How does the structural inhomogeneity influence the shear band behaviours of metallic glasses

ABSTRACT

Here we propose a model to understand the influence of structural inhomogeneity on the shear band behaviours of metallic glasses. By considering the inhomogeneous structure and stress concentration, the model predicts that the strain for shear band nucleation in metallic glasses can be variable and far below the theoretical elastic limit. During sliding, the shear band will approach a dynamic equilibrium state of balanced free volume generation and annihilation. By considering the accumulation of irreversible structure change, the shear band will finally develop into fracture. Under fluctuating load, the shear band shows an ‘activate-arrest’ behaviour, which results from a delayed response to the external load change. These results reasonably explain and correlate the physics behind the elastic limit, stick-slip shear band behaviour, implicit shear events, and shear band fracture in metallic glasses. The study can provide another perspective and platform to understand the correlations between structural inhomogeneity and shear band behaviours in metallic glasses, and further explore other shear band related phenomena not only in metallic glasses but also in the class of shear-softened materials.     

Shear band formation and ductility in bulk metallic glass

The variations in the chemical compositions of the metallic glasses reported in the literature, as well as the overall lack of experimental data concerning the inhomogeneous deformation behaviour of metallic glass, make the evaluation of the effects of shear band/fracture behaviour on the mechanical properties of metallic glasses difficult. Isolating the effect of local shear band formation on bulk inhomogeneous flow would appear to be a first step in approaching this problem. The mechanical behaviour of Vitreloy metallic glass at room temperature and at various strain rates in tension and compression was investigated. The formation of multiple shear bands was observed at high strain rates. An increase in strain rate leads to enhanced ductility in tension and compression. Some aspects of the deformation processes in tension and compression are discussed.     

Extraordinary plasticity of ductile bulk metallic glasses

Shear bands generally initiate strain softening and result in low ductility of metallic glasses. In this Letter, we report high-resolution electron microscope observations of shear bands in a ductile metallic glass. Strain softening caused by localized shearing was found to be effectively prevented by nanocrystallization that is in situ produced by plastic flow within the shear bands, leading to large plasticity and strain hardening. These atomic-scale observations not only well explain the extraordinary plasticity that was recently observed in some bulk metallic glasses, but also reveal a novel deformation mechanism that can effectively improve the ductility of monolithic metallic glasses.     

Ideal shear banding in metallic glass

As the most fundamental deformation mechanism in metallic glasses (MGs), the shear banding has attracted a lot of attention and interest over the years. However, the intrinsic properties of the shear band are affected and even substantially changed by the influence of non-rigid testing machine that cannot be completely removed in real compression tests. In particular, the duration of the shear banding event is prolonged due to the recovery of the stressed compliant frame of testing machine and therefore the temperature rise at the operating shear band is, more or less, underestimated in previous literatures. In this study, we propose a model for the ‘ideal’ shear banding in metallic glass. The compliance of the testing machine is eliminated, and the intrinsic shear banding process is extracted and investigated. Two important physical parameters, the sliding speed and the temperature of shear band, are calculated and analysed on the basis of the thermo-mechanical coupling. Strain-rate hardening is proposed to compensate thermal softening and stabilise the shear band. The maximum value of the sliding speed is found to be on the order of 10 m/s at least, and the critical temperature at which strain-rate hardening begins to take effect should reach as high as 0.9T_g (T_g is the glass transition temperature) for a stable shear banding event in metallic glass according to the early experimental data. This model can help to understand and control the shear banding and therefore the deformation in MGs.     

"Work-Hardenable" ductile bulk metallic glass

Usually, monolithic bulk metallic glasses undergo inhomogeneous plastic deformation and exhibit poor ductility (< 1%) at room temperature. We present a new class of bulk metallic glass, which exhibits high strength of up to 2265 MPa together with extensive "work hardening" and large ductility of 18%. Significant increase in the flow stress was observed during deformation. The "work-hardening" capability and ductility of this class of metallic glass is attributed to a unique structure correlated with atomic-scale inhomogeneity, leading to an inherent capability of extensive shear band formation, interactions, and multiplication of shear bands.     

