Plasticity Of Metallic Glasses Research Articles

Characterization of structural inhomogeneity in Al88Ce8Co4 metallic glass

Structural inhomogeneity is found to be a promising way to overcome intrinsic brittleness and to improve the plasticity of metallic glasses (MGs), although such inhomogeneity has not been well-understood. In this study, quantitative observations and theoretical explanations on the structural inhomogeneity were presented for Al88Ce8Co4 MG. Applying a technique called energy filtered electron diffraction (EFED), different medium-range orders (MROs) reflected from conspicuous and inconspicuous prepeaks, were observed, which, following reduced pair distribution functions obtained from the EFED measurement, correspond to different short-range orders (SROs). Then using reverse Monte Carlo (RMC) simulation, the structural inhomogeneity is attributed to diversity of AlCo clusters, which lead to loosely and densely packed local atomic structures and contribute to soft and hard regions, as evidenced by synchrotron radiation X-ray diffraction, dynamic mechanical analysis and nano-indentation. The obtained structural essence of intrinsic inhomogeneity in MGs lays the foundation for further understanding and designing MGs with high plasticity, by adjusting properly inhomogeneous structure.

The Critical Criterion on Runaway Shear Banding in Metallic Glasses.

The plastic flow of metallic glasses (MGs) in bulk is mediated by nanoscale shear bands, which is known to proceed in a stick-slip manner until reaching a transition state causing catastrophic failures. Such a slip-to-failure transition controls the plasticity of MGs and resembles many important phenomena in natural science and engineering, such as friction, lubrication and earthquake, therefore has attracted tremendous research interest over past decades. However, despite the fundamental and practical importance, the physical origin of this slip-to-failure transition is still poorly understood. By tracking the behavior of a single shear band, here we discover that the final fracture of various MGs during compression is triggered as the velocity of the dominant shear band rises to a critical value, the magnitude of which is independent of alloy composition, sample size, strain rate and testing frame stiffness. The critical shear band velocity is rationalized with the continuum theory of liquid instability, physically originating from a shear-induced cavitation process inside the shear band. Our current finding sheds a quantitative insight into deformation and fracture in disordered solids and, more importantly, is useful to the design of plastic/tough MG-based materials and structures.

The material-dependence of plasticity in metallic glasses: An origin from shear band thermology

The material origin of plasticity in metallic glasses is a puzzled question for decades. In this work, we tackled this question from a perspective of excessive heating. With a simultaneous heat generation and dissipation model to describe the spatial and temporal distribution of temperature-rise in shear bands, an upper bound of temperature-rise of about tens of Kelvins was noticed for the reported stable shear banding. These results implied the fracture of metallic glasses being triggered at some critical temperature-rise. Based on the thermal property data of a series of bulk metallic glasses (BMGs) with different plasticity, a trend of low temperature-rise in large plasticity BMGs was revealed. A new material-dependent parameter based on thermal properties was then suggested to estimate or predict the plasticity in metallic glasses. This study provides a reasonable physical picture to understand the origin of BMGs' fracture and their different plasticity, which sheds a light to the understanding of the complex mechanical properties of these materials.

An index of shear banding susceptibility of metallic glasses

By virtue of instability analysis, an index of shear banding susceptibility of metallic glasses (MGs): the critical strain localization factor to initiate an embryonic shear band:χ=γb/γm (γb shear strain in the embryonic shear band and γm in the matrix) is derived in analogue to the emergence of geometrically necessary dislocations (GND) in crystals. Physical properties, like Poisson ratioυ, Young's modulus E and the surface energy density ζ, are explicitly incorporated into the formulae of χ with which the origin of the plasticity of MGs are unraveled.

Size Effect Suppresses Brittle Failure in Hollow Cu60Zr40 Metallic Glass Nanolattices Deformed at Cryogenic Temperatures.

To harness "smaller is more ductile" behavior emergent at nanoscale and to proliferate it onto materials with macroscale dimensions, we produced hollow-tube Cu60Zr40 metallic glass nanolattices with the layer thicknesses of 120, 60, and 20 nm. They exhibit unique transitions in deformation mode with tube-wall thickness and temperature. Molecular dynamics simulations and analytical models were used to interpret these unique transitions in terms of size effects on the plasticity of metallic glasses and elastic instability.

A molecular dynamics analysis of internal friction effects on the plasticity of Zr65Cu35 metallic glass

Abstract The effects of internal friction (IF) on Zr 65 Cu 35 metallic glass plasticity are investigated through molecular dynamics simulations. Results show that the Voronoi polyhedron 〈0, 3, 6, 3〉 increases as IF increases, thereby effectively inhibiting localized deformation and improving metallic glass plasticity. The simulations allow reproduction of images of IF evolution in metallic glasses subjected to isothermal annealing at 730 K and 850 K respectively, which can help explain the experimental observations. IF could be adjusted by selecting suitable annealing temperatures and cooling rates. The results of this work provide a strong foundation for future metallic glass designs.

