Redistribution Of Free Volume Research Articles

Hidden shear bands of diversified structures in a bent heterogeneous metallic glass

It is highly disputed about the microstructure and evolution of shear bands (SBs) in metallic glasses (MGs) which affect their yielding and plasticity. We reconstruct the nanostructure of SBs in a bent heterogeneous Pd-based MG fiber with synchrotron X-ray nano-computed tomography and perform finite element analysis on their formation and evolution. It is found that apart from plain SBs (PSBs) under small shear offsets, a large amount of hidden SBs (HSBs) of density fluctuations appear inside the MG, which do not induce shear offsets. The average densities of the HSBs are ∼5%–∼25% smaller than the matrix around, and their thicknesses range from ∼100 nm to ∼530 nm. The diversified structures of the HSBs closely relate to the MG's multiscale structural heterogeneity and local strain rate. The HSBs can develop into PSBs that are densified near surface caused by structural relaxation under high temperatures, or be smeared by free volume redistribution in the low-density central area. The results could further the understanding of the formation and evolution of SBs' microstructure in heterogeneous glassy materials.

Atomic Force Microscopy Studies of Severely Deformed Amorphous TiNiCu Alloy

The amorphous Ti50Ni25Cu25 alloy was subjected to high pressure torsion (HPT) processing. TEM studies revealed in the structure of the HPT-processed samples the presence of nanocrystals with a size of about 5 nm and amorphous clusters with a size of about 10-30 nm. Atomic force microscopy (AFM) was used to study the surface morphology of foils prepared by ion polishing from the initial amorphous ribbons and HPT-processed samples. AFM images of the foil prepared from the initial ribbon revealed a smooth surface with an average roughness of 0.3 nm. A totally different surface morphology was observed for the foil prepared from the HPT-processed state by the same regime of ion polishing: the presence of holes with a depth of 2-4 nm and a width of 10-30 nm. The changes in the surface morphology, namely the holes-like surface morphology of the HPT-processed state, could be explained by a complex transformation of the amorphous structure, and probably by the variation and redistribution of free volume, which leads to the emergence of a cluster contrast in TEM images.

Influence of grain size and composition, topology and excess free volume on the deformation behavior of Cu-Zr nanoglasses.

The influence of grain size and composition on the mechanical properties of Cu–Zr nanoglasses (NGs) is investigated by molecular dynamics simulations using two model glasses of different alloy composition, namely Cu64Zr36 (Cu-rich) and Cu36Zr64 (Zr-rich). When the grain size is increased, or the fraction of interfaces in these NGs is decreased, we find a transition from a homogeneous to an inhomogeneous plastic deformation, because the softer interfaces are promoting the formation shear transformation zones. In case of the Cu-rich system, shear localization at the interfaces is most pronounced, since both the topological order and free volume content of the interfaces are very different from the bulk phase. After thermal treatment the redistribution of free volume leads to a more homogenous deformation behavior. The deformation behavior of the softer Zr-rich nanoglass, in contrast, is only weakly affected by the presence of glass–glass interfaces, since the interfaces don’t show topological disorder. Our results provide clear evidence that the mechanical properties of metallic NGs can be systematically tuned by controlling the size and the chemical composition of the glassy nanograins.

Effect of heat treatment on diffusion, internal friction, microstructure and mechanical properties of ultra-fine-grained nickel severely deformed by equal-channel angular pressing

Severe plastic deformation via equal-channel angular pressing was shown to induce characteristic ultra-fast diffusion paths in Ni (Divinski et al., 2011). The effect of heat treatment on these paths, which were found to be represented by deformation-modified general high-angle grain boundaries (GBs), is investigated by accurate radiotracer self-diffusion measurements applying the 63Ni isotope. Redistribution of free volume and segregation of residual impurities caused by the heat treatment triggers relaxation of the diffusion paths. A correlation between the GB diffusion kinetics, internal friction, microstructure evolution and microhardness changes is established and analyzed in detail. A phenomenological model of diffusion enhancement in deformation-modified GBs is proposed.

