Size-dependent shear fracture and global tensile plasticity of metallic glasses

The tensile ductility or brittleness of metallic glasses is found to depend strongly on the critical shear offset. Based on experimental observations, the tensile shear fracture processes of metallic glasses can be divided into three stages: multiplication and coalescence of the free volume, formation of void and the final fast propagation of a shear crack. Accordingly, the size effect on the tensile shear deformation processes of metallic glass can be well understood: with decreasing specimen size smaller than the equivalent critical shear offset, the shear deformation of metallic glass is changed from unstable to stable, which leads to a transition from global brittleness on the macroscale to large global plasticity or even necking on the microscale. These results are fundamentally useful in understanding the physical nature of tensile shear deformation of various metallic glasses and even in the design of new metallic glass materials with good plasticity.     

Shear transformation zone dynamics model for metallic glasses incorporating free volume as a state variable

A mesoscale model, shear transformation zone dynamics (STZ dynamics), is employed to investigate the connections between the structure and deformation of metallic glasses. The present STZ dynamics model is adapted to incorporate a structure-related state variable, and evolves via two competing processes: STZ activation, which creates free volume, vs. diffusive rearrangement, which annihilates it. The dynamical competition between these two processes gives rise to an equilibrium excess free volume that can be connected to flow viscosity via the phenomenological Vogel–Fulcher–Tammann relation in relaxed structures near the glass transition temperature. On the other hand, the excess free volume allows glasses to deform at low temperatures via shear localization into shear bands, even in the presence of internal stress distributions that arise upon cooling after processing.     

Mutual interaction of shear bands in metallic glasses

Shear banding is the main plastic deformation mode in metallic glasses. Even though there are many researches focused on the initiation and propagation of shear bands, the interaction among them has not been systematically studied. Using an atomic force microscope, we investigated the mutual interaction of shear bands at the surface of Cu₅₀Zr₅₀ metallic glass ribbons at the nanoscale. At the sites of the interaction, the propagation direction of one shear band can be changed by the pre-existing one, and the offset is the vector sum of the two bands. Under external stress, one shear band can be decomposed into several tiny bands and more materials could be taken into the deformation zones. Therefore, more energy can be dissipated and the deformation could be more homogeneous for the mutual interaction process. These results are useful for a mechanistic understanding of the evolution and suppression of shear band propagations, as well as the design of metallic glasses with improved plasticity.     

铜基非晶合金板弯曲塑性及剪切带间距的研究

制备了一种压缩断裂塑性应变接近2％的铜基块状非晶板材,并利用三点弯曲实验进行了其弯曲塑性、剪切带间距与试样厚度关系的研究．结果表明：非晶合金的弯曲断裂塑性应变明显依赖于试样厚度,即弯曲断裂塑性应变随试样厚度增加呈指数衰减关系减少;弯曲时剪切带间距随试样厚度增加而线性增加;剪切带间距与试样厚度的比值对于同一非晶合金为恒定值,但随非晶合金种类的不同而变化,与非晶合金的塑性变形能力有关．     

Low temperature uniform plastic deformation of metallic glasses during elastic iteration

Molecular dynamics simulations and dynamic mechanical analysis experiments were employed to investigate the mechanical behavior of metallic glasses subjected to iteration deformation in a nominally elastic region. It was found that cyclic deformation leads to the formation of irreversible shear transformation zones (STZs) and a permanent uniform strain. The initiation of STZs is directly correlated with the atomic heterogeneity of the metallic glass and the accumulated permanent strain has a linear relation with the number of STZs. This study reveals a new deformation mode and offers insights into the atomic mechanisms of STZ formation and low temperature uniform plastic deformation of metallic glasses.     

