Deformation Of Bulk Metallic Glass Research Articles

Mechanical annealing in the homogeneous deformation of bulk metallic glass under elastostatic compression

Elastostatic compression test on Zr60.14Cu22.31Fe4.85Al9.7Ag3 bulk metallic glass (BMG) is conducted at 90% of yielding strength at room temperature. Homogeneous deformation with 0.48% plastic strain is achieved after holding for 5 days. The deformed BMG shows decreased relaxation enthalpy and increased critical shear stress, yield strength and density under elastostatic compression, namely a mechanical annealing effect. This phenomenon is rationalized in terms of the atomic level stress theory and the coalescence of negative and positive free volume.

Structural features of plastic deformation in bulk metallic glasses

Spatially resolved strain maps of a plastically deformed bulk metallic glass (BMG) have been created by using high-energy X-ray diffraction. The results reveal that plastic deformation creates a spatially heterogeneous atomic arrangement, consisting of strong compressive and tensile strain fields. In addition, significant shear strain is introduced in the samples. The analysis of the eigenvalues and eigenvectors of the strain tensor indicates that considerable structural anisotropy occurs in both the magnitude and direction of the strain. These features are in contrast to the behavior observed in elastically deformed BMGs and represent a distinctive structural sign of plastic deformation in metallic glasses.

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.

Inhomogeneous deformation in bulk metallic glasses: FEM analysis

Abstract A modified coupled thermo-mechanical 3D constitutive model for bulk metallic glasses (BMGs), which integrates all the effects of free volume, temperature and hydrostatic stress on the deformation and failure of material, is introduced and implemented into LS-DYNA commercial software. With considering the inhomogeneity of the microstructure of BMGs, FEM geometrical model of the randomly distributed shear-band zones in the material is established. Then FEM simulations for different deformation scenarios, including quasi-static tension, compression, and bending as well as instrumented anvil-on-rod impact, are conducted, and the corresponding deformation and failure mechanisms of BMGs is analyzed. The inhomogeneous deformation in BMGs, especially the shear banding behaviors, including the nucleation and propagation of shear bands, and the shear banding-induced failure, is discussed in detail. Related analysis shows that BMGs exhibit a significant inhomogeneous deformation at the room temperature, and the corresponding mechanical behaviors are affected by the hydrostatic stress. The constitutive model combined with the geometrical models which contain the shear-band zones could represent the deformation and failure characteristics of BMGs relatively well.

Microscopic origin of nonlinear nonaffine deformation in bulk metallic glasses

The atomic theory of elasticity of amorphous solids, based on the nonaffine response formalism, is extended into the nonlinear stress-strain regime by coupling with the underlying irreversible many-body dynamics. The latter is implemented in compact analytical form using a qualitative method for the many-body Smoluchowski equation. The resulting nonlinear stress-strain (constitutive) relation is very simple, with few fitting parameters, yet contains all the microscopic physics. The theory is successfully tested against experimental data on metallic glasses, and it is able to reproduce the ubiquitous feature of stress-strain overshoot upon varying temperature and shear rate. A clear atomic-level interpretation is provided for the stress overshoot, in terms of the competition between the elastic instability caused by nonaffine deformation of the glassy cage and the stress buildup due to viscous dissipation.

Improving the mechanical properties of Zr-based bulk metallic glass by controlling the activation energy for β-relaxation through plastic deformation

