 3000 MPa and elevated plastic strain of 5.1%, together with high saturation magnetic induction of 1.51 T and low coercivity of 1.8 A/m. The Curie temperature up to 636 K was an intriguing finding. X-ray photoelectron spectroscopy results suggest that the brittle-ductile transition comes from the more s-like bonds between clusters because of the Ni addition. In terms of the inherent elastic nature, we consider that the high Poisson's ratio and the low G/K ratio of Ni lead to a large effective volume of the shear transition zone, which corresponds to the nucleation of shear bands. The significant compressive plastic behavior, large GFA and good soft magnetic properties indicated that the current Fe-based BMGs are promising for structural and magnetic functional applications.

Intrinsic and extrinsic effects on the fracture toughness of ductile metallic glasses

The potential application of metallic glasses (MGs) as structural materials demands accurate measurements of their toughness and the understanding of the underlying factors affecting it. Currently, it is challenging to precisely measure the toughness of MGs, especially ductile MGs. Measured toughness values are widely scattered, even for MGs with identical compositions. That is attributed to the combined effect of intrinsic and extrinsic factors including processing, sample geometries, and loading conditions. A fundamental understanding of the influences of these elements is thus of great significance. In the present study, molecular dynamics simulations are performed to investigate the influence of intrinsic and extrinsic effects on the fracture toughness of CuZr MGs. In particular, focused is placed on the effects of cooling rate, notch shape and size, strain rate, and temperature. The results indicate that the fracture toughness of a MG scales with the cooling rate used to prepare it. This is attributed to increased free volume content generated at high cooling rates, which enhances plastic deformation and amplifies the associated energy dissipation during plastic deformation events. Fracture toughness also increases with the strain rate, arguably due to strain rate-induced crack extension delay. Overall, the results demonstrate that the largest fracture toughness are achieved when MG samples are fabricated at high cooling rates and subjected to high strain rate deformation. In addition, results suggest that the fracture toughness decreases with increasing temperature, due to the significant decrease in strength. The correlations revealed between these crucial intrinsic and extrinsic parameters and the calculated MG fracture toughness support the development of a framework to understand the root of the discrepancies in the measurement of the toughness of MGs and provide insights into the design of tough MGs for structural applications.

Origin of strain hardening in monolithic metallic glasses

To overcome the brittleness of metallic glasses (MGs), their structure, chemistry, or loading conditions are usually controlled. Here, the local stress state in MGs was modulated without affecting their structure. The elastically designed MG heterostructures provide enhanced ductility together with strain hardening during loading. The stress heterogeneity leads to shear band multiplication that consequently enhances the macroscopic ductility of MGs. In addition, the residual compressive stress significantly increases the strength of the glass and is responsible for the observed strain hardening.

A concurrent high strength and ductility of 3D gyroidal nanoporous metallic glasses

By using molecular dynamics simulations, we investigate the tensile response of nanoporous (NP) Cu50Zr50 metallic glasses (MGs) with 3D gyroidal structure. Similar to nanoporous crystalline metals, NP–MGs fail by the plastic necking of ligaments, but shear bandings (SBs) prevail in those thick–ligament specimens. Particularly, with ligament diameter decreasing to 2.34 nm, SBs are hardly observed due to the limit of translation from shear transformation zones, resulting in superior strength and ductility of NP–MGs. The critical ligament size to suppress the formation of SBs is much smaller than those calculated value in MGs. The observations reveal a design strategy that a concurrent high strength and ductility of MGs could be obtained by designing 3D nanoporous structure with supra–nano size. The scaling laws for Young's modulus and strength as functions of relative density and ligament diameters are addressed, consistent with those in nanoporous crystalline materials.

Guiding shear bands in bulk metallic glasses using stress fields: A perspective from the activation of flow units

Controlling shear band propagation is the key to obtain ductile metallic glasses. Here, we use a residual stress field to vary the direction of shear band propagation. We ascribe this behavior to the effect of the stress field on the activation of shear transformation zones (STZs) along their characteristic direction and we quantify this contribution to the energy of the process. Because of the progressively adverse orientation of the stress field, the energy stored as shear in the STZ decreases to a level where shear band propagation at alternative angles becomes energetically more favorable.