Serrated flow and stick–slip deformation dynamics in the presence of shear-band interactions for a Zr-based metallic glass

In this paper, shear-band interactions (SBIs) were introduced by a simple method and their effect on the dynamics of shear bands and serrated flow was studied for a Zr-based metallic glass. Statistical analysis on serrations shows that the stick–slip dynamics of interacting shear bands is a complex, scale-free process, in which shear bands are highly correlated. Both the stress drop magnitude and the incubation time for serrations follow a power-law distribution, presenting a sharp contrast to the randomly generated, uncorrelated serrated flow events in the absence of SBIs. Observations on the fracture morphologies provide further evidence and insights into the deformation dynamics dominated by SBIs. A stick–slip model for multiple shear bands with interactions is also proposed and numerically calculated. The results, in good agreement with the experimental results, quantitatively show how multiple shear bands operate and correlate, especially for those with large serrated flow events. Our studies suggest that one serration in the stress–strain curve may correspond to collective stick–slip motions of multiple shear bands for those ductile bulk metallic glasses where a large number of shear bands are observed during deformation.     

Dynamic shear punching of metallic glass matrix composites

Dynamic shear punching is employed to process Zr_36.6Ti_31.4Nb₇Cu_5.9Be_19.1 metallic glass matrix composites. A brittle fracture dominates during punchforming, and no macroscopic cracks or ruffles can be found, indicative of a good surface finish. The softening instability for metallic glasses happens once the yielding is available, avoiding the existence of shear bands and achieving smooth surfaces. No crystallization is accompanied in the heavy deformation zones during dynamic shear punching. The thickness of punched sheets for various BMG systems is highly dependent on the shear strengths. Furthermore, it is deduced that larger glass-transition temperatures of BMGs correspond to the thinner punched sheets, which provides a theoretical guidance to rapidly process metallic glass sheets.     

Controlling shear band behavior in metallic glasses through microstructural design

Plastic deformation in metallic glasses is governed by the initiation and propagation of shear bands. The successful use of bulk metallic glasses in structural applications will depend on controlling these processes to improve ductility and toughness. In Zr–Cu–Ni–Al metallic glasses, the addition of Ta can influence the structure of the material and hence the shear band behavior in two ways. At low Ta contents (<4 at.%), the material is amorphous but has enhanced order over length scales of 5–15 Å Higher levels of Ta result in the precipitation of bcc Ta-rich solid solution particles in a metallic glass matrix. Under uniaxial compression, both of these materials show greater apparent plastic strain to failure than the glass without Ta. This appears to be the result of the influence of the structure on the initiation and propagation of shear bands in the amorphous matrix.     

Size-independent strength and deformation mode in compression of a Pd-based metallic glass

We present quasi-static, room temperature compression data for Pd₄₀Ni₄₀P₂₀ metallic glasses, with specimen sizes ranging from the submicron to several millimeters in diameter. We observe no change in deformation mode over this range. At all sizes, plastic flow is localized in shear bands, which are accompanied by sudden strain bursts. This metallic glass shows only a modest increase in strength in going from bulk to micrometer-sized specimens. We show that stress gradients in tapered specimens can complicate measurement of the yield strength of metallic glasses in microcompression. Estimates of yield strength based on the minimum cross-sectional area implicitly assume that yielding is controlled by a maximum effective shear stress criterion. An alternative is the shear plane yield criterion, in which the minimum shear stress on the shear band trajectory determines yield. Application of this criterion in tapered microspecimens reinforces the notion that metallic glasses possess relatively size-independent mechanical properties.     

Plastic deformability of metallic glass by artificial macroscopic notches

This paper reports that the plasticity of Zr-based metallic glass can be improved by creating two symmetrical semi-circular notches. Unlike the experimental findings of the samples without notches, a steady shear deformation can be created by the large-scale stress gradient around the two symmetrical notches and the plasticity of metallic glass can be enhanced to a high value of ～10% under compression tests. The improved plasticity may be due to the easy initiation of shear bands around the notches, and the consequent blocking effect of notches on the propagation of shear bands, similar to the dislocation mechanism in crystalline materials. To reveal the particular plastic deformation behavior of metallic glass, Ti₃SiC₂ ceramic and high-strength steel specimens with two symmetrical semi-circular notches were also conducted under compressive loadings; however, no enhancement in plasticity was found. It is suggested that creating a stress gradient is a particular strategy for designing metallic glasses in order to improve their plasticity.     

