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.     

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.     

Formation of chemical short range order and its influences on the dynamic/mechanical heterogeneity in amorphous Zr–Cu–Ag alloys: A molecular dynamics study

The chemical short range order of metallic glasses is expected to be correlated with their mechanical properties. In this article, classic molecular dynamics simulations of amorphous Zr₄₅Cu₄₅Ag₁₀ alloys were carried out to reveal such links in metallic glasses. Our calculations of Warren–Cowley parameter indicate the growth of chemical short range order during supercooling process, which also depends on the effective cooling rates. The chemical short range ordering is related to the energetic stability of the system. Based on the chemical preference or avoidance for different bonds, the model is separated into Cu-rich regions and Ag-rich regions. Simulated structural relaxation and shear loading process were performed to study how chemical bonds affect the distribution of dynamic and mechanical heterogeneity in our systems. The Cu-rich regions exhibit slower dynamics and higher shear resistance, whereas Ag-rich regions have faster dynamics and prefer to be plastically deformed.     

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.     

Mesoscale modeling of amorphous metals by shear transformation zone dynamics

A new mesoscale modeling technique for the thermo-mechanical behavior of metallic glasses is proposed. The modeling framework considers the shear transformation zone (STZ) as the fundamental unit of deformation, and coarse-grains an amorphous collection of atoms into an ensemble of STZs on a mesh. By employing finite element analysis and a kinetic Monte Carlo algorithm, the modeling technique is capable of simulating glass processing and deformation on time and length scales greater than those usually attainable by atomistic modeling. A thorough explanation of the framework is presented, along with a specific two-dimensional implementation for a model metallic glass. The model is shown to capture the basic behaviors of metallic glasses, including high-temperature homogeneous flow following the expected constitutive law, and low-temperature strain localization into shear bands. Details of the effects of processing and thermal history on the glass structure and properties are also discussed.     

Atomic scale calculation of the free volume in Zr2Ni metallic glass

How to evaluate the free volume in metallic glasses is a long-standing question in the field of metallic glasses. In this paper, a new approach at the atomic level to calculate free volume has been developed and applied in the Zr₂Ni metallic glass. The glass transition and volume change during the cooling of Zr₂Ni metallic liquid were investigated by molecular dynamics (MD). The MD resultant glass transition temperature (T_g) is about 700 K, which is close to the experimental value of 662.8 K obtained from differential scanning calorimetry (DSC). According to our method, the free volume in Zr₂Ni metallic glass was determined to be ～4.1% at T_g, which is comparable with the experimental results reported in other metallic glasses. Moreover, the probability distribution of free volume in our case can be fitted well by that given by Turnbull and Cohen, confirming the rationality of our method.     

Calorimetric study of β-relaxation in an amorphous alloy: An experimental technique for measuring the activation energy for shear transformation

When loaded at cryogenic temperature under stresses below the global yield, an amorphous alloy revealed two clearly distinguishable exothermic events during heating in calorimetric experiments. These exotherms, commonly known as the α- and β-relaxations, were attributed to the annealing out of two different local structures with different structural stability, presumably free volume and shear transformation zone (STZ). In this study, we introduced a simple and reliable experimental technique, which enables the evaluation of the activation energy corresponding to the β-relaxation, E_β. Based on experimental evidence and comparison with earlier studies, it is presumed that E_β is directly related to the potential energy barrier to shear transformation.     

金属玻璃剪切带中应力诱导的纳米缺陷形成

以非晶锆基金属玻璃薄带制备透射电子显微镜样品,并利用透射电镜找到在弯曲样品过程中产生的剪切带.高分辨透射电镜技术揭示在剪切带中存在着纳米级的空洞类和高密度区域缺陷.这些缺陷被认为是在机械负载和外应力移除过程中应力激发的自由体积演化而成的,其演化过程可以通过和在相同的金属玻璃样品上的热退火过程作类比来定性地理解.     

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.     

