Plastic Flow In Metallic Glasses Research Articles

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.

Various sizes of sliding event bursts in the plastic flow of metallic glasses based on a spatiotemporal dynamic model

This paper presents a spatiotemporal dynamic model based on the interaction between multiple shear bands in the plastic flow of metallic glasses during compressive deformation. Various sizes of sliding events burst in the plastic deformation as the generation of different scales of shear branches occurred; microscopic creep events and delocalized sliding events were analyzed based on the established model. This paper discusses the spatially uniform solutions and traveling wave solution. The phase space of the spatially uniform system applied in this study reflected the chaotic state of the system at a lower strain rate. Moreover, numerical simulation showed that the microscopic creep events were manifested at a lower strain rate, whereas the delocalized sliding events were manifested at a higher strain rate.

A quasi-phase perspective on flow units of glass transition and plastic flow in metallic glasses

We describe the activation and evolution processes of flow units for glass transition and plastic flow in metallic glasses using a quasi-phase model. We model the flow units as liquid-like quasi phases embedded in solid-like glassy substrate, and the formation and growth of the liquid-like phases in glasses are treated as a phase transition process. Our model can capture the main features of glass transition and mechanical behavior of metallic glasses and agree with experimental observations. The work may present a new perspective for structural origins of deformation and glass transition of metallic glasses.

Stress relaxation in metallic glasses of the system Pd – Cu – Ni – P prepared from melts with different glass-forming capacity

Shear stress relaxation is studied in glasses of similar chemical composition of the system Pd – Cu – Ni – P, whose melts differ considerably with respect to glass-forming capacity. It is demonstrated that glass, whose melt develops considerable glass-forming capacity, always relaxes more slowly. The result obtained is discussed in connection with the possible reasons for glass formation and homogeneous plastic flow of metallic glasses.

Stick-slip dynamics and recent insights into shear banding in metallic glasses

Abstract

Intrinsic correlation between dilatation and pressure sensitivity of plastic flow in metallic glasses

Taking shear-induced dilatation into consideration in shear transformation zone (STZ) operations, we derive a new yield criterion that reflects the pressure sensitivity in plastic flow in metallic glasses (MGs), which agrees well with experiments. Furthermore, an intrinsic theoretical correlation between the pressure sensitivity coefficient and the dilatation factor is revealed. It is found that the pressure sensitivity of plastic flow of MGs originates in the dilatation of microscale STZs.

Atomistic Origin of Rate-Dependent Serrated Plastic Flow in Metallic Glasses

Nanoindentation simulations on a binary metallic glass were performed under various strain rates by using molecular dynamics. The rate-dependent serrated plastic flow was clearly observed, and the spatiotemporal behavior of its underlying irreversible atomic rearrangement was probed. Our findings clearly validate that the serration is a temporally inhomogeneous characteristic of such rearrangements and not directly dependent on the resultant shear-banding spatiality. The unique spatiotemporal distribution of shear banding during nanoindentation is highlighted in terms of the potential energy landscape (PEL) theory.

Experimental characterization of shear transformation zones for plastic flow of bulk metallic glasses

We report experimental characterization of shear transformation zones (STZs) for plastic flow of bulk metallic glasses (BMGs) based on a newly developed cooperative shearing model [Johnson WL, Samwer K (2005) A universal criterion for plastic yielding of metallic glasses with a (T/T(g))(2/3) temperature dependence. Phys Rev Lett 95: 195501]. The good agreement between experimental measurements and theoretical predictions in the STZ volumes provides compelling evidence that the plastic flow of metallic glasses occurs through cooperative shearing of unstable STZs activated by shear stresses. Moreover, the ductility of BMGs was found to intrinsically correlate with their STZ volumes. The experiments presented herein pave a way to gain a quantitative insight into the atomic-scale mechanisms of BMG mechanical behavior.