Progressive shear band propagation in metallic glasses under compression

Shear band plays a key role in dominating the strength and plasticity of metallic glasses, and exhibits two kinds of propagation modes under compression, i.e., progressive and simultaneous propagation. These two different propagation modes of shear bands lie in different stages of plastic deformation. Prior to macroscopic yielding, shear bands have already been initiated yet not penetrated through the sample. These inserting shear bands exhibit linearly decreasing plastic strain from the end to tip, demonstrating a progressive propagation mode. Once the macroscopic yielding occurs, the major shear band fully transects the sample and propagates in a simultaneous sliding manner. The progressive propagation of shear bands causes an apparent work-hardening behavior, which can be well explained by assuming a higher critical stress for shear band initiation than propagation. The results demonstrate that metallic glasses with a smaller difference between critical conditions for initiation and propagation of shear band should have better plastic deformability, which can be reflected by the plastic strain to macroscopic yielding read from stress–strain curves.

Shear‐Band Dynamics in Metallic Glasses

The future of metallic glasses as an advanced structural and functional material will to a great extent depend on the understanding and control of their mesoscopic flow defects called shear bands. These defects are sweet‐and‐sour; sweet because they mediate macroscopic plasticity at room temperature, and sour because they quickly promote failure. In the past decade, fundamental research generated great progress in characterizing the role that shear bands play during plastic deformation of disordered systems, including metallic glasses. Similar to those in many other materials, shear bands in metallic glasses are only active for a very short time, which directed research focus towards topological, structural, chemical, and thermal properties of formed, but inactive shear bands. In this paper, recent progress in directly characterizing the shear‐band dynamics in situ during straining experiments is presented. Various shear‐banding stages are outlined, including formation, propagation, and arrest, as well as shear‐band creep and aging. The results are discussed in a more general context of disordered materials, concluding with a summarizing overview of time‐scales involved in shear banding, and describing future research directions that may lead to controlled shear‐band plasticity in metallic glasses.

Composition effect on intrinsic plasticity or brittleness in metallic glasses.

The high plasticity of metallic glasses is highly desirable for a wide range of novel engineering applications. However, the physical origin of the ductile/brittle behaviour of metallic glasses with various compositions and thermal histories has not been fully clarified. Here we have found that metallic glasses with compositions at or near intermetallic compounds, in contrast to the ones at or near eutectics, are extremely ductile and also insensitive to annealing-induced embrittlement. We have also proposed a close correlation between the element distribution features and the plasticity of metallic glasses by tracing the evolutions of the element distribution rearrangement and the corresponding potential energy change within the sliding shear band. These novel results provide useful and universal guidelines to search for new ductile metallic glasses at or near the intermetallic compound compositions in a number of glass-forming alloy systems.

Structural perspectives on the elastic and mechanical properties of metallic glasses

The flow units of glasses are generally perceived to be a local rearrangement of atoms that are microstructural origin for plastic deformation and relaxations in metallic glasses. We find a relationship of the effective concentration of flow units and some properties such as elastic moduli, micro-hardness and plasticity of metallic glasses. The relationship helps in understanding the softening phenomenon, structural heterogeneous, evolution process of flow units, and widespread mechanical behavior of metallic glasses and can reveal the essential structural mechanism of the Poisson's ratio criterion for plasticity in metallic glasses. The relationship also indicates that the flow unit is a key structural parameter for understanding and controlling the properties and the performance of metallic glasses.

Correction: Corrigendum: Critical fictive temperature for plasticity in metallic glasses

Nature Communications 4: Article number: 1536 (2013); Published: 26 February 2013; Updated: 17 October 2013. The funding for this Article was not fully acknowledged. The Acknowledgements should have read: This work was funded by DOE, Office of Basics Energy Sciences through DE SC 0004889, and by theNational Science Foundation through MRSEC DMR-1119826.

Enhancing the plasticity of metallic glasses: Shear band formation, nanocomposites and nanoglasses investigated by molecular dynamics simulations

We investigate the influence of various microstructural features on the deformation behavior of binary Cu64Zr36 glasses by molecular dynamics computer simulations and discuss how and why the very same modifications established for enhancing the strengths of crystalline materials, namely the insertion of solutes, precipitates and grain boundaries, can be used for tuning the mechanical properties of metallic glasses. First, by testing bulk samples with and without open surfaces under tensile load, we show that the condensation of shear transformation zones into shear bands can occur as heterogeneous but also as a homogeneous nucleation process. Then, the influence of crystalline nanoprecipitates on shear band nucleation and propagation is investigated. Finally, we study the effect of grain size and composition on the deformation behavior of nanoglasses and nanoglass composites. The results reveal that glass–glass interfaces act as structural heterogeneities ,which promote shear band formation and prevent strain localization.