Bicontinuous nanoporous design induced homogenization of strain localization in metallic glasses

Bicontinuous nanoporous metallic glasses (MG) synergize the outstanding properties of MGs and open-cell nanoporous materials. The low-density and high-specific-surface-area of bicontinuous nanoporous structures have the potential to enhance the applicability of MGs in catalysis, sensors, and lightweight structural designs. Here, we report molecular dynamics simulations of tensile loading deformation and failure of bicontinuous nanoporous Cu64Zr36 MG with 55% porosity and 4.4 nm ligament size. Results indicate an anomalous mechanical behavior featuring delocalized plastic deformation preceding ductile failure. The deformation follows two mechanisms: i) Necking of ligaments aligned with the loading direction and ii) progressive alignment of randomly oriented ligaments. Failure occurs at 0.16 strain, following massive rupture of ligaments. This work indicates that a bicontinuous nanoporous design is able to effectively delocalize strain localization in a MG due to a combination of size effect on the ductility of MGs resulting in nano ligaments necking and progressive asynchronous alignment of ligaments.

Intrinsic and extrinsic effects on the brittle-to-ductile transition in metallic glasses

The effects of cooling rate, temperature, and applied strain rate on the tensile deformation behavior of a Cu64Zr36 metallic glass (MG) are investigated using large-scale molecular dynamics simulations. An increase in the quenching rate during sample preparation, as well as an increase of the temperature or the applied strain rate, affects the activation of shear transformation zones (STZs) and, consequently, the shear-banding processes, which ultimately causes a brittle-to-ductile transition in the deformation behavior of MGs. A quantitative interpretation for the observed enhanced ductility in MGs with an increasing quenching rate is obtained by sampling the saddle points on the potential energy surface. High quenching rates lead to lower energy barriers for activation of a local atomic rearrangement (STZ) as compared to those MGs obtained at low quenching rates. Although the glassy structure does not show significant variations with increasing temperature, the kinetic energy of the atoms increases dramatically, which allows the atoms to rearrange easily; therefore, the probability of homogeneous thermal activation of STZs increases. Finally, a large number of STZs can also be activated by deformation at high strain rates when a large amount of elastic energy is stored in the glassy matrix. Consequently, a high density of STZ events and, therefore, a more complex percolation process results in a low probability for strain localization and formation of critical shear bands. Our results provide an atomistic understanding for the strain localization mechanisms in metallic glasses and shed more light on the brittle-to-ductile transition.

Signature of local stress states in the deformation behavior of metallic glasses

The design of ductile heterogeneous metallic glasses (MGs) with enhanced deformability by purposely controlling the shear-band dynamics via modulation of the atomic-scale structures and local stress states remains a significant challenge. Here, we correlate the changes in the local atomic structure when cooling to cryogenic temperature with the observed improved shear stability. The enhanced atomic-level structural and elastic heterogeneities related to the nonaffine thermal contraction of the short-range order (SRO) and medium-range order (MRO) change the characteristics of the activation process of the shear transformation zones (STZs). The experimental observations corroborated by Eshelby inclusion analysis and molecular dynamics simulations disclose the correlation between the structural fluctuations and the change in the stress field around the STZ. The variations in the inclination axes of the STZs alter their percolation mechanism, affect the shear-band dynamics and kinetics, and consequently delay shear failure. These results expand the understanding of the correlation between the atomic-level structure and elementary plastic events in monolithic MGs and thereby pave the way for the design of new ductile metallic alloys.

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.

Non-trivial avalanches triggered by shear banding in compression of metallic glass foams

Ductile metallic glass foams (DMGFs) are a new type of structural material with a perfect combination of high strength and toughness. Owing to their disordered atomic-scale microstructures and randomly distributed macroscopic voids, the compressive deformation of DMGFs proceeds through multiple nanoscale shear bands accompanied by local fracture of cellular structures, which induces avalanche-like intermittences in stress–strain curves. In this paper, we present a statistical analysis, including distributions of avalanche size, energy dissipation, waiting times and aftershock sequence, on such a complex dynamic process, which is dominated by shear banding. After eliminating the influence of structural disorder, we demonstrate that, in contrast to the mean-field results of their brittle counterparts, scaling laws in DMGFs are characterized by different exponents. It is shown that the occurrence of non-trivial scaling behaviours is attributed to the localized plastic yielding, which effectively prevents the system from building up a long-range correlation. This accounts for the high structural stability and energy absorption performance of DMGFs. Furthermore, our results suggest that such shear banding dynamics introduce an additional characteristic time scale, which leads to a universal gamma distribution of waiting times.