Correlations between the relaxed excess free volume and the plasticity in Zr-based bulk metallic glasses

Zr-based bulk metallic glasses (BMGs) with Nb minor alloying have been fabricated with different free volume (FV) trapped in. FV is evaluated by the relaxed excess free volume (REFV) after annealing just below T_g through loop thermal expansion tests. The results show that there is a qualitative correlation between the plasticity and REFV in Zr-based BMGs. The larger amount of excess FV the BMGs relax, the better plasticity they exhibit. With 1.5% Nb addition, the brittle Zr₆₅Cu₁₅Ni₁₀Al₁₀ BMGs possess REFV up to about 0.428% and exhibit the relatively good plasticity up to 25.6%. This provides a promising way to estimate the plasticity of BMGs and design new ductile BMGs through the minor alloying.     

Introducing a strain-hardening capability to improve the ductility of bulk metallic glasses via severe plastic deformation

The great technological potential for bulk metallic glasses (BMGs) arises primarily because of their superior mechanical properties. To realize this potential, it is essential to overcome the severe ductility limitations of BMGs which are generally attributed to shear localization and strain softening. Despite much international effort, progress in improving the ductility of BMGs has been limited to certain alloys with specific compositions. Here, we report that severe plastic deformation of a quasi-constrained volume, which prevents brittle materials from fracture during the plastic deformation, can be used to induce strain hardening and to reduce shear localization in BMGs, thereby giving a significant enhancement in their ductility. Structural characterizations reveal the increased free volume and nanoscale heterogeneity induced by severe plastic deformation are responsible for the improved ductility. This finding opens a new and important pathway towards enhanced ductility of BMGs.     

Relation between the structural parameters of metallic glasses at the onset crystallization temperatures and threshold values of the effective diffusion coefficients

Using the results of differential scanning calorimetry and X-ray diffractometry, an analysis has been carried out of the initial stages of the eutectic and primary mechanisms of crystallization of a series of metallic glasses based on Fe and Al with the established temperature dependences of the effective diffusion coefficients. Analytical relationships, which relate the volume density of crystallites formed in the glasses at the temperatures of the onset of crystallization with the values of the effective diffusion coefficients at these temperatures have been proposed. It has been established that, in the glasses, the crystallization of which begins at the lower boundary of the threshold values of the effective diffusion coefficients (~10^–20 m²/s), structures are formed with the volume density of crystallites on the order of 10²³–10²⁴ m^–3 and, at the upper boundary (10^–18 m²/s), of the order of 10¹⁸ and 10²⁰ m^–3 in the glasses that are crystallized via the eutectic and primary mechanisms, respectively. Good agreement between the calculated and experimental estimates indicates that the threshold values of the effective diffusion coefficients are the main factors that determine the structure of glasses at the initial stages of crystallization.     

Flow serration and shear-band viscosity during inhomogeneous deformation of a Zr-based bulk metallic glass

Uniaxial compressive behavior of Zr_64.13Cu_15.75Ni_10.12Al₁₀ bulk metallic glass at room temperature was characterized with high-sensitivity strain gauges directly attached to test samples. Displacement–time curves exhibited micron-size serrations (or bursts) after the onset of yielding, apparently associated with discrete shear band formation. Each displacement burst disclosed three-step (acceleration, steady-state, and deceleration) process in shear band propagation. The viscosity of a propagating shear band was found to be relatively low and, actually, in a similar range usually measured in the supercooled liquid region. A detailed analysis of the experimental results using a self-consistent Vogel–Fulcher–Tamann (VFT) equation based on free volume model suggested that shear band propagation was mainly resulted from free volume accumulation.     

Temperature dependence of the crystal–melt interfacial energy of metals

A model to express the dependence of the crystal–melt interfacial energy on the temperature for metals is proposed. The crystal–melt interfacial energies, the homogeneous nucleation undercoolings and the critical cooling rates to form ideal metallic glasses of silver, copper and nickel have been predicted according to the present model and simulated by the molecular dynamics method. The results show that the crystal–melt interfacial energy of metals increases nonlinearly with temperature. Over a wide temperature range from the melting point to the glass transition temperature the predicted results for the crystal–melt interfacial energy, the homogeneous nucleation undercooling and the critical cooling rate to form ideal metallic glasses from the present crystal–melt interfacial energy model are in good agreement with the experimental results reported, as well as the results of molecular dynamics simulations based on different EAM potentials of the metals.     

