Deformation Mechanisms Of Metallic Glasses Research Articles

Mechanical Properties and Deformation Mechanisms of Metallic Glasses Under Hydrostatic Pressure

Mechanical Properties and Deformation Mechanisms of Metallic Glasses Under Hydrostatic Pressure

A hierarchically correlated flow defect model for metallic glass: Universal understanding of stress relaxation and creep

Due to the structurally disordered arrangement of atoms and deviation from thermodynamic equilibrium, the physical and mechanical properties of metallic glasses can vary with time, temperature and magnitude of strain or stress. The current work provides a theoretical framework based on the hierarchically correlated atomic theory, which allows a quantitative description of the non-elastic deformation in metallic glasses. The defect concentration is adopted as an order parameter, which can evolve with temperature and non-elastic strain owing to correlated atomic movements. Through our hierarchically correlated atomic theory, we derive the characteristic times for local shear events in metallic glasses that entail activation, growth and/or annihilation of flow defects, which however are not accounted for in the previous mean field theories. Finally, we demonstrate that the current theoretical framework can be validated by the stress relaxation and creep experiments on typical La-based metallic glasses, which in turn provides quantitative insights into the non-elastic deformation mechanisms in metallic glasses.

Manipulation of relaxation processes in a metallic glass through cryogenic treatment

The relaxation spectrum of glassy solids has long been considered to probe their structural features and deformation mechanisms. Here, by systematically investigating the structural evolution, dynamical relaxations and mechanical properties of a Zr-based metallic glass subjected to different cryogenic treatment time, we build a bridge to connect the relaxation processes and mechanical properties. It is found that the β-relaxation triggered by local excitations has no memory effect of the thermal history or initial free volume content of the system. However, some local denser areas with a high potential energy created by CT might contribute to β′-relaxation that activated at a much lower energy, which may play a crucial important role in the fundamental deformation mechanisms of metallic glasses. The relaxation behaviors and the related improved mechanical properties can be rationalized in terms of the atomic-level stress theory. These findings will advance our understanding of the intrinsic correlation between local excitations and mechanical properties of metallic glasses, thereby may help guide to develop amorphous alloys with high mechanical performance.

Pressure-induced maximum shear strength and transition from shear banding to uniform plasticity in metallic glass

The effect of pressure on the mechanical and physical properties of metallic glasses (MGs) has been widely reported. In experiments, it remains challenging to mechanically loading the materials under extreme hydrostatic pressures and carrying out the corresponding measurements. Here, we investigate the pressure dependence of shear response of a typical brittle FeP MG by using molecular dynamics (MD) simulations. We show that the plastic deformation of this MG loaded at a pressure below 10 GPa is highly localized in the form of a dominant shear band. The plastic flow stress during shear banding is measured to increase with the applied pressure, leading to a pressure-induced strengthening behavior at pressures in the range of 0–10 GPa. As the pressure is increased to a critical pressure of approximately 15 GPa, there occurs a transition in the deformation mechanism from localized shear banding to a delocalized shear deformation mechanism governed by the uniform activation of individual shearing events in the material. At this critical pressure, both the yield strength and the average flow stress are maximized. Further increasing the pressure lowers the strength of the material. The pressure effect on the evolution of different types of atomic clusters is analyzed, revealing that the pressure-sensitivity of atomic clusters in MGs is related to their structural symmetry. Under high hydrostatic pressures, low-symmetry clusters become sluggish due to their larger pressure sensitivity. In such circumstances, high-symmetry clusters with lower pressure sensitivity are relatively more active during shear deformation and evenly distributed in the whole sample, leading to the uniform plastic deformation at high pressures. Our findings calls for further experimental investigations of the effect of pressure on mechanical properties and deformation mechanisms of MGs.

