Free Volume Generation Research Articles

Understanding the influence of free-volume generation, thermal instabilities and fracture energy on shear localization in micromachining of bulk metallic glass

Bulk metallic glasses (BMGs) are a new class of amorphous metallic alloys having enhanced mechanical and tribological properties. However, the chip formation behavior of BMGs is not as well established as that of the crystalline materials, which hinders its prospective applications. In this paper, an orthogonal micromachining experiment is conducted, and chip morphology, force signature, hardness, and enthalpy of the machined surface are analyzed. The chips show segmented morphology with distinct fractures. The chip exhibits primary shear zones along with several secondary shear bands emanating within a single segmentation, which evidenced shear localization. Additionally, the machined surface exhibits softening as compared to the undeformed material. The shear localization, responsible for chip segmentation in BMG machining, is assumed to be driven primarily by free volume and the shear zone temperature. Hence, a localized chip segmentation model has been formulated by incorporating the effects of temperature and free volume generation-induced instabilities. The proposed chip formation model accounts for the contributions of the fracture and the new surface creation energies, in addition to the shear and friction energies used traditionally. The contribution of fracture energy was observed to be significant in the chip formation of BMGs. To differentiate between the relative contributions of temperature-induced instabilities and free volume generation on shear localization, four distinct regimes were identified based on the critical value of free volume flow coefficient (FVFC) and heat flow coefficients (HFC). The findings demonstrate that if the FVFC is greater than the critical value, significant shear localization takes place. Conversely, the shear localization becomes less pronounced if the FVFC is smaller than the critical values. Thus, it can be deduced that temperature has an insignificant role in shear localization, and free volume acts as the main driver.

Leveraging molecular scale free volume generation to improve gas separation performance of carbon molecular sieve membranes

Carbon molecular sieve (CMS) membranes have proven to be promising candidates for next-generation gas separations. Modification of polymeric precursors is a critical tool that permits fine-tuning of CMS structure and performance for a wide variety of gas mixtures. Here, we make a targeted alteration to a polyimide CMS precursor through substitution of free-volume-generating trifluoromethyl groups for aliphatic methyl groups on the polymer backbone. Gas separation performance shows a vast improvement, demonstrated by up to an eightfold increase in gas permeability as well as higher mixed gas separation factors in some cases. We investigate these properties, and their dependence on pyrolysis temperature, with detailed measurements of gas sorption and permeation in CMS dense film membranes with additional analysis through classical materials characterization methods. Our observations indicate that addition of free-volume-generating groups into polymeric precursors is a powerful tool for developing state-of-the-art CMS membranes, especially in cases when high permeability is an important design parameter.

Unravelling the relation between free volume gradient and shear band deflection induced extra plasticity in metallic glasses

Previous experiments have revealed that the controllable introduction of structural gradients in metallic glasses (MGs) can endow the materials with extra plasticity due to the gradient-induced deflection of shear bands. However, the relation between the spatial structural gradient and the initiation of shear band deflection remains unclear. The current study has been focused on investigating the relationship between the improved mechanical properties of MGs and structural gradients specified by the distribution of the intrinsic free volume. Molecular dynamics (MD) simulations are firstly performed on homogeneous MG models containing various initial free volume values, showing that the shear band angle increases with decreasing free volume under uniaxial compression, whereas higher shear band angle is observed under uniaxial tension with increasing free volume. Based on the asymmetric behaviors of MGs under compression and tension, a theoretical model is developed to quantitatively characterize the influence of free volume on the mechanical response of MGs, which incorporates a failure criterion based on free volume generation during external loadings. The model can be further utilized to interpret and predict the fracture strain, shear band angle, maximum stress, and fracture surface morphology of gradient structured MGs in both simulations and experiments. The relationship between free volume gradient and shear band deflection induced extra plasticity established in this study provides valuable guidance for the structural design of MGs with enhanced mechanical properties.

Chitosan‐containing electrospun poly(ethylene oxide)‐polybutadiene‐CNT fibers

AbstractCross‐linked polymer composite fibers are gaining more and more interest in the fields of drug delivery, bone regeneration and biomineralization. This study focuses on the replacement of the radical initiator by photoinitiation via benzophenone, resulting in poly(ethylene oxide)‐polybutadiene‐carbon nanotube (PEO‐PBu‐CNT) nanofibers. The inclusion of small amounts of CNTs increases the rupture temperature and the first glass transition temperature Tg1 of the composite by 28 and 23 K, respectively, while decreasing the second glass transition temperature Tg2 by 20 K determined by dynamic mechanical analysis. A twice higher initial storage modulus of ~44 MPa can be detected for the CNT‐containing samples, indicating the stiffening effect due to CNT addition. Primary X‐ray diffraction peaks of PEO shift by 0.42° towards lower angles by the inclusion of CNTs, accounting for the generation of free volume within the polymer composite. Two orders of magnitude increase in Bode magnitude with a rise in Bode angle from 6° to 56° is detected by electrochemical impedance spectroscopy for the UV‐cured PEO‐PBu‐CNT. Entanglement between the fibers, as well as a rough surface with a rigid structure, is observed upon small CNT additions. Finally, attenuated total reflectance—Fourier transform infrared and Raman spectroscopy allow the detection of the main peaks and intensity changes of these polymer composite structures for the UV‐cured and uncured states.