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.

Critical fictive temperature for plasticity in metallic glasses

A long-sought goal in metallic glasses is to impart ductility without conceding their strength and elastic limit. The rational design of tough metallic glasses, however, remains challenging because of the inability of existing theories to capture the correlation between plasticity, composition and processing for a wide range of glass-forming alloys. Here we propose a phenomenological criterion based on a critical fictive temperature, Tfc, which can rationalize the effect of composition, cooling rate and annealing on room-temperature plasticity of metallic glasses. Such criterion helps in understanding the widespread mechanical behaviour of metallic glasses and reveals alloy-specific preparation conditions to circumvent brittleness.

Tensile fracture dynamics and intrinsic plasticity of metallic glasses

Under tensile tests of metallic glasses (MGs) subjected to annealing below glass transition temperature Tg, ductile-to-brittle transition (DBT) occurs due to structural relaxation, which results in more ordered atomic packing and decrease of glass fraction φ. DBT is observed simultaneously with fracture mechanism transition: shear banding to cracking. All MG samples annealed under different temperature were also restricted to shear banding and cracking separately under small-aspect-ratio compression and compact tension avoiding DBT. Experimental results prove that as annealing temperature increases (or glass fraction φ decreases), strength for shear banding increases, while strength for cracking decreases; as φ becomes less than critical state φDBT, MG samples tend to cracking instead of shear banding. So, φDBT is proposed as an important parameter to characterize the intrinsic plasticity of various MGs and to conform to the previous factors soundly.

Towards more uniform deformation in metallic glasses: The role of Poisson's ratio

We develop a quantitative analysis of how the plastic deformation in a metallic glass is more uniform if its Poisson ratio ν is higher. The plasticity of metallic glasses under ambient conditions is mediated by shear localized in thin bands, and can be characterized by experiments on the bending of thin plates. We extend the analysis by Conner et al. (Conner et al., J. Appl. Phys. 94 (2003), 904–911) of bands in bent plates to include the micromechanics of individual shear bands. Expressions are derived for the shear-band spacing and the offset on each band. Both these quantities are predicted to decrease as ν is increased. The predictions are tested against measurements on metallic glasses with a wide range of ν. Good agreement is found, supporting the new model for the shear-band spacing, and pointing the way towards more diffuse deformation, and consequently improved plasticity and toughness, of metallic glasses as ν increases toward the limiting value of 0.5.

Atomistic mechanisms of cyclic hardening in metallic glass

Molecular dynamics with an embedded-atom method potential is used to simulate the nanoindentation of Cu63.5Zr36.5 metallic glasses. In particular, the effects of cyclic loading within the nominal elastic range on the overall strength and plasticity of metallic glass are studied. The simulated results are in line with the characteristics of experimentally observed hardening effects. In addition, analysis based on local von Mises strain suggests that the hardening is induced by confined microplasticity and stiffening in regions of the originally preferred yielding path, requiring a higher applied load to trigger a secondary one.

Inherent parameters governing ductile-brittle transition in metallic glasses

The ductile-brittle transition and its mechanism are the hot research topics in the field of material and mechanics. In recent hundred years, the critical conditions for cracking and the governing parameters on ductility have developed for crystalline solids. Metallic glasses (MGs) as an emerging class of structural materials in recent years, show a broad potential applications due to their excellent mechanical properties. However, the low plasticity caused by highly strain localization greatly restrains them from engineering application. Therefore, to clarify the plastic behavior and the ductile-brittle transition in MGs becomes significant. Based on the classical theories, researchers have developed some important conditions to predict the plasticity in MGs, by taking their inherent properties into account. These theories usually include single or two governing parameters. In this paper, a brief review is carried out on these previous work, and further the underlying correlations between these governing parameters and the physics behind the ductile-brittle transition are revealed.

Effect of cooling rate on the bending plasticity of Zr55Al10Ni5Cu30 bulk metallic glass

The effect of cooling rate on the bending plasticity of Zr55Al10Ni5Cu30 bulk metallic glass was investigated by varying the thickness of the as-cast sample. It turned out that as the thickness of the as-cast sample increases from 1 to 3mm, short-range ordering in the microstructures is enhanced until noticeable nanocrystallization occurs in the thickest sample, but the bending plasticity measured under the same test specification first decreases and then increases. It is suggested that both free volume and structural heterogeneity dominate the plasticity of metallic glasses.

Pressure Sensitivity of Plasticity in Metallic Glasses below Glass Transition: A Literature Review

This paper deals with the pressure dependence of plasticity in metallic glasses below glass transition. Recent results indicate that some metallic glasses have such a dependence and that it increases with temperature (Keryvin et al., Phil. Mag., 88, 1773, 2008). We investigate the possibility that such a situation could be a common feature for all metallic glasses by performing a literature review. Results indicate that it is not strightforward to draw decisive conclusions.