Effect of free volume redistribution on the diffusivity of water and benzene in poly(vinyl alcohol)

We used molecular dynamics simulation to study the diffusion of two selected penetrants, water and benzene, in poly(vinyl alcohol) (PVA) over a wide range of temperatures and concentrations to gain insights into their diffusion mechanisms under different conditions. To help understand the effect of free volume on the diffusion behavior of water and benzene, we used the technique of Voronoi tessellation to determine key characteristics of free volume redistribution. Unlike the Connolly surface analysis method, Voronoi tessellation yields information not only on the mean fractional free volume but also on the size distribution of the free volume surrounding each penetrant molecule and its time evolution. In the case of water, we showed that it was the free volume redistribution frequency that led to an observation, previously reported in the literature, that the self-diffusion coefficient of water increases with increasing water concentration despite the fact that increase in water concentration decreases the mean fractional free volume in PVA. In the case of benzene (non-polar) diffusing in PVA (polar), we demonstrated that the failure of the Macki–Meares model was not entirely due to the dissimilarity between the intermolecular interactions. Rather, one of the reasons was the inability of the benzene molecules to break the hydrogen bonds between the PVA chains which are essential to increase the polymer segments mobility. Higher polymer segmental mobility is needed to create larger fluctuations in free volume holes for benzene to diffuse. This would in turn prevent the benzene molecules, with a relatively larger size compared to water molecules, from diffusing by a fluid-like streaming mechanism that is required by the model. In fact, the Macki–Meares model also failed for water at low temperatures and concentrations, as water diffused by a hopping mechanism under such conditions.

Free volume fragmentation in glassy chalcogenides during natural physical ageing as probed by PAL spectroscopy

Free volume redistribution, caused by structural relaxation during long-term natural physical ageing, is revealed by positron annihilation lifetime technique for AsxSe100-x and GeySe100-y glasses. It is shown, that two-state positron trapping model is appropriate for an adequate description of changes caused by long-term physical ageing in the atomic-deficient subsystem of these materials. The observed behaviour of defect-related component in the fit of the experimentally measured PAL spectra for aged and rejuvenated glasses testifies in favour of fragmentation of larger free volume entities.

Low‐Temperature Annealing Induced Amorphization in Nanocrystalline NiW Alloy Films

Annealing induced amorphization in sputtered glass‐forming thin films was generally observed in the supercooled liquid region. Based on X‐ray diffraction and transmission electron microscope (TEM) analysis, however, here, we demonstrate that nearly full amorphization could occur in nanocrystalline (NC) sputtered NiW alloy films annealed at relatively low temperature. Whilst the supersaturation of W content caused by the formation of Ni4W phase played a crucial role in the amorphization process of NiW alloy films annealed at 473 K for 30 min, nearly full amorphization occurred upon further annealing of the film for 60 min. The redistribution of free volume from amorphous regions into crystalline regions was proposed as the possible mechanism underlying the nearly full amorphization observed in NiW alloys.

Ultra-fast diffusion channels in pure Ni severely deformed by equal-channel angular pressing

Grain boundary self-diffusion in Ni severely deformed by equal-channel angular pressing has been investigated. The radiotracer technique was applied by using the 63Ni isotope and high-precision mechanical grinding. Ultra-fast diffusion rates, which exceed those along general high-angle grain boundaries in annealed coarse-grained Ni by orders of magnitude, were observed. Such high diffusivities were attributed to a non-equilibrium state of grain boundaries produced by severe plastic deformation. A significant downward deviation from an Arrhenius plot for ultra-fast diffusivity for temperatures above 400 K was found. This was interpreted in terms of a partial relaxation of the non-equilibrium grain boundary structure. Additionally, the formation of ultra-fast diffusion channels, presumably in the form of percolating porosity as a result of stepwise annealing of the material, was observed. This phenomenon is discussed in terms of the free volume redistribution in grain boundaries. It is proposed to quantify the excess energy of interfaces in terms of the observed enhancement of diffusivity. This approach allows the excess energy of non-equilibrium grain boundaries to be determined from diffusion measurements.