A non-viscous-featured fractograph in metallic glasses

A fractograph of non-viscous feature but pure shear-offsets was found in three-point bending samples of a ductile Pd–Cu–Si metallic glass. A sustainable shear band multiplication with large plasticity during notch propagation was observed. Such non-viscous-featured fractograph was formed by a crack propagation manner of continual multiple shear bands formation in front of the crack-tip, instead of the conventional rapid fracture along shear bands. With a 2D model of crack propagation by multiple shear bands, we showed that such fracture process was achieved by a faster stress relaxation than shear-softening effect in the sample. This study confirmed that the viscous fracture along shear bands could be not a necessary process in ductile metallic glasses fracture, and could provide new ways to understand the plasticity in the shear-softened metallic glasses.     

非晶合金及合金液体的局域五次对称性

简要回顾了从20世纪30年代至今,有关非晶合金及合金液体的局域结构五次对称性的实验、理论和模拟研究.在简单液体的早期研究中,人们已经意识到五次对称性在简单液体的无序结构、过冷和晶化等起着重要作用,二十面体短程序作为五次对称性的典型代表受到了广泛关注.自从Frank提出简单液体中二十面体短程序的结构单元,大量的理论和实验研究已经明确在简单液体、合金液体和金属玻璃中存在局域五次对称性,并且建立了局域五次对称性与合金液体复杂动力学行为、玻璃转变、液体-液体相变以及非晶合金的形变等统一的定量描述和物理图像,表明了局域五次对称性作为结构参量的简单、普遍和有效性.     

Metallic liquids and glasses: atomic order and global packing

In this Letter, we have revealed the common structural behavior of metallic glasses through scrutinizing the evolution of pair distribution functions from metallic liquids to glasses and statistically analyzing pair distribution functions of 64 metallic glasses. It is found that the complex atomic configuration in metallic glasses can be interpreted globally as a combination of the spherical-periodic order and local translational symmetry. The implications of our study suggest that the glass transition could be visualized mainly as a process involving in local translational symmetry increased from the liquid to glassy states.     

Dual self-organised shear banding behaviours and enhanced ductility in phase separating Zr-based bulk metallic glasses

The multiplication and interaction of self-organised shear bands often transform to a stick-slip behaviour of a major shear band along the primary shear plane, and ultimately the major shear band becomes runaway and terminates the plasticity of bulk metallic glasses (BMGs). Here, we examined the deformation behaviours of the nanoscale phase-separating Zr_65–xCu₂₅Al₁₀Fe_x (x = 5 and 7.5 at.%) BMGs. The formation of multi-step phase separation, being mainly governed by nucleation and growth, results in the microstructural inhomogeneity on a wide range of length-scales and leads to obviously macroscopic and repeatable ductility. The good deformability can be attributed to two mechanisms for stabilizing shear banding process, i.e. the mutual interaction of multiple shear bands away from the major shear band and the delaying slip-to-failure of dense fine shear bands around the major shear band, both of which show a self-organised criticality yet with different power-law exponents. The two mechanisms could come into effect in the intermediate (stable) and later plastic deformation regime, respectively. Our findings provide a possibility to enhance the shear banding stability over the whole plastic deformation through a proper design of microstructure heterogeneities.     