The mechanism of plastic deformation in bulk metallic glasses (BMGs) is widely believed to be based on a shear transformation zone (STZ). This model assumes that a shear-induced atomic rearrangement occurs at local clusters that are a few to hundreds of atoms in size. It was recently postulated that the potential energy barrier for STZ activation, WSTZ, calculated using the cooperative shear model, is equivalent to the activation energy for β-relaxation, Eβ. This result suggested that the fundamental process for STZ activation is the mechanically activated β-relaxation. Since the Eβ value and the glass transition temperature Tg of BMGs have a linear relation, that is, because Eβ ≈ 26RTg, the composition of the BMG determines the ease with which the STZ can be activated. Enthalpy relaxation experiments revealed that the BMG Zr50Cu40Al10 when deformed by high-pressure torsion (HPT) has a lower Eβ of 101 kJ/mol. The HPT-processed samples accordingly exhibited tensile plastic elongation (0.34%) and marked decreases in their yield strength (330 MPa). These results suggest that mechanically induced structural defects (i.e., the free volume and the anti-free volume) effectively act to reduce WSTZ and increase the number of STZs activated during tensile testing to accommodate the plastic strain without requiring a change in the composition of the BMG. Thus, this study shows quantitatively that mechanically induced structural defects can overcome the compositional limitations of Eβ (or WSTZ) and result in improvements in the mechanical properties of the BMG.

Correlation between the Enhanced Plasticity of Glassy Matrix Composites and the Volume Fraction, Size and Intrinsic Mechanical Property of Reinforcements

In recent years, bulk metallic glasses (BMGs) have received considerable attention due to their unique mechanical properties. However, the deformation of BMGs is highly localized in a few shear bands so that many of them exhibit poor plasticity. As such, more and more researchers have focused on improving the plasticity by in-situ or ex-situ introducing of nanoor micro-scale crystalline phases into the metallic glassy matrix in order to formation of multiple shear bands.

Statistical analysis of acoustic emission events in torsional deformation of a Vitreloy bulk metallic glass

The characteristics of acoustic emission events produced during torsional deformation of bulk metallic glass samples were statistically analyzed. The amplitudes of these events, and waiting time between them, follow power-law distributions, indicating correlated shear band initialization processes. Time correlation within groups of events is demonstrated statistically. This correlation can be traced back to sequences of events with exponentially decreasing waiting times. The appearance of event sequences is explained by considering the stress field around a shear band of finite length and by the resulting intermittent formation of the bands.

On plastic deformation of bulk metallic glasses in Bridgman anvils

The paper demonstrates the possibility, in principle, of plastic deformation of bulk metallic Ti- and Zr-based glasses with a fully amorphous structure using a method of severe deformation in a copper shell in Bridgmann’s anvils. The presence in the structure of the crystalline phase inclusions in the form of a pile-up of lamellae leads to embrittlement of metallic glass.

Indentation-induced deformation localisation in Zr–Cu-based metallic glass

It has been well documented that the plastic deformation of bulk metallic glasses is localised in thin shear bands. In order to comprehensively understand deformation mechanisms of BMGs, it is necessary to investigate formation of shear bands and related deformation. Nano-indentation studies with a spherical indenter were carried out to characterise shear-band localisation on a surface of Zr–Cu-based metallic glass. The load–displacement diagram of an indentation cycle reflects an elastic deformation followed by the first pop-in at around 4mN. In addition, an alternative technique is proposed to characterise a plasticity mechanism through the evolution of localised shear bands beneath the indentation – wedge indentation. It is found that a hemicylindrical region of shear bands is formed beneath the indentation zone in this case.

Influence of free volume and medium-range order on the deformation response of rapidly solidified and bulk Zr-based (Zr52Ti6Al10Cu18Ni14) metallic glass

Free volume and medium-range order (MRO) present in rapidly solidified ribbons (RSRs) and bulk metallic glasses (BMGs) of Zr52Ti6Al10Cu18Ni14 have been probed by high resolution electron microscopy, fluctuation microscopy, positron annihilation and differential scanning calorimetry. In the as-solidified condition, RSRs showed higher free volume and lower MRO in comparison to BMGs. Within BMGs, the central regions showed higher MRO and lower free volume than the peripheral regions. Uniform deformation of BMGs and RSRs modified their structures, where in, free volume increased in the former and reduced in the latter. These changes in structures led to work hardening in RSRs and work softening in BMGs. Such behaviour could be explained by invoking a concept of critical free volume in the glass phase. For samples (in as-solidified condition) having free volume higher than the critical value, free volume decreased with deformation and showed work-hardening behaviour. In contrast, the work softening behaviour was noticed in samples having free volume lower than the critical free volume.