Local order influences initiation of plastic flow in metallic glass: Effects of alloy composition and sample cooling history

Bulk metallic glasses (MGs) with tunable plasticity and strength have been reported recently. Using Cu–Zr and Cu–Zr–Al MG models, here we illustrate how and why alloy composition and cooling history influence the initial flow behavior in the early stage of plasticity. Starting from Cu 46Zr 54, either increasing the Cu concentration, or substituting Al for a few percent of Zr, increases the resistance to the initiation of plastic flow, the softening after the local yielding, and the propensity for strain localization. These effects are shown to be intrinsic to the uniform, fully amorphous MGs and rooted in their internal structure. Our quantitative monitoring of the local environment, especially the role of full-icosahedral clusters in shear transformations, identifies the fertile and resistant structural entities controlling deformation. The structural mechanisms have implications for macroscopic plasticity, and the alloy dependence of the MG structure reveals a microscopic origin underlying the varying mechanical properties.

Plastic flow and failure resistance of metallic glass: Insight fromin situcompression of nanopillars

We report in situ nanocompression tests of Cu-Zr-Al metallic glass (MG) pillars in a transmission electron microscope. This technique is capable of spatially and temporally resolving the plastic flow in MGs. The observations reveal the intrinsic ability of fully glassy MGs to sustain large plastic strains, which would otherwise be preempted by catastrophic instability in macroscopic samples and conventional tests. The high ductility in volume-limited MGs and the sample size effects in suppressing the rapid failure common to MGs are analyzed by modeling the evolution of the collectivity of flow defects toward localization.

Local temperature rises during mechanical testing of metallic glasses

Under ambient conditions, plastic flow in metallic glasses is sharply localized into shear bands. The heat content of, and consequent temperature rise at, shear bands in three bulk metallic glasses are compared using a recently reported fusible coating method. The minimum shear offsets necessary to detect local heating are determined. It is shown that the dependence of heat content on offset is consistent with frictional heating in the band. The effective stress on the band undergoing shear is 50–70% of the macroscopic shear stress, a ratio compared with simulations of shear-band initiation and operation. It is also noted that frictional heating can occur not only at shear bands, but also at mixed-mode cracks.

The Mohr–Coulomb criterion from unit shear processes in metallic glass

The experimental and simulation-based evidence for asymmetric plastic yielding of metallic glasses is reviewed, and discussed within the framework of the Mohr–Coulomb criterion. The weight of evidence is supportive of normal stress-dependent yielding, although it is difficult to ascribe the origin of this effect to a particular mechanism given the myriad of complex physical processes that occur during plastic flow of metallic glasses. Using simple unit models of four- or nine-atom ‘shear transformation zones’, we show that Mohr–Coulomb yielding arises even at this very fine scale. Furthermore, by considering the topological difference between the four- and nine-atom models, we illustrate why the degree of the yield asymmetry in metallic glasses can vary over an appreciable range. We also discuss a potential breakdown of the Mohr–Coulomb criterion for states of high tensile stress in light of these unit models.

The Kinetics of Irreversible Structural Relaxation and Homogeneous Plastic Flow of Metallic Glasses

The paper is devoted to a comprehensive review of the Directional Structural Relaxation model developed recently. The model treats homogeneous plastic flow of metallic glasses as a result of structural relaxation oriented by the external stress. The analytical expressions for the macroscopic plastic strain rate due to irreversible structural relaxation of metallic glasses are derived. A comparison of the predictions of the model with the experimental data is given. It is concluded that the model provides a good basis for the understanding of homogeneous plastic flow of metallic glasses under various experimental conditions.

The Kinetics of Irreversible Structural Relaxation and Homogeneous Plastic Flow of Metallic Glasses

The Kinetics of Irreversible Structural Relaxation and Homogeneous Plastic Flow of Metallic Glasses

Kinetics of structural relaxation and regularities of plastic flow of metallic glasses

On the basis of a calculation of the structural relaxation rate and an experimental acoustical-emission determination of the temperature of the transition from localized to uniform flow it is argued that the type of plastic deformation of metallic glasses is uniquely determined by the kinetic structure of the relaxation. In the case of a kinetically hindered structural relaxation, which is characteristic for tests of initial samples at temperatures T<380–420 K, a localized dislocational deformation is realized. At higher temperatures, “memory” of the thermal prehistory of the samples is lost (aging at room temperature), the structural relaxation rate grows abruptly and plastic flow becomes uniform viscoplastic flow.