Multiscale simulations on the grain growth process in nanostructured materials

Abstract In this work, multi-phase field and molecular dynamics simulations have been used to investigate nanoscale grain growth mechanisms. Based on experimental observations, the combination of grain boundary expansion and vacancy diffusion has been considered in the multi-phase field model. The atomistic mechanism of boundary movement and the free volume redistribution during the growth process have been investigated using molecular dynamics simulations. According to the multi-phase field results, linear grain growth in nanostructured materials at low temperature can be explained by vacancy diffusion in the stress field around the grain boundaries. Molecular dynamics simulations confirm the observation of linear grain growth for nanometresized grains. The activation energy of grain boundary motion in this regime has been determined to be of the order of onetenth of the self-diffusion activation energy, which is consistent with experimental data. Based on the simulation results, the transition from linear to normal grain growth is discussed in detail and a criterion for this transition is proposed.

Molecular dynamics of shear transformation zones in metallic glasses

Molecular dynamics simulations have been employed to investigate the dynamics of atomic rearrangements in Ni 50Zr 50 amorphous alloys sheared at different pressures. Local atomic dynamics has been characterized in terms of number of displacing atoms as well as of apparent activation volume and energy. The dimensionality of rearranging atom clusters has been also evaluated. Numerical findings, although not conclusive, indicate that atomic rearrangements are probably triggered by a free volume redistribution. Highly localized and cooperative, such plastic rearrangements occur in regions tending to a two-dimensional occupancy of space.

Positron annihilation lifetime response and relaxation dynamics in glycerol

Positron annihilation lifetime spectroscopy (PALS) data on glycerol are related to the results of broadband dielectric studies from the literature. Specifically, the temperature variation of the ortho-positronium ( o-Ps) lifetime, τ 3, and of the relative intensity, I 3, from 100 K up to 300 K have been compared with the spectral features of the α and β relaxation from the decomposed dielectric loss spectra. First, it is found that in the τ 3 vs. temperature plot, in addition to the universal characteristic PALS temperatures: T g PALS and T b 2 L = 1.53 T g PALS where large discontinuities in slope are observed and which are associated with the glass–liquid transition and the onset of a quasi-plateau level, respectively, a discontinuity in the slope occurs at T b 1 L = 1.28 T g PALS . Further, it is found that τ 3 at T b 1 L is approximately the same as the mean secondary β relaxation time, τ 3 ( T b 1 L ) ≈ τ β ( T b 1 L ) . Next, it is suggested that the decrease in the I 3 vs. T plot as temperature increases above T g PALS can be related to the β and α relaxations qualitatively. These findings are consistent with the notion that free volume redistribution in liquid glycerol is due to the local β process as well as the high-frequency part of cooperative α process causing a partial elimination of smaller holes from the PALS spectrum as temperature increases. An alternative or perhaps additional explanation is that as temperature increases, some of the small, detectable holes break up into holes too small for the PALS measurement.

Investigations on conductivity anomalies in Li 2O·4MnO 1+ x·4B 2O 3 glasses

Glassy material Li 2O·4MnO 1+ x ·4B 2O 3 (0< x<0.5) has been prepared by quenching the melt in air, and its electrical conduction behavior has been studied. Two conductivity anomalies are observed around the glass transition temperature. One appears in the dependence of the isothermal conductivity vs. the annealing time. This anomaly is attributed to the oxidation of Mn 2+ by oxygen ions penetrating into the glass matrix, leading to the increase of the charge carrier density. The other anomaly is observed in the dependence of the conductivity vs. the temperature for different times of annealing. The abnormal increase and decrease of the conductivity vs. annealing time might result from a complicated structural variation in the glass. Below 200°C, the conductivity obeys an Arrhenius relation. Above 350°C, the formation of crystalline MnBO 3 causes a conductivity decrease by one order of magnitude. The conductivity is enhanced by the redistribution of free volume between these two temperatures.

Generalized model of holographic recording in photopolymer materials

The generalized diffusion model of holographic recording in photopolymer materials has been offered. The theoretical description of hologram formation process is based on the concept of free volume redistribution and using the generalized diffusion equation. Free volume is produced in photopolymer medium due to the polymer shrinkage effect. The developed theory allows to take into account influence of inhomogeneous monomer distribution during recording process and shrinkage rate on a kinetics of a hologram formation. The principal influence of the diffusion/polymerization rates ratio on the hologram properties has been shown. The offered model allows also to describe relief formation process on the surface of a recording layer.