Origin of Intermittent Plastic Flow and Instability of Shear Band Sliding in Bulk Metallic Glasses

Intermittent or serrated plastic flow is widely observed in the deformation of bulk metallic glasses (BMGs) or other disordered solids at low temperatures. However, the underlying physical process responsible for the phenomena is still poorly understood. Here, we give an interpretation of the serrated flow behavior in BMGs by relating the atomic-scale deformation with the macroscopic shear band behavior. Our theoretical analysis shows that serrated flow in fact arises from an intrinsic dynamic instability of the shear band sliding, which is determined by a critical stiffness parameter in stick-slip dynamics. Based on this, the transition from serrated to nonserrated flow with the strain rate or the temperature is well predicted and the effects of various extrinsic and intrinsic factors on shear band stability can be quantitatively analyzed in BMGs. Our results, which are verified by a series of compression tests on various BMGs, provide key ingredients to fundamentally understand serrated flow and may bridge the gap between the atomic-scale physics and the larger-scale shear band dynamics governing the deformation of BMGs.

Computational modeling of plastic deformation and shear banding in bulk metallic glasses

The study of plastic deformation of bulk metallic glasses (BMGs) reveals two modes. At low temperatures and high stresses plastic deformation is inhomogeneous, in which case the failure of BMGs is mostly due to the formation of a very thin shear band. Inside this band, the evolving strain is highly localized. At high temperatures and low stresses, plastic deformation is homogeneous and the BMGs exhibit a Newtonian viscosity. It is generally acknowledged that the shear-induced free volume evolution plays an important role in the failure of BMGs under both modes of deformation. In this paper, a free volume-based constitutive model is proposed, which accounts for the transition of inhomogeneous to homogeneous deformation and non-Newtonian to Newtonian viscosity. A time integration algorithm is presented in this paper with a description of its implementation in the commercial code ABAQUS through the user-defined material window UMAT. 2 and 3-Dimensional finite element simulations are performed and the model is validated with results of uniaxial compression and micro-indentation experiments for a wide range of temperatures and strain rates. The simulation results exhibit hydrostatic pressure dependence, and suggest that a mixed-power-strain-rate-dependent free volume creation mode is more effective than the commonly used strain-rate-dependent free volume creation law, when considering large temperature ranges.

The relationship between shear bands and deformability in bulk metallic glasses

It has been well documented that the plastic deformation of bulk metallic glasses (BMGs) is localized in narrow shear bands, resulting in rapid propagation of these shear bands and catastrophic failures of BMGs subjected to loads. Shear bands in BMGs play a dominant role in their deformation behavior and macroscopic properties. In order to comprehensively understand the deformation mechanism of BMGs, it is necessary to investigate the formation of shear bands and consequent deformation. In present paper, Zr62.55Cu17.55Ni9.9Al10 and Zr64.80Cu14.85Ni10.35Al10 BMGs were prepared and the effect of indentation on the formation of shear bands was studied at room temperature. The results showed that Zr62.55Cu17.55Ni9.9Al10 BMG deforms through formation of radiation-like and circular shear bands, while for Zr64.80Cu14.85Ni10.35Al10 BMG, basically only radiation-like shear bands can be found after indentation deformation. The density of shear bands in the former is larger than that in the latter with a same loading condition. The different indentation deformation behavior in the two BMGs might be attributed to the differences in thermodynamic stabilities and intrinsic natures. The formation mechanism of the shear bands as well as the deformation mechanism of the BMGs were analyzed and discussed.

Investigation on the inhomogeneous structure of metallic glasses based on the initial elastic deformation in nanoindentation

The initial elastic deformation of bulk metallic glasses (BMGs) with various casting diameters and annealing conditions was investigated by nanoindentation test with a spherical indenter. It was found that with the increasing of the glassy rod diameter and the annealing temperature, the onset slope of nanoindentation plot remarkably increased. The BMGs with various casting diameters and annealing temperatures are of different atomic configuration due to their different cooling rate during casting and different structural relaxation processes during annealing, respectively, indicating that the onset slope of nanoindentation plot is strangely dependent on the atomic configuration. Therefore, the onset slope of nanoindentation plot may serve as an indicator of the initial deformation behavior of BMGs. The structual origin of the initial elastic deformation was discussed in terms of short order nature in glasses and the distribution of free volume.