Investigation of bulk metallic glass structure by means of electron irradiation

The sensitivity of radiation effects on structural features in metallic glasses (MG) provides a way of investigating the structure and structural defects of MG through the studies of accumulation and relaxation processes of radiation damages in the glasses. In particular, it is a way to verify the validity of models proposed for the MG structure. Currently, there are two theoretical structural models of MG, based on completely different concepts. On the one hand, the model of random closely packed spheres + free volume approach is based on the idea of “the ultimate disorder”. On the other hand, in the polycluster model the idea of altering and distorted short-range order is used. Each of the models implies fairly different kinds of primary radiation damages, relaxation kinetics, and other properties. We have studied the accumulation and recovery kinetics of radiation defects in ZrTiCuNiBe and ZrTiCuNiAl bulk metallic glasses irradiated with 2.5 MeV electrons at T ～ 80 K. Electrical resistance measurements of the irradiated samples were performed. Dose dependences at ～80 K and the recovery spectrum of irradiation-induced electrical resistance in the 85–300 K temperature range were obtained. The data suggest that the point defects are stable in the metallic glasses, and the defect mobility is a thermally activated process. The results obtained are evidently in accord with the polycluster model of the MG structure.     

Temperature-dependent structural and transport properties of liquid transition metals

The evolution of the atomic structures and diffusivity of liquid transition metals with increasing temperature has been studied here using molecular dynamics. An analysis of Honeycutt and Andersen (HA) Indices indicates that relatively low order atomic clusters of rhombohedra-related structures increase with an increase in temperature. The results illustrate that the distortion in the local structural order with a predominant rhombohedra character enhances the diffusivity in liquid metals. The excess entropy approximated by the two-body contribution increases with the distortion of the local structural order traced by the HA Indices. The relationship between the excess entropy and the reduced diffusion coefficient supports the universal scaling law proposed by M. Dzugutov. The calculated diffusivities were compared with predictions of four diffusion models. The temperature dependence of the diffusivity cannot be described by the Arrhenius Law, the moving oscillator model or the free volume model but rather by the density fluctuation model with the square proportionality of temperature.     

Contributions to the modelling of the thermodynamic behaviour of Se---Te liquid solutions

The contributions from the changes in volume which occur on isothermally mixing at constant pressure, or excess volume, in the analytical formulation of the difference between the thermodynamic quantities of mixing, such as enthalpy, entropy and Gibbs free energy, derived at constant volume and those derived at constant pressure are considered. The procedure followed is to express this difference in a power series expansion, in terms of the excess volume. Numerical calculations are performed to assess the effect of the excess volume on the thermodynamic mixing quantities. Finally, the treatment is applied to the calculation of the thermodynamic mixing quantities of Se---Te liquid alloys at 773 K. The analysis shows that the agreement between the calculated and the experimental data is significantly improved when the excess volume contributions are considered.     

On factors influencing the ductile-to-brittle transition in a bulk metallic glass

An experimental study to ascertain the ductile-to-brittle transition (DBT) in a bulk metallic glass (BMG) was conducted. Results of the impact toughness tests conducted at various temperatures on as-cast and structurally relaxed Zr-based BMG show a sharp DBT. The DBT temperature was found to be sensitive to the free-volume content in the alloy. Possible factors that result in the DBT were critically examined. It was found that the postulate of a critical free volume required for the amorphous alloy to exhibit good toughness cannot rationalize the experimental trends. Likewise, the Poisson’s ratio–toughness correlations, which suggest a critical Poisson’s ratio above which all glasses are tough, were found not to hold good. Viscoplasticity theories, developed using the concept of shear transformation zones and which describe the temperature and strain rate dependence of the crack-tip plasticity in BMGs, appear to be capable of capturing the essence of the experiments. Our results highlight the need for a more generalized theory to understand the origins of toughness in BMGs.