Shear Bands In Metallic Glasses Research Articles

Effective temperature dynamics of shear bands in metallic glasses.

We study the plastic deformation of bulk metallic glasses with shear transformation zone (STZ) theory, a physical model for plasticity in amorphous systems, and compare it with experimental data. In STZ theory, plastic deformation occurs when localized regions rearrange due to applied stress and the density of these regions is determined by a dynamically evolving effective disorder temperature. We compare the predictions of STZ theory to experiments that explore the low-temperature deformation of Zr-based bulk metallic glasses via shear bands at various thermal temperatures and strain rates. By following the evolution of effective temperature with time, strain rate, and temperature through a series of approximate and numerical solutions to the STZ equations, we successfully model a suite of experimentally observed phenomena, including shear-band aging as apparent from slide-hold-slide tests, a temperature-dependent steady-state flow stress, and a strain-rate- and temperature-dependent transition from stick-slip (serrated flow) to steady-sliding (nonserrated flow). We find that STZ theory quantitatively matches the observed experimental data and provides a framework for relating the experimentally measured energy scales to different types of atomic rearrangements.

A single shear band in a metallic glass: Local core and wide soft zone

Two dimensional mapping of structural properties near a single shear band in a Zr-based bulk metallic glass reveals the presence of hardness and modulus reductions at a micrometer length scale. The isolated shear band had carried all the macroscopic plastic strain and the material near the shear-band exhibits structural variations both along and normal to the shear plane. Analyzing the nanoindentation data indicates that long range internal stresses are the primary cause of the spatially varying structure. The results demonstrate that a nano-scale defect in a metallic glass may have a micrometer range signature.

Shear bands in metallic glasses are not necessarily hot

We have used the fusible tin coating method to detect shear band heating in amorphous Zr57Ti5Cu20Ni8Al10 loaded under quasi-static uniaxial compression. High-rate load data allowed a precise determination of the duration of shearing events and final fracture. When loading was halted prior to fracture we saw no evidence of melted tin despite the presence of shear offsets up to 6 μm on some shear bands. Samples loaded to fracture showed evidence of tin melting near the fracture surface. We attribute the difference to the duration of the events, which is much longer for shear banding (milliseconds) than for fracture (microseconds).

Note: High-speed optical imaging powered by acoustic emission triggering.

Aiming at high speed video recording of rapid events, which occur in loaded solids with unpredictable timing, the efficient triggering method is proposed for optical imaging systems with an aid from the highly sensitive acoustic emission technique. The efficiency of the proposed method is demonstrated on two examples: observation of initiation and propagation of highly localized shear bands in metallic glasses and mechanical twinning in Mg polycrystals.

Crossover from random three-dimensional avalanches to correlated nano shear bands in metallic glasses

When applying mechanical stress to a bulk metallic glass it responds with elastic and/or plastic deformations. A comprehensive microscopic theory for the plasticity of amorphous solids remains an open task. Shear transformation zones consisting of dozens of atoms have been identified as smallest units of deformation. The connexion between local formation of shear transformations zones and the creation of macroscopic shear bands can be made using statistical analysis of stress/energy drops or strain slips during mechanical loading. Numerical work has proposed a power law dependence of those energy drops. Here we present an approach to circumvent the experimental resolution problem using a waiting time analysis. We report on the power law-distributed deformation behaviour and the observation of a crossover in the waiting times statistics. This crossover indicates a transition in the plastic deformation behaviour from three-dimensional random activity to a two-dimensional nano shear band sliding.

A Novel Experimental Approach to Determine Density Changes in Shear Bands of Metallic Glass by Correlative Analytical TEM

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Strength softening at shear bands in metallic glasses

The plasticity of metallic glasses (MGs) is mediated by localized shearing in thin bands, degrading the mechanical properties at shear bands (SBs), which can cause shear localization and poor deformability. Quantifying the strength loss at SBs in MGs is important for both further material improvement and failure prediction. By applying a combination of microscale bending with in situ tensile tests, we successfully determine the strength loss at SBs in two typical MGs, namely Zr41Ti14Cu12.5Ni10Be22.5 and (Cu50Zr50)95Al5, that are about 20% lower than those of their intact counterparts. We further notice, based on fractographic examinations, that stable SBs may induce only a negligible heat, but unstable SBs give rise to a high temperature increase in SBs. This observation sheds light on the controversy about whether SBs in MGs must be hot.