Serration behaviours in metallic glasses with different plasticity

We present a statistical study of serration behaviours in Pd_77.5Cu₆Si_16.5, Ti₄₁Zr₂₅Be₂₆Ag₈, Zr_41.2Ti_13.8Cu_12.5Ni_10.0Be_22.5 and Fe₅₀Ni₃₀P₁₃C₇ metallic glasses with different plasticity. The four samples show similar serration patterns in the beginning of yielding, and different patterns during later loading. These results indicate that the shear band initiation process in metallic glasses follow some similar dynamics. And the later serration process follows different dynamics and will lead to different plasticity. Here we interpret these serration behaviours from a perspective of inhomogeneity. The different serration patterns and shear band dynamics could be reasonably understood. The serration pattern of the Fe-based sample suggests that the brittleness of metallic glasses might result from a lower degree of inhomogeneity, and a less tendency of forming shear band intersections. This study might provide new experimental evidences for different micro-structures (or inhomogeneity) and dynamic behaviours in metallic glasses with different plasticity.     

Mechanisms of inelastic deformation in metallic glasses

The phenomenology of inelastic deformation in metallic glasses is briefly reviewed. This is followed by a short review of methods used to characterize the glassy structure. Considerable emphasis is put on the amorphous soap bubble raft as a two-dimensional analog medium that can give quantitatively accurate simulations of both the structural properties ofmetallic glasses and the mechanisms of their inelastic deformation. Analysis of such sheared bubble rafts has established that the basic ingredient of inelastic deformation in these materials is shear transformations occurring in small, atomic size volume elements that are only weakly interacting. Through the use of an inter-bubble potential, the changes in energy of these shear transformations can be analyzed in great detail. This has disclosed a remarkable capability in the model to parallel and predict complex relaxation phenomena in metallic glasses down to the detail of simulating the distributed nature of the structure and the free energies for transformation. This has led further to a very satisfactory model for the kinetics of anelastic and viscoplastic response of the structure for processes on shear localization, and nas led even to a semi-quantitative model for embrittlement resulting from physical ageing.     

非晶态物质原子局域连接度与弛豫动力学

非晶态物质的本质及形成过程是凝聚态物理领域最困难也是最有趣的问题之一.非晶形成过程在原子结构上不会衍生出人们在传统晶体结构里所熟悉的长程有序性,因此对于此类在自然界中广泛存在的物质形态,至今还没有有效的实验表征手段和理论研究方法.非晶态物质的原子结构及其构效关系的研究是凝聚态物理和材料科学等众多研究领域所关注的热点问题之一.随着对非晶态物质物性研究的深入,人们逐渐意识到非晶态物质中原子中程序对系统性质的重要影响,建立以中程序为基础的结构-动力学关系对于理解玻璃及玻璃转变的本质起着重要的作用.本文简要综述了基于图论提出的原子局域连接度这一新的结构序参量在液体和玻璃的结构及构效关系研究中的应用.新的结构序参量从过去侧重于关注局域原子团簇的种类和分布,转移到更加关注某一类具有特殊对称性的原子的空间连接情况,即更多地尝试从原子中程序的角度来建立非晶态物质中的构效关系.新的研究结果表明,局域连接度可与非晶态物质中原子的短时或长时动力学行为、输运方式、以及振动模态等一系列物理性质建立联系.     

Ductile bulk metallic glass

We report on experimental evidence of pronounced global plasticity measured in monolithic Pt57.5Cu14.7Ni5.3P22.5 bulk metallic glass under both bending and unconfined compression loading conditions. A plastic strain of 20% is measured, never before seen in metallic glasses. Also, permanent deformation and a strain exceeding 3% before failure is observed during bending of 4 mm thick samples. To date, no monolithic metallic material has exhibited such a combination of high strength, extensive ductility, and high elastic limit. The large plasticity is reflected in a high Poisson ratio of 0.42, which causes the tip of a shear band to extend rather than initiate a crack. This results in the formation of multiple shear bands and is the origin of the observed large global ductility and very high fracture toughness, approximately 80 MPa m(-1/2).     