Elastic moduli, density, and structural relaxation in bulk amorphous Zr 41.2Ti 13.8Cu 12.5Ni 10Be 22.5 alloy

The thermal expansion and elastic properties of bulk amorphous Zr 41.2Ti 13.8Cu 12.5Ni 10Be 22.5 alloy are reported. The thermal expansion was measured between 80 K and 773 K, which includes the glass transition and crystallization temperatures. The elastic moduli were measured at room temperature using a novel resonant ultrasonic spectroscopy technique in samples annealed at increasing temperatures between 300 K to 773 K. The thermal expansion data were used to deduce the Debye temperature Θ D, and the Grüneisen parameter, γ. The changes in elastic properties are discussed in terms of the annihilation and redistribution of free volume in the bulk amorphous state.

Light scattering and dielectric studies on glass forming liquids

The α-relaxation behaviour of polymers and glass forming viscous liquids is well described by the Vogel-Fulcher-Tammann equation, introducing a Vogel temperature T 0 at which the relaxation time τ α diverges ( T 0 ≈ T g - 50 K). With the recent development of the mode coupling theory a new critical temperature T c is introduced located about 50 to 80 K above T g. The relevance of the various temperatures is discussed on the basis of dynamic light scattering studies and dielectric relaxation data. The dynamic and static light scattering experiments revealed some unexpected features, which cannot be explained on the basis of conventional liquid state theories: (1) In static light scattering the intensity I( q→0) is no longer proportional to the isothermal compressibility. (2) This excess scattering I exc shows a strong q-dependence ( q = (4π n/λ)sin(θ/2)) corresponding to a correlation length ξ in the range of 20–200 nm. (3) The Landau-Placzek ratio I Rayleigh/2 I Brillouin is much too high, compared with the results of light scattering theories. (4) In photon correlation spectroscopy a new ultraslow hydrodynamic mode (Γ∼ q 2) is detected with relaxation rates Γ about 10 -4 to 10 -7 lower than those of the α-process at a given temperature. These effects are caused by long density fluctuations indicating a nonhomogeneous distribution of free volume. The redistribution of free volume in space causes the new ultraslow mode. A tentative model is proposed which describes the long range density fluctuations as a result of the coexistence of molecules with two different dynamic states, which show up in the Fabry-Perot and Raman spectroscopy.

Non-equilibrium microstructural and transport characteristics of glassy poly (ethylene terephthalate)

In this work the effects of penetrant-induced microstructural changes in amorphous poly (ethylene terephthalate) (PET)) are examined via CO 2 transport, Fourier transform infrared spectroscopy and differential scanning calorimetry employed in coordination. The effects of sub- T g thermal annealing are examined as well, in both as-received and pre-conditioned samples. In the CO 2 transport study, a new parameter estimation procedure is introduced which makes use of differential permeation data to obtain the relevant sorption/transport parameters embedded in the dual-mode sorption/transport model. Exposure of amorphous PET to CO 2 is found to increase the overall free volume of the sample as reflected by increases in the penetrant saturation constant, C' H, with CO 2 pressure and time of exposure. At sufficiently high penetrant activities and/or long exposure times, the sample becomes fully plasticized; this effect is also evident in the accelerated consolidation process apparent by differential permeation. From the FTIR data, changes in local conformational isomer ratios of the ethylene glycol moieties of ET chains near and below the glass transition region due to thermal effects and that resulting from CO 2 exposure are isolated and linked to changes in free volume. The elevated gauche levels resulting from conditioning treatments are paralleled by variations in C' H until full plasticization occurs. The parallel reduction in the gauche fraction and C' H resulting from thermal annealing, can, on the other hand, be ascribed to conversion of the isomer to the higher energy trans conformation. In DSC studies, while C' H continues to increase with CO 2 exposure time, the endothermic peak also increases, after passing through a minimum. This behavior is attributed to the redistribution of free volume during the heating cycle. Similar to the case of as-received films, annealing of pre-conditioned films subsequently reduces the intensity of the endothermic peak after a period of free volume readjustment.