New regime of homogeneous flow in the deformation map of metallic glasses: elevated temperature nanoindentation experiments and mechanistic modeling

The character of plastic deformation in metallic glasses is investigated through instrumented nanoindentation experiments on amorphous Pd₄₀Ni₄₀P₂₀ and Mg₆₅Cu₂₅Gd₁₀. Using a customized experimental apparatus, nanoindentation experiments have been conducted over four decades of indentation strain rate and from ambient temperature up to the glass transition, allowing rapid evaluation of an extensive deformation map with only small volumes of experimental material. At low rates and temperatures, inhomogeneous or serrated flow is observed, owing to the discrete operation of individual shear bands. Two distinct regimes of homogeneous flow can be identified. The first, expected, regime of homogeneous flow corresponds to the onset of viscous deformation at high temperatures and low rates, and is well described by existing mechanistic models. The second homogeneous regime occurs at high deformation rates even well below the glass transition, and arises when deformation rates exceed the characteristic rate for shear band nucleation, kinetically forcing strain distribution. By extending an existing model for glass deformation to explore shear band nucleation kinetics, this second regime is quantitatively rationalized and the natural frequency for shear band nucleation is extracted from the data. From this analysis the critical radius of a shear band as it transitions from nucleation to propagation is estimated to be in the submicron range.     

Theoretical prediction of temperature dependent shear modulus of bulk metallic glasses

A model of temperature dependent shear modulus and Young's modulus in bulk metallic glasses is established. The inherent relationship between the glass transition temperatures, the Debye temperature and shear modulus of bulk metallic glasses is revealed. The temperature dependent shear modulus can be predicted by our model without any fitting parameter. The model is presented based on a critical energy density criterion for plastic yielding which is derived from fundamental thermodynamics. This critical energy density consists of two parts: the heat added to the system and the input of mechanical energy, which are not completely equivalent. The agreement between theoretical results and experimental results is striking. And it is found that the temperature dependent Young's modulus could also be predicted pretty well by our model.     

Mechanical behavior of amorphous alloys

The mechanical properties of amorphous alloys have proven both scientifically unique and of potential practical interest, although the underlying deformation physics of these materials remain less firmly established as compared with crystalline alloys. In this article, we review recent advances in understanding the mechanical behavior of metallic glasses, with particular emphasis on the deformation and fracture mechanisms. Atomistic as well as continuum modeling and experimental work on elasticity, plastic flow and localization, fracture and fatigue are all discussed, and theoretical developments are connected, where possible, with macroscopic experimental responses. The role of glass structure on mechanical properties, and conversely, the effect of deformation upon glass structure, are also described. The mechanical properties of metallic glass-derivative materials – including in situ and ex situ composites, foams and nanocrystal-reinforced glasses – are reviewed as well. Finally, we identify a number of important unresolved issues for the field.     

An electron microscopy appraisal of tensile fracture in metallic glasses

Three glass-forming alloy compositions were chosen for ribbon production and subsequent electron microscopy studies. In situ tensile testing with transmission electron microscopy (TEM), followed by ex situ TEM and ex situ scanning electron microscopy (SEM), allowed the deformation processes in tensile fracture of metallic glasses to be analysed. In situ shear band propagation was found to be jump-like, with the jump sites correlating with the formation of secondary shear bands. The effect of structural relaxation by in situ heating is also discussed. Nanocrystallization near the fracture surface was observed; however, no crystallization was also reported in the same sample and the reasons for this are discussed. Both the TEM and the SEM observations confirmed the presence of a liquid-like layer on or near the fracture surface of the ribbons. The formation of a liquid-like layer was characterized by the vein geometries and vein densities on the fracture surfaces and its dependence on shear displacement, δ, is discussed. A simple model is adapted to relate the temperature rise during shear banding to the glass transition and melting temperatures and this is used to explain the variety of fracture surfaces which are developed for macroscopically identical tensile testing of metallic glasses together with features which exhibit local melting.     