SHEAR-BAND DYNAMICS IN METALLIC GLASSES

Shear banding is a localized plastic deformation form that wildly exist in amorphous systems, and plays the decisive role in controlling the failure and plasticity. Due to the localization in space and instantaneous in motions, clarifying the map of shear bands is still a challenge mission. For traditional amorphous systems as rocks, colloid, oxide glasses and polymers, the studies on shear bands are impeded due to the poor mechanical properties or complex structures of these systems. In recent years, the developments of metallic glasses (MGs) overcome this barrier with the abundant mechanical experiments on shear bands in laboratories, which greatly promotes the insights to shear bands. In addition, the dynamics of shear bands have a significant influence to the mechanical properties and behaviors of MGs, which is also of significant importance for understanding the deformation mechanisms of MGs. The insights for the structural origin, morphology, affect area, properties and motions of shear bands has made great progress. With the investigations on MGs, shear mands are found inhomogenous in space distribution and non-steady in time. The behaviors of shear bands show a similarity with that of the complex systems in nature as well as physic areas. This article mainly reviews our recent studies on the complex behaviors of shear bands in MGs, including the serration follow behaviors and the self-organizing criticality (SOC) behaviors of shear bands. The models that quantify these behaviors of shear bands, such as stick-slip model, machine-sample coupling model, are also introduced. The review also identified the key questions remaining to be answered, and presents an outlook for the field.

Temperature-dependent plasticity and fracture mechanism under shear loading in metallic glass

The fracture surface morphology under shear loading is important for understanding the plasticity and the deformation mechanism of metallic glasses (MGs). However, it is difficult to carry out the shear fracture test for most MGs by conventional test methods due to their limited glass forming ability. In this work, through a unique anti-symmetrical four-point bend method, the shear fracture test is carried out under a quasi-static loading at a wide temperature range from 220 K (0.31Tg, Tg: glass transition temperature) to 620 K (0.88Tg) for a Zr-based MG. Basing on the examination on the fracture surface morphologies, we found three different fracture mechanisms with temperature: at lower temperatures (220 ~ 260 K, 0.31 ~ 0.37 Tg), both flower-like and shear-driven vein patterns appear on the fracture surface, which displays a mix of shear- and dilatation- dominated fracture; at intermediate temperatures (300 ~ 570 K, 0.42 ~ 0.80 Tg), the shear-driven vein patterns dominate on the fracture surface, indicating the shear-dominated fracture; at higher temperatures (600 ~ 620 K, >0.80 Tg), a mass of melt droplets indicate the transition from inhomogeneous to homogeneous deformation. From the Weibull statistical analysis on the main fracture features at intermediate temperatures, we found that the size of shear-driven vein patterns increases and the data distribution is sparser with the temperature increasing. The temperature dependence of fracture surface morphology as well as the relationship between the fracture surface feature and the microstructural evolution in MG were also interpreted from the shear transformation zone theory.

Deformation-Enhanced Hierarchical Multiscale Structure Heterogeneity in a Pd-Si Bulk Metallic Glass

The multiscale structures in a Pd82Si18 binary bulk metallic glass before and after bending were studied using electron microscopy, high-energy synchrotron X-ray diffraction, and small-angle X-ray scattering. The experimental results revealed the enhancement of hierarchical structure heterogeneities on multiple length scales after deformation. Hierarchical multiple shear bands of high number density can be observed after bending, introducing complex but periodically distributed residual strain. Pair distribution function analysis revealed that the connectivity of the short-range cluster on the medium-range scale determines the packing density difference between the tension side and the compression side in the sample after bending. The electron density fluctuations on the nanoscale observed in the deformed part of the sample are indicative of a nanoscale amorphous phase separation induced by plastic deformation. Our findings will deepen the understanding of the plastic deformation mechanism of metallic glasses and shed light on designing glassy alloys of excellent plasticity and toughness through hierarchical multiscale structure heterogeneities.

Structural characteristics in deformation mechanism transformation in nanoscale metallic glasses

Deformation of metallic glasses is closely related to their microstructures which depend on the composition, processing method, and the size of the materials. This subtle structure-property relation is fairly complex and remains to be explored. Here, we scrutinize the microstructural evolution in relation to the mechanical properties in metallic glass nanowires with the same composition and size but subtle microstructural differences by controlling the preparing process using molecular dynamics simulations. The results suggest that a structural threshold exists for the transformation of deformation mechanisms in metallic glasses: when the structural feature exceeds the threshold, the deformation changes from homogeneous flow to shear localized deformation.

