Shear Bands In Bulk Metallic Glass Research Articles

A maximum temperature rise model of the shear band in bulk metallic glasses

Based on mechanisms applicable to bulk metallic glasses (BMGs), such as a superposition principle of an instantaneous finite surface heat source temperature field and a flow model about the shear-band velocity, a maximum temperature rise model of shear bands in BMGs was deduced, ΔTmax=σycpΔupl·vSB2πα. The model describes the dependence of the maximum temperature rise on the yield strength and the shear-band velocity in BMGs. It can be calculated that the maximum temperature rise of BMGs is between 1 K ∼ 30 K at room temperature through these two parameters. A critical temperature, TC, of almost 0.8∼0.9Tg is introduced, at T<TC, the change of the yield strength with temperature in BMGs basically conforms a cooperative shear model (CSM), and at T>TC, a significant reduction of the yield strength in BMGs can be attributed to avoiding failure due to localized material softening.

A flow model in bulk metallic glasses

Based on the microscopic mechanisms applicable to bulk metallic glasses (BMGs), such as the free-volume theory and fracture theory of brittle solids, a constitutive equation for the crack-like propagation velocity of shear bands in BMGs is deduced, νSBcl=ν0a0exp(−ES/kT). Furthermore, a flow model of monolithic BMGs is successfully established, logνSB=log(ν0a0)+logeES/(kTl)−logeES/(kT). The model explains clearly the fundamental deformation of BMGs at the temperature far below the liquidus temperature, i.e., the mechanism of serrated flows. Not only does the model accurately predict the transition between serrated and non-serrated flows, but the Arrhenius equation and activation energy received by the current model for most BMGs are consistent with the experimental results. The current study paves a way to plastically process BMGs, avoiding catastrophic plasticity instability.

Shear Bands Topology in the Deformed Bulk Metallic Glasses

Recent experimental studies revealed the presence of Volterra dislocation-type long-range elastic strain/stress field around a shear band (SB) terminated in a bulk metallic glass (BMG). The corollary from this finding is that shear bands can interact with these stress fields. In other words, the mutual behaviour of SBs should be affected by their stress fields superimposed with the external stresses. In order to verify this suggestion, the topography of the regions surrounding SBs terminated in the BMGs was carefully analysed. The surfaces of several BMGs, deformed by compression and indentation, were investigated with a high spatial resolution by means of scanning white-light interferometry (SWLI). Along with the evidence for the interaction between SBs, different scenarios of the SB propagation have been observed. Specifically, the SB path deviation, mutual blocking, and deflection of SBs were revealed along with the significant differences between the topologies of the mode II (in-plane) and mode III (out of plane) SBs. While the type II shear manifests a linear propagation path and a monotonically increasing shear offset, the type III shear is associated with a curved, segmented path and a non-monotonically varying shear offset. The systematic application of the “classic” elastic Volterra’s theory of dislocations to the behaviour of SBs in BMGs provides new insight into the widely reported experimental phenomena concerning the SB morphology, which is further detailed in the present work.

Collective evolution dynamics of multiple shear bands in bulk metallic glasses

The collective behavior of multiple shear bands was investigated under in situ four-point bending tests of a Zr-based bulk metallic glass (BMG) over a wide range of sample scales. The self-organization of shear band pattern, characterized by shear band spacing and shear offset, is observed with the variation of sample size and bend curvature, which presents significant size effect and tension–compression asymmetry. To unveil these fundamental behaviors, an analytical model for the evolution dynamics of multiple shear banding is developed for BMGs. In this model, both micro-structural evolution and pressure sensitivity are taken into account by introducing a new law for the stress softening of BMGs within the framework of continuum mechanics; the collective evolution of shear bands is regarded as the coupling result of the structural softening, the momentum diffusion, and the energy conservation. Applying the proposed theoretical model to the bending deformation of BMGs, the analytical solutions of shear band spacing, shear offset and failure strain are obtained. The fundamental behaviors of multiple shear bands are uncovered, in line with the experimental observations: notable scaling laws are found in the evolution of shear band spacing and shear offset, and the inhomogeneous size effect of plasticity is revealed by a transition from weak to strong size-dependence of failure strain with decreasing sample thickness. To be further, a competing map of shear band nucleation and propagation is established based on energy dissipation. The underlying mechanism of these size dependent behaviors of multiple shear bands in BMGs is found to be ascribed to the energy dissipation competition between the nucleation and propagation of shear bands.