Approach to the composition dependence of the glass transition temperature of compatible polymer blends: 1

Starting with the idea that, besides conformational energy barriers, surface contacts are responsible for both conformation and ‘free’ volume distribution, a new concept is developed to describe the glass transition in compatible polymer blends. An extended Gordon-Taylor equation results if both the effective contact probabilities of the blend components and the effect of molecular surroundings on the contact contribution to the glass transition of the blend are considered. Free volume redistribution due to surface contacts is included. The Gordon-Taylor constant K of the relation obtained is now not a fitting parameter, but is related to the ratio of the different expansion coefficients of the free volume. The relation introduces two fitting parameters, K 1 and K 2, which are related to the intensity of polymer-polymer interaction and to the effect of immediate molecular surroundings on the interaction. Data analysis suggests that these fitting parameters are not only polymer-specific, but also molecular-weight-dependent.

Ionic conductivity enhancement during pre-crystallization in amorphous lithium ionic conductor Li 2B 2O 4

It is found from the temperature dependence of the ionic conductivity of amorphous Li 2B 2O 4 that the ion transport process is rather complicated. At temperature below T k (≈310°C), the ionic conductivity obeys an Arrhenius relation. Above the crystallization temperature T c (≈410°C) ion transport is dominated by the process in the crystalline state. In between the ionic conductivity is anomalously enhanced. In this pre-crystallization range two distinguishable steps with a transition temperature at T p (≈380°C) can be identified. At temperature between T k and T p, the conductivity increase is due to the redistribution of free volume. For temperature between T p and T c the specimen contains a small amount of crystallites but the crystallinity is less than 5%. The 7Li NMR spectra show that the line shape of partially crystallized amorphous material is quite similar to that of LiCl containing DSPP but in this case the second-phase particles are crystallites of the parent material.

Development of visco-plastic deformation in metallic glasses

Inelastic deformation of metallic glasses above 0.6 T g is envisioned to be the result of uncorrelated shear transformations. In the early stages of deformation when all transformations are spatially isolated, back stresses set up in the transformation sites by the undeformed surroundings can fully reverse the deformation upon removal of the applied stress, and the deformation is purely anelastic. With larger accumulations of inelastic strain, the probability increases for transformed volume elements to acquire additional, transformed near neighbors. Interpreting experimental results with the help of a probabilistic theoretical model that is proposed has suggested that when the number of contacts between transformed near neighbor regions exceeds two, back stresses in the central transformations are lost and they become visco-plastic. Substantial relief of back stresses in two or more adjacent contacting transformed volume elements is expected to be by free volume redistribution that becomes possible when the deformed region is large enough. Based on the model it is predicted that transient inelastic shear strains in excess of 0.02 are required before steady state flow can be achieved. Finally, the model permits also the reaching of some basic conclusions on structural relaxations vis-a-vis unrestrained visco-plastic flow.

IONIC CONDUCTION DURING PRE-CRYSTALLIZATION PROCESS IN AMORPHOUS IONIC CONDUCTOR Li2B2O4

The temperature dependence of ionic conductivity, especially the ion transport property during pre-crystallization process in amorphous ionic conductor Li2B2O4 has been studied. When the temperature is lower than Tk(≈310℃), the ionic conduction obeys Arrhenius relation. Above the crystallization temperature Tc(≈411℃), the ion transport behaviour is dominated by the process in crystalline state. In the range Tkc, the ionic conductivity deviates from thermal excitation mechanism and is enhanced anomalously. This process is now called pre-crystallization process. It is found that the free volume model could be used to describe the ion transport properties. In pre-crystallization process there are two distinguishable steps. Below Tp(≈380℃) the ionic conductivity increase is due to the redistribution of free volume. While above Tp considerable enhancement of the ionic conductivity is caused by the in-terfacial effect between the crystallites and amorphous matrix because small amount of crystallites (less than 5%) were emerged. The pre-crystallization status of the amorphous could be remained at ambient condition by quenching the specimen to room temperature. Therefore it is possible to prepare material with higher ionic conductivity than that of pure amorphous.