On the applicability of a mesoscopic interface sliding controlled model for understanding superplastic flow in bulk metallic glasses

It is demonstrated that a mesoscopic interface sliding controlled flow model, which has already been shown to account for superplastic deformation in different types of crystalline materials, is also capable of describing superplastic flow in bulk metallic glasses. The only difference is that the random high-angle grain boundaries in crystalline materials along which deformation is concentrated, have to be replaced by the transient interfaces which are formed by interconnecting shear transformation zones in the region of homogeneous flow in bulk metallic glasses. Comparison with experimental results concerning superplastic flow in eight bulk metallic glasses shows that the numerical solutions obtained in the paper for the transcendental stress–strain rate equation of superplastic deformation lead to accurate predictions.     

Deformation of metallic glasses with special emphasis in supercooled liquid region

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Interpretation of viscous deformation of Zr-based bulk metallic glass alloys based on Nabarro-Herring creep model

Superplastic-like viscous deformation of bulk metallic glass alloys around the glass transition temperature (Tg) was analyzed based on the Nabarro-Herring creep model, a classical creep model, where the diffusional motion of atoms or vacancies through the lattice (atomic configuration) is considered. The amorphous matrix of bulk metallic glasses that has a randomly-packed atomic configuration was assumed to behave in a manner similar to the grain boundary in polycrystalline metals so as to approximate the diffusivity of the major constituent element. In spite of rough approximation of the parameters in the Nabarro-Herring creep equation, a reasonable value of the diffusion path (d) could be obtained from the experimentally-obtained metal flow data, including the steady state stress and the strain rate. Due to the absence of vacancy sources such as grain boundaries in homogeneous metallic glasses, the diffusion path, which, in polycrystalline materials, generally is the average distance between vacancy sources such as grain boundaries, was considered in this work as the average distance between tunneling centers in bulk metallic glass alloys. The calculated diffusion path was comparable to the density of tunneling centers around Tg, proposed by M. H. Cohen and G. S. Grest based on free volume theory. The calculated diffusion path showed monotonous decrease with temperature over Tg for Zr-based bulk metallic glass alloys. Based on this analysis, a schematic model for viscous deformation of bulk metallic glass was proposed.     

High strength ductile Cu-base metallic glass

Usually, bulk metallic glasses exhibit strength values superior to conventional crystalline alloys, often combined with a large elastic limit and rather low Young's modulus. This combination of properties renders such alloys quite unique when compared to commercial materials. However, the major drawback for engineering applications is their limited room temperature ductility and toughness due to the localized deformation processes linked to shear banding, where high plastic deformation is accumulated in a very narrow region without contributing to macroscopic deformation, work hardening or yielding. In this work we report on a new class of metallic glass in a simple Cu-base alloy. Addition of 5 at.% Al increases the glass-forming ability of binary Cu₅₀Zr₅₀. The resulting Cu_47.5Zr_47.5Al₅ glass exhibits high strength (2265 MPa) together with large room temperature ductility up to 18%. After yielding a strong increase in the flow stress is observed during deformation. The structure of the metallic glass exhibits atomic-scale heterogeneities that enable easy nucleation and continuous multiplication of shear bands. The interaction and intersection of shear bands increases the flow stress of the material with further deformation, leading to a ‘work hardening’-like behavior and yields a continuous rotation of the shear angle up to fracture resulting in a high compressive ductility.     

Effect of Al Addition on the Mechanical Properties and Microstructure of Zr35Ti30Cu7.5Be27.5 Bulk Metallic Glass

利用单轴压缩试验研究了用部分Al替代Zr对Zr35-xTi30Cu7.5Be27.5Alx(x=0,1,1.5,2,2.5,5,at%)块体金属玻璃力学性能的影响。研究表明,当添加Al含量为1.5at%和2at%时,所得到的块体金属玻璃的压缩塑性从0.95%(x=0)分别提高至15.10%(x=1.5)和3.45%(x=2)。利用扫描电子显微镜(SEM)对金属玻璃样品的断裂形貌进行了表面分析。利用透射电子显微镜(TEM)对不同Al含量的块体金属玻璃试样进行了微观结构表征,结果显示,Al含量为1.5at%和2at%的金属玻璃样品的微观结构呈现出了纳米级别的"微观不均匀性"。最后,结合临界剪切应力(CSS)讨论了微观结构与塑性变形行为之间的关联性。