Mesostructural effects on the mechanical properties of Zr-based bulk metallic glasses

Structural and elemental mapping analyses were applied to characterize the mesostructure in the bulk metallic glass-forming Zr60FexCu30−xAl10 and Zr60+xFe5Cu25−xAl10 alloys. It is found that the observed meso-/microstructure is adjustable by the addition of Fe and tunes the acoustic anelasticity of the samples, while the relaxation time derived from our ultrasound measurements inversely scales with the mechanical properties and ductility of the alloys. The measurements also give data for the thermal diffusivity and coefficient of thermal expansion. Together with mechanical compression tests the results are further discussed on the basis of the thermo-elastic effect and the related role of heat dissipation on the propagation of shear bands in metallic glasses during plastic deformation.

Probing Shear-Band Initiation in Metallic Glasses

Insitu acoustic emission monitoring is shown to capture the initiation of shear bands in metallic glasses. A model picture is inferred from stick-slip flow in granular media such that the origin of acoustic emission is attributed to a mechanism of structural dilatation. By employing a quantitative approach, the critical volume change associated with shear-band initiation in a metallic glass is estimated to be a few percent only. This result agrees with typical values of excess free volume found in the supercooled liquid regime near the glass transition temperature.

Quasi phase transition model of shear bands in metallic glasses

A quasi phase transition model of shear bands in metallic glasses (MGs) is presented from the thermodynamic viewpoint. Energy changes during shear banding in a sample–machine system are analyzed following fundamental energy theorems. Three characteristic parameters, i.e. the critical initiation energy ΔGc, the shear band stability index k0, and the critical shear band length lc, are derived to elucidate the initiation and propagation of shear bands. The criteria for good plasticity in MGs with predominant thermodynamic arrest of shear bands are proposed as low ΔGc, large k0, and small lc. The model, combined with experimental results, is used to analyze some controversial phenomena of deformation behavior in MGs, such as the size effect, the effect of testing machine stiffness and the relationship between elastic modulus and plasticity. This study has important implications for a fundamental understanding of shear banding as well as deformation mechanisms in MGs and provides a theoretical basis for improving the ductility of MGs.

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

Abstract

How does the initial free volume distribution affect shear band formation in metallic glass?

Introducing heterogeneities into the structure is an effective way to enhance the plasticity in metallic glasses (MGs). As natural heterogeneity, the original randomly distributed free volume in MGs has been found to be in favor of plasticity. However, the exact correlation between the free volume distribution and mechanical response is still unclear. In this paper, we investigate the shear banding in MGs with different structural disorders, characterized by both the free volume concentration (FVC) and the free volume dispersion (FVD). It is found that, either high FVC or wide FVD leads to low activation stress of shear band; wide FVD promotes the multiplication of shear bands but high FVC restricts it. It reveals that the yield strength in MGs is dependent on the amount of free volume while the plasticity mainly relies on the distribution. An optimum combination of the two aspects probably helps to design a MG of both good plasticity and high strength.

On the shear-band direction in metallic glasses

This paper shows that it is inappropriate to relate the angle between the loading axis and the shear-band (or fracture) plane in metallic glasses under uniaxial loading conditions to the coefficient of internal friction in the Mohr–Coulomb model. Shear bands in metallic glasses are a result of material instability (which can be predicted from constitutive parameters and loading conditions), which does not correspond to the material yield condition. Specifically, the shear-band directions depend on the Poisson’s ratio, the ratios of three deviatoric principal stresses to the von Mises stress, the coefficient of internal friction, and the dilatancy factor. The last parameter describes whether the plastic flow is associative or non-associative. Theoretical predictions based on the classic Rudnicki–Rice model agree well with a compilation of observations in uniaxial mechanical tests. Furthermore, using the elastic contact solutions and the Rudnicki–Rice model, we identify three (two) regimes under the two-dimensional cylindrical (three-dimensional spherical) contact where different shear-band directions may occur. When using a bonded-interface technique to visualize shear bands under three-dimensional contacts, it should be noted that the stress component normal to the bonded interface is released, resulting in the commonly observed semicircular shear bands whose directions are predicted to follow the larger in-plane principal stress.