The role of structural relaxation in the plastic flow of metallic glasses

The role of structural relaxation in the plastic flow behavior of metallic glasses is analyzed both theoretically and experimentally. The characteristic time of structural relaxation is calculated as a function of glass thermal prehistory. It is revealed that heating above the room temperature by several tens of Kelvins results in a sharp, by several orders of magnitude, decrease of this time. It is argued that localized “inhomogeneous” dislocation-like flow occurs on loading if the characteristic time of structural relaxation is much greater than the characteristic loading time, while “homogeneous” viscous deformation is observed in the opposite case. Precise measurements of acoustic emission in a Co-based metallic glass being loaded at different temperatures and strain rates are employed for verification of this statement. It is shown that the inhomogeneous → homogeneous flow transition occurs at temperatures somewhat higher than T=400 K, and the transition temperature increases by ≈ 40 K as the strain rate increases by two orders of magnitude. Theoretical estimations show that for the inhomogeneous flow the characteristic time of structural relaxation in the loaded state is indeed much greater than the characteristic loading time. It is concluded that the kinetics of structural relaxation determines the flow mode of metallic glasses in a unique manner. The kinetically “frozen” structural relaxation gives rise to a crystalline-like localized flow under load while intensive structural relaxation facilitates a viscous glass-like behavior.

Acoustic emission during heterogeneous and homogeneous plastic flow of a metallic glass

Measurements have been made of the acoustic emission that occurs during the plastic deformation of metallic glass subjected to various heat treatments. It is shown that for the same temperature and deformation rate it is possible to have either homogeneous flow with no acoustic emission or localized flow accompanied by intense discrete acoustic emission, depending on the preliminary annealing conditions. From an analysis of the experimental results, it is deduced that the nature of the plastic flow of metallic glasses is determined by the structural relaxation rate at the experimental temperature and the conditions of intense structural relaxation, that plastic flow is homogeneous viscoelastic, and, under the conditions of kinetically inhibited structural relaxation, that it is localized and occurs by a dislocation-like mechanism.

Kinetics of low temperature stress relaxation in metallic glasses

Stress relaxation and strain recovery investigations give forcible arguments that room temperature plays a particular role in the physics of plastic flow of metallic glasses (MGs). The present paper is devoted to the experimental investigation of stress relaxation in two MGs in the range 5--293 K and the determination of the activation volume of relaxation. Soft magnetic MGs Co{sub 70}Fe{sub 5}Si{sub 15}B{sub 10} and Fe{sub 40}Ni{sub 40}P{sub 14}B{sub 6} studied in many aspects were chosen for the investigation. The alloys were prepared by a standard melt spinning technique with ribbons 25--30 {mu}m thick. X-ray diffraction was performed to ensure that the alloys would be entirely amorphous. The specimens for the investigation were prepared by cutting g off the ribbon into small strips (1--2 mm wide and {approx} 25 mm long) with the help of a special cutting-out press. Mechanical testing was performed using a string-type tensile testing machine.

Internal friction in cold-rolled metallic glasses Cu50Ti50 and Ni78Si8B14

The influence of cold rolling on the low temperature (30 to 300 K) internal friction of metallic glasses Cu50Ti50 and Ni78Si8B14 is investigated. It is shown that cold rolling of both metallic glasses up to 2 to 6% results in the appearance of a high relaxation damping peak aroung 260 to 280 K. The internal friction background below the peak shows a strong amplitude dependence. In highly predeformed specimens (≈ 16%) the internal friction peak is absent. Electron irradiation (2 MeV, 1019 cm−2) leads to the suppression of the deformation-induced internal friction peak. The results are interpreted in the framework of the dislocation models of plastic flow of metallic glasses. [Russian Text Ignored].

A yield criterion for Pd 40Ni 40P 20 metallic glass

The yield strength of Pd 40Ni 40P 20 metallic glass undergoing low-temperature, localised plastic flow has been measured in uniaxial compression, plane-strain compression, pure shear and tension. The results demonstrate that yielding in Pd 40Ni 40P 20 follows a Mohr-Coulomb criterion, rather than the von Mises criterion which is appropriate to polycrystalline metals. The sign of the normal stress acting across the slip-plane has a significant effect on the yield strength so that the yield strength in compression is substantially higher than in tension, but the hydrostatic pressure has only a small effect on yielding. This result is discussed in terms of the micromechanism of plastic flow in metallic glasses.