Atomistic understanding of deformation-induced heterogeneities in wire drawing and their effects on the tensile ductility of metallic glass wires

Compared to crystalline metals, metallic glasses (MGs) show an exceptional feature of improved ductility after being mechanically processed by wire drawing. However, its underlying mechanisms have not yet been fully elucidated. In this study, with the aid of atomistic simulations, wire drawing and subsequent tensile loading were performed on MG nanowires to systematically investigate the deformation mechanisms of MGs in wire drawing, the deformation-induced heterogeneities and their influences on the tensile ductility. The results revealed that the deformation mechanisms of MGs in wire drawing are closely associated with the area reduction ratio (R): at a small R of 4.7%, the area reduction is realized via shear transformations of atoms near the surface, leaving the core intact; while at a large R of 9.3%, it relies on the formation of multiple spatially distributed shear bands that redistributes the plasticity throughout the sample. The deformation-induced heterogeneities were understood through the detailed analysis of the resultant residual strain and stresses, the gradient rejuvenated amorphous structures, the unique free volume distribution and spatially distributed shear bands. Moreover, the tensile simulations revealed improved ductility synchronized with decreased yield strength of the drawn samples. The improved ductility is attributed to the synergistic effects of three beneficial factors: 1) The surface compressive residual axial stress leads to a shift of the yield sites from the surface to the core, suppressing the rapid formation of shear bands; 2) The rejuvenated structures near the surface constrain and accommodate the plastic deformation in the core; 3) The spatially distributed shear bands, generated at large R, serve as heterogeneous nucleation sites for highly dispersed plastic shearing. The findings provide a comprehensive elucidation of the deformation-induced heterogeneities of MGs in wire drawing and establish a physical relationship between these heterogeneities and mechanical properties, which can serve to interpret the experimental results.

Bond-breaking analyses on the characteristics of flow defects in metallic glasses under plastic deformation

Bond breaking related with plastic deformation in a deformed metallic glass Zr50Cu50 is investigated by molecular dynamics simulations. The results show that the spatial distributions of broken bonds are closely correlated with local shear strains, and the clustering behaviors of atoms with broken bonds (flow defects) are characterized with different stages of plastic deformation. The average distance among those clusters of flow defects decreases as the strains increase, which follows the curvature quadrupole model for flow defects in ideal amorphous solids. For the first time, the features of bond breaking processes are quantitatively measured with the chemical composition, bond length and orientation, bond pairs, local five-fold symmetry and quasi-nearest atoms, whose threshold values are important factors that could characterize the flow defects in metallic glasses under plastic deformation. The shape, orientation and energetics of flow defects quantitatively characterized by the bond breaking analysis thus facilitate our understanding on the deformation mechanisms in metallic glasses.

Voronoi volume recovery during plastic deformation in deep-notched metallic glasses

Many previous simulations of the plastic deformation in metallic glasses showed that shear banding is the dominant mode accompanied by Voronoi volume continuous generation and softening. In the present work, we report a contrary phenomenon where Voronoi volume recovery is dominant during plastic deformation in CuZr metallic glasses under tension. By creating a notched geometry, the deformation mode of notched samples transits from shear banding to necking due to the introduction of triaxial stress state. With the suppressing of shear banding, a notch strengthening and structure ordering phenomenon together with the recovery of Cu-centered full-icosahedra fraction are also observed. The present work enriches our understanding on the mechanical behavior and deformation mechanism of metallic glasses.

Plastic Deformation Mechanism of Ductile Fe50Ni30P13C7 Metallic Glass

Shear band (SB) multiplication is considered as an essential characteristic of the plastic deformation in metallic glasses (MGs). In this work, the evolutionary characteristics of SBs and serrated behavior in ductile Fe50Ni30P13C7 MGs were studied by the finite element method simulation. The study demonstrated that the stress field would redistribute and become inhomogeneous during SB sliding, where the stress perpendicular to the original SB gradually accumulates until reaching the magnitude of yielding strength and triggering new SBs. The results of simulation are in good agreement with the SBs intersection morphologies observed in SEM images and the serration flows on the stress–strain curves. Furthermore, several factors affecting stress field distribution in MGs, such as contact friction, aspect ratio, and boundary confinement, were also analyzed and discussed. The overall results indicate that the ductility of Fe-based MGs could be achieved without any inhomogeneous structure, and scientifically explicate the dependence of SB multiplication on both material properties and loading condition, which can provide us with a deeper understanding on plastic deformation mechanism of MGs.

In-situ synchrotron X-ray diffraction study of dual-step strain variation in laser shock peened metallic glasses

Atomic-structure evolution is significant in understanding the deformation mechanism of metallic glasses. Here, we firstly find a dual-step atomic strain variation in laser-shock-peened (LSPed) metallic glasses during compression tests by using in-situ synchrotron X-ray diffraction. Under low compressive load, LSP-deformed zone's atomic-structure shows low Young's Modulus (E); with load increase, atomic-structure are re-hardened, showing high E. An atomic deformation mechanism is proposed by using flow unit model, that LSP could induce interconnected flow units and homogenize the atomic-structure. These interconnected flow units are metastable and start to annihilate during compressive loading, causing the dual-step atomic strain variation.