Critical Free Volume Concentration of Shear Banding Instability in Metallic Glasses

We present a model which predicts the critical free volume concentration of shear banding instability in metallic glasses (MGs). From the stability map, this model demonstrates that the prediction of shear band thickness is valid only for a short time after shear instability, and the diffusion of defects should be included in the mature shear band in MGs. The results agree well with the experimental observations and simulations.

Shear bands in metallic glasses: Size effects on thermal profiles

The characteristic dimensions of the hot, liquid zone behind a moving shear-band front in a metallic glass are analyzed. In addition to the expected dependence on material constants, the thickness of the zone is proportional to the shear offset, while its width is proportional to the square of the offset. Considering the bending of a plate, the size and shape of the hot zone are found to be strongly dependent on plate thickness. For shear offsets ≪ 1 μ m , typical of plate thickness ≪ 100 μ m , local temperature rises are insignificant. For larger dimensions, local temperature rises give a liquid zone centered on the shear plane with width comparable to the sample dimensions. The scaling of characteristic lengths and times with plate thickness facilitates the interpretation of the transitions observed in mechanical behavior, and the variation in behavior from glass to glass.

Propagation of shear bands in metallic glasses and transition from serrated to non-serrated plastic flow at low temperatures

The mechanical behavior and the kinetics of shear banding in bulk metallic glasses was investigated at room and liquid nitrogen temperature using the acoustic emission (AE) technique. It was demonstrated that the intensive AE reflecting the activity of strongly localized shear bands at room temperature vanishes at the transition of plastic flow from serrated to non-serrated with a reduction in temperature. The disappearance of AE clearly suggests that the shear band propagation velocity significantly decreases at low temperature, and sliding along the principle shear band is observed at the machine-driven rate.

Effects of pores on shear bands in metallic glasses: A molecular dynamics study

The effects of nanoscale pores on the strength and ductility of porous Cu 46Zr 54 metallic glasses during nanoindentation and uniaxial compression tests are modelled and investigated using molecular dynamics (MD) simulations. In the MD simulations, atomistic amorphous samples were digitally prepared through fast quenching from the liquid states of copper and zirconium alloy. In both of the nanoindentation and uniaxial compression simulations, shear transformation zones and shear bands are observed through the local deviatoric shear strains in the samples. The results show that the existence of pores causes strain concentrations and greatly promotes the initialization and propagation of shear bands. Importantly, only pores reaching critical size can effectively facilitate the formation of multiple shear bands. It is also observed that hardening occurs through pore annihilation and the shear band stops in porous metallic glasses.

WITHDRAWN: Shear banding in metallic glasses

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Prediction of shear-band thickness in metallic glasses

We derive an explicit expression for predicting the thicknesses of shear bands in metallic glasses. The model demonstrates that the shear-band thickness is mainly dominated by the activation size of the shear transformation zone (STZ) and its activation free volume concentration. The predicted thicknesses agree well with the results of measurements and simulations. The underlying physics is attributed to the local topological instability of the activated STZ. The result is of significance in understanding the origin of inhomogeneous flow in metallic glasses.

1008 金属ガラス中のせん断帯の生成・成長挙動に関するモデル解析(OS10.電子・原子・マルチシミュレーションに基づく材料特性評価(2:局所性),オーガナイズドセッション)

1008 金属ガラス中のせん断帯の生成・成長挙動に関するモデル解析(OS10.電子・原子・マルチシミュレーションに基づく材料特性評価(2:局所性),オーガナイズドセッション)