Long-range strain correlations in sheared colloidal glasses

Glasses behave as solids on experimental time scales due to their slow relaxation. Growing dynamic length scales due to cooperative motion of particles are believed to be central to this slow response. For quiescent glasses, however, the size of the cooperatively rearranging regions has never been observed to exceed a few particle diameters, and the observation of long-range correlations has remained elusive. Here, we provide direct experimental evidence of long-range correlations during the deformation of a dense colloidal glass. By imposing an external stress, we force structural rearrangements, and we identify long-range correlations in the fluctuations of microscopic strain and elucidate their scaling and spatial symmetry. The applied shear induces a transition from homogeneous to inhomogeneous flow at a critical shear rate, and we investigate the role of strain correlations in this transition.     

Deformation behavior and enhanced plasticity of Ti-based metallic glasses with notches

The plasticity of Ti-based metallic glasses with different aspect ratios can be improved by introducing two semicircular notches on the edges of the samples, owing to the interactions of shear bands (SBs) under conventional compression tests. The interaction of SBs can be ascribed to the easy initiation of SBs around the notches due to the large stress gradient, and the consequent blocking effect of notches on the propagation of shear bands. Additionally, the current findings provide a new way to understand the physical nature for the plastic deformation behavior of some brittle metallic glasses and supply an effective approach to enhance the plasticity to some extent.     

Secondary relaxation and dynamic heterogeneity in metallic glasses:A brief review

Understanding mechanical relaxation, such as primary(α) and secondary(β) relaxation, is key to unravel the intertwined relation between the atomic dynamics and non-equilibrium thermodynamics in metallic glasses. At a fundamental level, relaxation, plastic deformation, glass transition, and crystallization of metallic glasses are intimately linked to each other, which can be related to atomic packing, inter-atomic diffusion, and cooperative atom movement. Conceptually, βrelaxation is usually associated with structural heterogeneities intrinsic to metallic glasses. However, the details of such structural heterogeneities, being masked by the meta-stable disordered long-range structure, are yet to be understood. In this paper, we briefly review the recent experimental and simulation results that were attempted to elucidate structural heterogeneities in metallic glasses within the framework of β relaxation. In particular, we will discuss the correlation amongβ relaxation, structural heterogeneity, and mechanical properties of metallic glasses.     

金属玻璃流变的扩展弹性模型

玻璃-液体转变现象,简称玻璃转变,被诺贝尔物理学奖获得者安德森教授评为最深奥与重要的凝聚态物理问题之一.金属玻璃作为典型的非晶态物质,具有与液体相似的无序原子结构,因此又称为冻结了的液态金属,是研究玻璃转变问题的理想模型材料.当加热至玻璃转变温度,或者加载到力学屈服点附近时,金属玻璃将会发生流动.由于热或应力导致的流动现象对金属玻璃的应用具有重要意义.本文简要回顾了金属玻璃流变现象,综述了流变扩展弹性模型的研究进展和未来发展趋势.     

Nature of nanocrystallization in shear bands created by megaplastic deformation in amorphous alloys

A theoretical study is made of the process of nanocrystallization upon the formation of shear bands created by megaplastic deformation in amorphous metallic alloys. Such nanocrystallization is shown to be caused by a considerable increase in temperature inside the shear bands, which in turn is associated with the stored energy of megaplastic deformation. The temperature increment depends on the degree of deformation, the rate of propagation of the shear band, and the physical parameters that determine the thermal characteristics of an amorphous matrix in the range of the shear band.     

Effect of annealing on the deformation and fracture of metallic glass under local loading

An investigation is undertaken into the variations observed in the cracking resistance, the plasticity, and the structure of an 82K3KhSR metallic glass upon annealing. A method of evaluating the mechanical properties and the structural state of metallic glasses is proposed. This method is based on the indentation of the metallic glass deposited onto a substrate prepared from a polyester material and a metal. The critical annealing temperature that corresponds to drastic changes in the mechanical properties of the metallic glass is determined. It is found that dependences of the cracking resistance of metallic glasses on the indenter load exhibit a linear behavior at annealing temperatures above the critical point. An exponential decrease in the cracking resistance upon indentation is observed with an increase in the annealing temperature of metallic glasses.