Plasticity Of Glasses Research Articles

Correlation between supercooled liquid relaxation and glass Poisson's ratio.

We report on a correlation between the supercooled liquid (SL) relaxation and glass Poisson's ratio (v) by comparing the activation energy ratio (r) of the α and the slow β relaxations and the v values for both metallic and nonmetallic glasses. Poisson's ratio v generally increases with an increase in the ratio r and this relation can be described by the empirical function v = 0.5 - A*exp(-B*r), where A and B are constants. This correlation might imply that glass plasticity is associated with the competition between the α and the slow β relaxations in SLs. The underlying physics of this correlation lies in the heredity of the structural heterogeneity from liquid to glass. This work gives insights into both the microscopic mechanism of glass deformation through the SL dynamics and the complex structural evolution during liquid-glass transition.

Impact of micro-alloying on the plasticity of Pd-based bulk metallic glasses

Micro-alloying was performed using additions of Co and Fe to monolithic Pd40Ni40P20 bulk metallic glass. Compression tests showed a plastic strain of 13% for the Co addition (1at.%), whereas the Fe addition (0.6at.%) led to immediate failure after reaching the elastic limit. Plasticity is not reflected by the high Poisson’s ratio of 0.4 since it remained unaffected by minor alloying. Applying the fictive temperature concept to analyze the impact of minor alloying suggests that the total amount of free volume frozen-in during vitrification is less important for the mechanical properties of bulk metallic glasses than its local distribution.

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.

Evaluate the effect of laser shock peening on plasticity of Zr-based bulk metallic glass

Abstract The effect of laser shock peening (LSP) on mechanical properties, especially the plasticity of a typical Zr-based bulk metallic glass (BMG) had been investigated. The surface treatment introduced by successive laser pulses facilitated the formation of shear bands and promoted the plastic deformation of BMG. Experimental investigations of the bending deformation behavior were performed by three-point bending test at room temperature. It was found that the LSP pretreatment could improve the plasticity. The shear bands in fracture surface of laser peened sample were increased which could be observed by SEM. Logarithmic strain and stress distributions during bending deformation were predicted by finite element analysis. The high level of strain concentration in peened sample represented the increasing possibility of shear band formation which was beneficial to enhance plasticity. Corresponding with the strain distribution, the stress in peened sample was also increased which would enhance the ability of resisting a larger bending load force. Besides, simulation results clearly showed the logarithmic strain evolution of the beams with different laser irradiated regions throughout the bending process. The results indicated that the larger the laser irradiated region was, the better the plasticity of BMG would be.

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.

Hierarchical amorphous structures in a Zr50Cu42Al8 bulk metallic glass

The mechanism for the room-temperature plasticity of bulk metallic glasses (BMGs), in most cases adopting the microscopic point of view, has been extensively studied. However, less attention has been paid to the macro-structural heterogeneity of BMGs. In this work, a hierarchical amorphous structure, from the full amorphous surface to the phase-separated central region, has been observed in a Zr50Cu42Al8 glassy rod, displaying excellent room-temperature plasticity under compression. The formation mechanism for the hierarchical structure and its effect on the deformation behavior of the glassy rod have also been investigated.

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.

Plasticity, crack initiation and defect resistance in alkali-borosilicate glasses: From normal to anomalous behavior

We provide a comprehensive description of the defect tolerance of sodium-borosilicate glasses upon sharp contact loading. This is motivated by the key role which is taken by this particular glass system in a wide variety of applications, ranging from electronic substrates, display covers and substrates for biomedical imaging and sensing to, e.g., radioactive waste vitrification. The present report covers the mechanical properties of glasses in the Na2O–B2O3–SiO2 ternary over the broad range of compositions from pure SiO2 to binary sodium-borates, and crossing the regions of various commercially relevant specialty borosilicate glasses, such as the multi-component Duran-, Pyrex- and BK7-type compositions and typical soda-lime silicate glasses, which are also included in this study. In terms of structure, the considered glasses may be separated into two groups, that is, one series which contains only bridging oxygen atoms, and another series which is designed with an increasing number of non-bridging oxygen ions. Elastic moduli, Poisson ratio, hardness as well as creep and crack resistance were evaluated, as well as the contribution of densification to the overall amount of indentation deformation. Correlations between the mechanical properties and structural characteristics of near- and mid-range order are discussed, from which we obtain a mechanistic view at the molecular reactions which govern the overall deformation reaction and, ultimately, contact cracking.