Simulation of shear banding in bulk metallic glass composites containing dendrite phases

• Dendrite formation in BMG composite is simulated using phase-field modeling. • Phase field models are developed to investigate shear banding in BMG matrix and fracture in crystalline dendrites. • Geometry and fracture energy of the dendrites affect the branching, multiplication and detour of the shear bands. The mechanisms of shear banding in Zr-based bulk metallic glasses (BMGs) containing dendrite phase such as β-Zr 2 Cu are numerically investigated using the phase-field simulation approaches. The growth of dendrite is simulated based on Elder’s solidification theory. The interactions between the shear bands and the dendrites are studied using the phase-field model for shear banding in BMGs. It is found that the properties of dendrites such as the rotation angle representing their dispersion patterns and their fracture energy significantly affect the branching, multiplication and detour of shear bands, which result in the improved ductility of the composites. The simulation evaluates quantitatively the properties of dendrites that determine the features of shear banding in the composites.

Effect of hydrostatic pressure on shear banding behaviors in bulk metallic glasses

We report the effect of hydrostatic pressure on shear banding behaviors of a bulk metallic glass (BMG) Zr41.2Ti13.8Cu12.5Ni10Be22.5 (Vit 1). A confining sleeve technique was used to apply multiaxial compression. The density of shear bands increased along a direction with the increase of hydrostatic pressure from 0MPa to 550MPa. Interestingly, as the hydrostatic pressure increased to 680MPa, branching phenomenon was emerged in the shear band, and shear bands were intersected with the hydrostatic pressure further increased to 960MPa. At the maximum hydrostatic pressure of 1940MPa, intersection sites increased and branching phenomenon could be also observed simultaneously. The present results suggest that the hydrostatic pressure has an important influence on the formation and propagation of the shear band in BMGs.

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.

Shear-band toughness of bulk metallic glasses

Dynamic “forced” shear and quasi-static tensile tests were performed on a typical Vitreloy 1 bulk metallic glass (BMG). The nanoscale shear band is found to initiate in a local plastic region, then propagates with a well-defined front. A theoretical model that takes into account momentum, energy and free-volume balance is developed to quantitatively describe the dissipation system of shear-band propagation in the local plastic region. The analytical expressions for shear-localization time, shear-band thickness and critical energy dissipated within the band are obtained. Calculations demonstrate that the shear-band propagation process is dominated by the free-volume softening, rather than the thermal softening. The latter increases the band thickness slightly but decreases the corresponding critical dissipation energy somewhat. The concept of shear-band toughness is further introduced to measure the inherent resistance capability of materials to the propagation of shear bands in BMGs. These results assist in more comprehensively understanding the evolution mechanism of the shear bands, and in guiding alloy design to enhance the toughness of BMGs.

Comparison of shear banding in BMGs due to thermal-softening and free volume creation

This paper reports a comparative study of shear banding in BMGs resulting from thermal softening and free volume creation. Firstly, the effects of thermal softening and free volume creation on shear instability are discussed. It is known that thermal softening governs thermal shear banding, hence it is essentially energy related. However, compound free volume creation is the key factor to the other instability, though void-induced softening seems to be the counterpart of thermal softening. So, the driving force for shear instability owing to free volume creation is very different from the thermally assisted one. In particular, long wave perturbations are always unstable owing to compound free volume creation. Therefore, the shear instability resulting from coupled compound free volume creation and thermal softening may start more like that due to free volume creation. Also, the compound free volume creation implies a specific and intrinsic characteristic growth time of shear instability. Finally, the mature shear band width is governed by the corresponding diffusions (thermal or void diffusion) within the band. As a rough guide, the dimensionless numbers: Thermal softening related number B, Deborah number (denoting the relation of instability growth rate owing to compound free volume and loading time) and Lewis number (denoting the competition of different diffusions) show us their relative importance of thermal softening and free volume creation in shear banding. All these results are of particular significance in understanding the mechanism of shear banding in bulk metallic glasses (BMGs).

Basic mechanical behaviors and mechanics of shear banding in BMGs

In this paper, some basic mechanical behaviors of bulk metallic glasses (BMGs) were discussed. It can be found from the discussions that the mechanical behaviors of BMGs are mainly due to the formation and operation of shear bands in BMGs. Furthermore, the relevant mechanics of shear banding were investigated in the paper. The theoretical analysis of deformation coupling thermal softening and free volume creation softening demonstrates that the free volume creation and thermal softening can jointly promote the formation of shear bands in BMGs, and the observed post mortem shear band width looks more like that governed by free volume creation.

Adiabatic shear banding instability in bulk metallic glasses

In this letter, a linear instability analysis was performed to highlight the mechanism of formation of adiabatic shear banding instabilities in bulk metallic glasses (BMGs). It is found that this instability is determined by the free volume coalescence-diffusion Deborah number. The most important findings are that both free volume coalescence softening and adiabatic heating softening exert an influence on the formation of adiabatic shear banding instability in BMGs, and higher strain rate promotes the growth of instability. These results are of particular significance in understanding the mechanism of formation of shear bands in BMGs.