Ion irradiation of metallic glasses

Metallic glasses (MGs), as new disordered materials prepared by rapidly quenching melted alloys, have attracted tremendous attention in the material science community. Due to their long-ranged disorderd and short-ranged ordered structures, MGs usually exhibit uniquely physical, chemical and mechanical properties, which give rise to promising applications in many fields, and especially they are expected to be potentially structural materials used in irradiation conditions, such as in nuclear reactors and aerospace.In this paper, the effects of ion irradiation on the microstructure, mechanical properties, physical, and chemical properties of MGs are reviewed. It is found that the effects of ion irradiation on the microstructures and mechanical properties depend on the ion energy as well as the composition of MG. When high energy ions interact with a solid, the collisions take place between the incident ions and atoms of the solid, which are dominated by inelastic processes (electronic stopping) and elastic processes (nuclear stopping). The inelastic processes result in the excitation and ionization of substrate atoms. In contrast, the elastic processes lead to ballistic atomic displacements. Nuclear stopping can produce structure defects and irradiation damage in glassy phase. The collisions between the incident ions and the target atoms in MGs can cause the target atoms to deviate from their original positions, and leave a large number of vacancies and interstitial atoms behind. The separations between the vacancies and the interstitial atoms form displacement cascades. The interstitial atoms with a low kinetic energy can transfer self-energies to thermal energies, resulting in a thermal spike due to the accumulation of a large quantity of the thermal energies from interstitial atoms. Such a thermal spike will cause MGs to melt and resolidify, which therefore makes the structure of glassy phase changed. Furthermore, the ion irradiation can modify the structures of MGs by introducing excessive free volumes and promoting the mobilities of atoms, which leads to the dilatation of the glassy phase and nanocrystallization. The increase of free volumes softens the MGs, and then causes the plastic deformation mechanism to transform from a heterogeneous deformation to a homogeneous deformation, which significantly enhances the plastic deformation ability.This review paper can not only improve the understanding of the relationship between microstructure evolution and macroscopic mechanical properties, and provide an experimental and fundamental basis to understand the deformation mechanism of MGs, but also summarize the performances of MGs under high dosage of ion irradiation. Moreover, it is of fundamental and practical importance for engineering applications of such advanced materials.

Brittle-to-Ductile Transition in Metallic Glass Nanowires.

When reducing the size of metallic glass samples down to the nanoscale regime, experimental studies on the plasticity under uniaxial tension show a wide range of failure modes ranging from brittle to ductile ones. Simulations on the deformation behavior of nanoscaled metallic glasses report an unusual extended strain softening and are not able to reproduce the brittle-like fracture deformation as found in experiments. Using large-scale molecular dynamics simulations we provide an atomistic understanding of the deformation mechanisms of metallic glass nanowires and differentiate the extrinsic size effects and aspect ratio contribution to plasticity. A model for predicting the critical nanowire aspect ratio for the ductile-to-brittle transition is developed. Furthermore, the structure of brittle nanowires can be tuned to a softer phase characterized by a defective short-range order and an excess free volume upon systematic structural rejuvenation, leading to enhanced tensile ductility. The presented results shed light on the fundamental deformation mechanisms of nanoscaled metallic glasses and demarcate ductile and catastrophic failure.

Tuning the performance of bulk metallic glasses by milling artificial holes

The mechanical performance of materials is greatly affected and could be tuned by artificial defects, especially for amorphous alloys. In present work, specially designed holes are created for bulk metallic glass and apparent mechanical performance improvement is obtained when compared with the intact ones. The fracture characterization discovers that the inner wall of the artificial hole has a blocking effect to shear bands (SBs), leading to an apparent enhancement of mechanical property. Our results demonstrate that the blocking effect of SBs induced by the designing artificial hole may provide some new sights on the plastic deformation mechanism of metallic glasses rather than the improved plasticity itself.