Microstructure evolution and mechanical properties of Nb-alloyed Cu-based bulk metallic glasses and composites

Abstract This paper reports the microstructure evolution of Cu50.2 Zr40.8Ti9−xNbx (x = 0.5, 1.0, and 2.0 at.%) bulk metallic glass and bulk metallic glass composites accompanied with the addition of Nb and the corresponding mechanical properties. The X-ray diffraction and characterization of microstructures demonstrate that the microstructures of as-cast alloys undergo a composite-amorphous evolution. DSC analysis indicates that the glass-forming ability of as-cast alloys increases with addition of Nb. The microstructure evolution can be contributed to the combination of the stabilization of Nb on precipitated crystalline phases and cooling time. 1.0 at.% Nb-alloyed sample has the best plasticity (15.1%) and the highest fracture strength (2205 MPa) among three as-cast alloys. This work suggests that the uniformly dispersed tiny crystalline phases in glassy matrix can enhance the plasticity of bulk metallic glasses.

Enhanced plasticity of silica glass at high pressure

We have measured shear flow and residual structural anisotropy of $\mathrm{Si}{\mathrm{O}}_{2}$ glass uniaxially compressed in a diamond-anvil cell at room temperature with in situ optical-microscope and ex situ x-ray diffraction techniques, respectively. Large shear flow began at $8\ensuremath{-}10\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ and continued at least up to 20 GPa, where the macroscopic differential strain reached 70%. Recovered samples after shear flow were in the fully densified state and showed a large microscopic differential strain of 3% only in the intermediate-range network structure. These phenomena may be attributable to the changes in the Si-O bond covalency and the Si-O-Si bond angle with pressure and stresses.

PHOTOINDUCED CHANGES IN MICROHARDNES OF AMORPHOUS THIN FILMS OF THE Ge-As-Se SYSTEM

The results of experimental studies of the kinetics of photoinduced changes in microhardness of amorphous films Ge х As у Se 100-х-у under the irradiation with a laser of 50 mW power and 655 nm wavelength are presented. It is shown that the intense (10-27%) photoinduced changes of hardness of the films are observed for 25-30 minutes exposure: when the irradiation is enabled the microhardness decreases exponentially, and when it's disabled - increases exponentially. The value of photoinduced changes of microhardness depends on the average coordination number Z and has maximum at Z = 2.8. Minimum of the photoinduced microhardness changes in the topological structural 2D-3D transition at Z = 2.67 was observed. Giant growth of photoinduced changes of microhardness at Z > 2.67 is explained within the intramolecular structural model of photoinduced plasticity in chalcogenide glasses.

Effects of Aspect Ratio on the Shear Band Arrangements of Zr-Based Metallic Glasses

Effects of aspect ratios of Zr-based metallic glasses on the shear band arrangements are investigated through molecular dynamics simulations and experiments. It is found that as the aspect ratio decreases, the dense multiple shear bands form, effectively depressing the formation of the penetrating shear bands, which improves plasticity and strength of metallic glass. Simulation reproduces the images of the evolution of the shear bands in metallic glass, explaining the experimental observation. It is found that as the aspect ratio decreases, shear transformation zones disperse evenly in the entire model, restraining connecting into penetrating shear bands.

Mechanical properties and plasticity of a model glass loaded under stress control.

Much of the progress achieved in understanding plasticity and failure in amorphous solids had been achieved using experiments and simulations in which the materials were loaded using strain control. There is paucity of results under stress control. Here we present a method that was carefully geared to allow loading under stress control either at T=0 or at any other temperature, using Monte Carlo techniques. The method is applied to a model-perfect crystalline solid, to a crystalline solid contaminated with topological defects, and to a generic glass. The highest yield stress belongs to the crystal, the lowest to the crystal with a few defects, with the glass in between. Although the glass is more disordered than the crystal with a few defects, it yields stress much higher than that of the latter. We explain this fact by considering the actual microscopic interactions that are typical of glass-forming materials, pointing out the reasons for the higher cohesive nature of the glass. The main conclusion of this paper is that the instabilities encountered in stress-control condition are the identical saddle-node bifurcation seen in strain control. Accordingly one can use the latter condition to infer the former. Finally we discuss temperature effects and comment on the time needed to see a stress-controlled material failure.