Temperature rise and flow of Zr-based bulk metallic glasses under high shearing stress

Deformation of the bulk metallic glasses (BMGs) and the creation and propagation of the shear bands are closely interconnected. Shearing force was loaded on Zr41.2Ti13.8Cu12.5Ni10.0Be22.5(Vit.1) BMGs by cutting during the turning of the BMG rod. The temperature rise of alloy on the shear bands was calculated and the result showed that it could reach the temperature of the super-cooled liquid zone or exceed the melting point. The temperature rise caused viscous fluid flow and brought about the deformation of BMGs. This suggested that the deformation of BMGs was derived, at least to some extent, from the adiabatic shear temperature rise.

Study of mechanical deformation of Zr55Cu30Al10Ni5 bulk metallic glass through instrumented indentation

Abstract Instrumented sharp indentation experiments using both conical and Vickers diamond pyramidal indenters were carried out to study deformation characteristics of Zr55Cu30Al10Ni5 bulk metallic glass. Finite element simulations of instrumented indentation were also performed to formulate an overall constitutive response. Comparing the experimentally obtained results with the finite element predictions, it can be stated that mechanical deformation of the bulk metallic glass can be described well by both Mohr–Coulomb and Drucker–Prager constitutive criteria. Using these criteria, the extent of material pile-up observed around the indenter was also estimated very well.

Atomic-level structural modifications induced by severe plastic shear deformation in bulk metallic glasses

Structural evolution of an Au-based bulk metallic glass (BMG) after severe plastic deformation (SPD) was investigated. The newly formed glass contains high-density shear bands, a reduced ordering and a concomitant excess free volume. Moreover, it exhibits a less-changed local structure even in the supercooled liquid region, but a reduced thermal stability reflected in an accelerated crystal nucleation and growth process. These results suggest that SPD modifies the atomic structure of BMGs by localized shear band formation, thus producing so-called nanoglasses.

Finite-element analysis for high-temperature deformation of bulk metallic glasses in a supercooled liquid region based on the free volume constitutive model

A constitutive relation based on the free volume model was developed to describe the strain-rate-dependent deformation behavior of bulk metallic glasses (BMG) at temperatures within the supercooled liquid region (SLR). Validity of the present approach has consequently been assessed by comparing the numerical results obtained from finite-element analyses with the experimental results previously obtained from compression tests of Vitreloy-1 BMG alloy. Finite-element-method simulations combined with free volume constitutive relations were found to reproduce well the plastic deformation behavior of Vitreloy-1 alloy, exhibiting a Newtonian viscous flow without stress overshoot and also a non-Newtonian viscous flow with stress overshoot at temperatures within the SLR. The present approach appears to provide a powerful means of understanding plastic deformation behavior in relation to localized and uniform deformation and also of making reliable formability estimations of BMG alloys.

Stick–slip behavior of serrated flow during inhomogeneous deformation of bulk metallic glasses

Accurate compression tests with a piezoelectric load cell and an acquisition rate of up to 10 kHz were performed on a Zr-based bulk metallic glass in the temperature range 210–320 K at a strain rate of 10 −3 s −1. Information about the stress drop magnitude and the associated size of shear displacements as a function of temperature and strain provides detailed insights into the shear band characteristics, which can be described by a stick–slip process. The average shear slip displacement is on average about 1–2 μm, irrespective of temperature, whereas the associated slip time (or stress drop time) increases from ∼1 ms at 320 K to ∼0.4 s at 213 K, yielding values on the deformation kinetics and the shear viscosity. Scanning electron microscopy investigations on shear surfaces and in situ acoustic emission measurements provide further understanding into the complex multistep shear slip process.