Research on viscoelastic behavior and rheological constitutive parameters of metallic glasses based on fractional-differential rheological model

Metallic glasses offer novel physical, chemical and mechanical properties and have promising potential applications. Recently, exploring the structure and deformation mechanism of metallic glasses according to the rheological mechanical behavior in the nominal elastic region has been the object of intensive research. Physically the mechanical analogues of fractional elements can be represented by self-similarity spring-dashpot fractal networks. In light of the fractal distribution features of the structural heterogeneities, a fractional differential rheological model is introduced to explore the viscoelastic a behavior of metallic glasses in this paper. To investigate the viscoelastic deformation mechanism, carefully designed nanoindentation tests at ambient temperature are proposed in this study. Three kinds of metallic glasses with different Poisson's ratio and glass transition temperature, which have the chemical compositions of Pd40Cu30Ni10P20, Zr48Cu34Pd2Al8Ag8, and (Fe0.432Co0.288B0.192Si0.048Nb0.04) 96Cr4 are selected as the model materials. Experimental and theoretical results clearly indicate that in the nominal elastic region, these metallic glasses exhibit linear viscoelasticity, implying a loading rate-dependent character. Based on the fractional calculus and Riemann-Liouville definition, experimental results are analyzed by the fractional-differential and integer order rheology models respectively. According to the stability of the fitting parameters, here we show that the fractional-differential Kelvin model, which consists of a spring and a fractional dashpot element, can fit the experimental viscoelastic deformation data of the investigated metallic glasses better than that with integer order rheological model. The extracted elastic modulis E1 of the spring in the fractional-differential Kelvin model are comparable to those of samples measured by traditional methods. Such a similarity can be well explained by the mechanical analogue of fractal model proposed for describing the distribution features of the structural heterogeneities in metallic glasses. The rheological parameters obtained here including viscosity index A and fractional order are capable of reflecting the rheological features and the flowing tendency of the above-mentioned metallic glasses. It is found that there exists a clear relationship between the rheological parameters and the reduced glass transition temperature as well as Poisson's ratio, which is helpful for understanding the correlation between plasticity and Poisson's ratio from the micro-structural point of view. The current work provides a rheological model-structure-property relation that may be applicable to a wide range of glassy materials.

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.

分子動力学・連続場変換法によるCu-Zr系金属ガラスのせん断変形機構解析

We conducted molecular dynamics (MD) simulation on simple shear deformation of Cu-Zr metallic glass. A metallic glass model is prepared by rapid quench from an equilibrium melting state. Shear deformation process is simulated by applying stepwise affine-displacement which is followed by structural relaxation for a certain time interval. Present MD simulation demonstrated typical deformation behavior of metallic glasses including elastic response, yielding and nucleation and growth of shear bands in the atomistic scale. To obtain a course-grained picture of the deformation, we transformed the atomistic relative displacements into a continuously differentiable field using the Gauss-type radial basis function (RBF). This analysis revealed that local structural relaxation and their percolation play a dominant role on the formation of shear band. We also revealed that source and sink of divergence of the displacement velocity have a side-by-side configuration due to accommodative motion for relaxation. These results indicate that the continuous field transformation by RBF is effective to understand the plastic deformation mechanism of metallic glasses.

Evolution of atomic rearrangements in deformation in metallic glasses.

Atomic rearrangements induced by shear stress are fundamental for understanding deformation mechanisms in metallic glasses (MGs). Using molecular dynamic simulation, the atomic rearrangements characterized by nonaffine displacements (NADs) and their spatial distribution and evolution with tensile stress in Cu50Zr50 MG were investigated. It was found that in the elastic regime the atomic rearrangements with the largest NADs are relatively homogeneous in space, but exhibit strong spatial correlation, become localized and inhomogeneous, and form large clusters as strain increases, which may facilitate the so-called shear transformation zones. Furthermore, initially they prefer to take place around Cu atoms which have more nonicosahedral configurations. As strain increases, the preference decays and disappears in the plastic regime. The atomic rearrangements with the smallest NADs are preferentially located around Cu atoms, too, but with more icosahedral or icosahedral-like atomic configurations. The preference is maintained in the whole deformation process. In contrast, the atomic rearrangements with moderate NADs distribute homogeneously, and do not show explicit preference or spatial correlation, acting as matrix during deformation. Among the atomic rearrangements with different NADs, those with largest and smallest NADs are nearest neighbors initially, but separating with increasing strain, while those with largest and moderate NADs always avoid to each other. The correlations in the fluctuations of the NADs confirm the long-range strain correlation and the scale-free characteristic of NADs in both elastic and plastic deformation, which suggests a universality of the scaling in the plastic flow in MGs.