Enhancement of plasticity in Zr-based bulk metallic glasses electroplated with copper coatings

Electrodeposition was used to coat copper films on the surface of the BMG pillars (bulk metallic glasses) of Zr52.5Cu17.9Ni14.6Al10Ti5 (Vit. 105) with the film thicknesses of 71.5 and 161.1 μm. The experimental results of the compression tests of the bare Vit. 105 pillars and the coated Vit. 105 pillars revealed that the copper costing increased the density of shear bands in the Vit. 105 pillars formed during the tests, resulting in the improvement of plasticity. The plastic strain was 6.1% for the coated pillars with a coating thickness of 161.1 μm, which is 3.59 times of 1.7% of the bare Vit. 105 pillars. The deformation of the copper films dissipated the strain energy and limited the propagation of shear bands, which led to the initiation and formation of multiple shear bands. The technique developed in this work provides an effective way to enhance the plasticity of BMGs at room temperature.

Elasticity and plasticity in stiff and flexible oligomeric glasses.

In this paper we focus on the mechanical properties of oligomeric glasses (waxes), employing a microscopic model that provides, via numerical simulations, information about the shear modulus of such materials, the failure mechanism via plastic instabilities, and the geometric responses of the oligomers themselves to a mechanical load. We present a microscopic theory that explains the numerically observed phenomena, including an exact theory of the shear modulus and of the plastic instabilities, both local and system spanning. In addition we present a model to explain the geometric changes in the oligomeric chains under increasing strains.

Improved plasticity of bulk metallic glasses by electrodeposition

Zr52.5Cu17.9Ni14.6Al10Ti5 (Vit 105) bulk metallic glasses (BMGs) are coated with Ni films, costing different time for coating. The plasticity of coated Vit 105 BMGs is greatly improved. Experimental results reveal that coating thickness of Ni film is responsible for modifying the mechanical properties of BMG. For compressive tests, the corresponding plastic deformation of 3 hours-coating sample is 9%, which improves 7% than the uncoated sample. However, the compressive plasticity is reduced by 6 hours-coating with the thicker (100μm) film. For bending tests, bending strength and fracture deflection are increasingly improved with the coating thickness of Ni-layer. The deformation mechanisms are investigated in detail.

Flow unit perspective on room temperature homogeneous plastic deformation in metallic glasses.

A mandrel winding method, which can realize remarkable homogeneous plastic deformation at room temperature for various metallic glasses, is applied to characterize plastic flow units and study their relationship with macroscopic deformations and relaxations. The method can provide information on the activation energy, activation time, size, intrinsic relaxation time, distribution, and density of flow units. We find the plasticity of a metallic glass can be controlled through modulating the features of flow units. The results have benefits for better understanding the structural origins of deformations and relaxations, and for designing metallic glasses with improved performances.

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.

Linking high- and low-temperature plasticity in bulk metallic glasses II: use of a log-normal barrier energy distribution and a mean-field description of high-temperature plasticity

A thermal activation model to describe the plasticity of bulk metallic glasses (Derlet and Maaß, Phil. Mag. 93 (2013) p.4232) which uses a distribution of barrier energies and some aspects of under-cooled liquid physics is developed further. In particular, a log-normal distribution is now employed to describe the statistics of barrier energies. A high-temperature mean-field description of homogeneous macroplasticity is then developed and is shown to be similar to a thermal activation picture employing a single characteristic activation energy and activation volume. In making this comparison, the activation volume is interpreted as being proportional to the average mean-square-value of the plastic shear strain magnitude within the material. Also, the kinetic fragility at the glass transition temperature is shown to represent the effective number of irreversible structural transformations available at that temperature.