Modeling and analysis of chip segmentation in micro-cutting of Zr-based bulk metallic glass (BMG)

Bulk metallic glasses (BMG) exhibit an amorphous structure and possess excellent properties, such as high hardness (∼8 GPa), high elastic and strain limit (∼2 %), and wear/corrosion resistance. Chip formation behavior of BMG during machining is distinct from the crystalline materials, and is not very well understood. Hence, this paper presents experimental and analytical investigations to explore the underlying chip formation mechanism in the micro-cutting of BMG. The experimental study was carried out by analyzing the chip morphology, force signature, and hardness of the workpiece. BMG machining yields segmented chips due to the shear localization. The shear localization is clearly observed in terms of periodic primary shear bands and multiple secondary shear bands. The machined surface also shows work softening due to machining-induced deformation. It has been observed that free volume and the shear zone temperature affect the shear localization, which is reflected in the chip segmentation. A shear localization model has been developed, which includes the effects of new surface creation energy, shear energy, and friction energy. Based on a bifurcation analysis, critical free volume and temperature flow coefficients have been calculated. Based on the critical free volume and temperature flow coefficients, four distinct regimes of shear localization have been identified. The shear localization dampens out if the free volume coefficient is less than the critical free volume flow coefficient. Substantial shear localization is observed if the free volume flow coefficient exceeds the critical value. This study shows that free volume generation primarily drives the chip segmentation and shear localization, and the temperature effects play a secondary role.

Heterogeneity of microstructures in a Cu–Zr based amorphous alloy composite reinforced by crystalline phases

Amorphous alloy composites (AACs) comprising reinforcing crystalline phases exhibit far better mechanical properties than their base amorphous alloy counterparts, and Cu–Zr based alloys consisting of B2 phase particles embedded in an amorphous matrix are well-researched AACs in this respect. Traditionally, the B2 particles in Cu–Zr based AACs may appear to be homogeneously crystalline (without any amorphous phase inside), the glassy matrix homogeneously amorphous, and their interfaces atomically sharp. Yet, such a picture would pose difficulties in understanding how crystallization can happen in the glassy matrix, since the crystallized product is denser than but iso-stoichiometric with the matrix. The present work provides insights into this puzzle, by using electron microscopy, nanoindentation and molecular dynamics (MD) simulations to investigate the structures and mechanical properties of the crystalline and amorphous phases, and their interfaces in a Cu62Zr34.5Al3Nb0.5 AAC. Surprisingly, in this AAC, the superficially crystalline particles with tens to hundreds of microns large are found to be heterogeneous at the sub-micron scale, comprising nano-crystallites embedded in a glassy matrix, and the amorphous phase outside also exhibits a gradient in free volume content towards the interface. Smaller particles containing only B2 nano-crystallites are found to be softer than the amorphous matrix, while larger particles are harder than the amorphous phase due to martensitic transformation into the B19’ phase. MD simulations indicate that the B2 phase is marginally more stable than the glassy state, and on crystallization the densification leads to free-volume generation in the surrounding glassy matrix as observed experimentally. Mechanics analysis indicates that the strain energy in the glassy matrix associated with free-volume generation scales with the volume of the crystallization, as would be the crystallization energy itself. Hence, crystallization in the AAC leading to a two-phase structure is a spontaneous but kinetically limiting process arising from the net energy difference between the crystallization energy and free-volume energy produced in the glassy matrix. This work provides a detailed understanding of the heterogeneity in the microstructures in Cu–Zr based AACs, to shed light on the crystallization of the iso-stoichiometric B2 phase from the amorphous phase.

Strain rate response and rheological characteristics of ZrCuNiAl bulk metallic glass in supercooled liquid region

In this study, the plastic deformation behavior of Zr55.7Cu22.4Ni7.2Al14.7 bulk metallic glass (BMG) in the supercooled liquid region (SLR) was studied by high temperature compression experiments at various strain rates. The results show that the high temperature plastic deformation behavior of the BMG in SLR is highly sensitive to the strain rate. With the decrease in strain rate, the steady-state flow stress and peak stress decreases, and the viscosity increases. Meanwhile the stress overshoot gradually disappears, the deformation of the BMG specimen changes from non-Newtonian flow to near-Newtonian flow, and there is a critical strain rate of 5 × 10−4 s−1. The surface deformation profile changes from an "S" shaped truncated cone to a true truncated cone. The plastic deformation of BMGs at room temperature is completed by the initiation and expansion of the shear bands, while the high temperature plastic deformation is caused by the generation and annihilation of free volume.

Crystallization Kinetics on Melt Spun and HPT-Processed Zr62Cu22Al10Fe5Dy1 Metallic Glass

The influence of conventional and accumulative HPT (Acc. HPT) process on structural properties of Zr62Cu22Al10Fe5Dy1 metallic glass (MG) phase is examined by X-ray diffraction (XRD) technique. The XRD results showed that HPT process leads to free volume generation in MG. Free volume increases with the number of anvil turns in the conventional HPT process, while the highest free volume increment was observed in Acc. HPT process. To understand the role of latent crystalline phases on free volume, thermal stability and crystallization kinetics of Zr62Cu22Al10Fe5Dy1 MG are investigated by non-isothermal heating using differential scanning calorimetry (DSC). Thermal investigation indicates Dy addition results in increased activation energy along with lower nucleation rate during crystallization. As the initial amorphous phase significantly influences crystallization behavior, d-spacing analysis of amorphous hump is carried out to predict latent crystalline phases. Strong correlation was observed between latent crystalline phases and intermetallic compounds in devitrified Zr62Cu22Al10Fe5Dy1 alloy.

Exploring frictional performance of diamond nanothread reinforced polymer composites from the atomistic simulation and density functional theory

Diamond nanothread (DNT), which has been recently found to outperform other conventional carbon-based nanomaterials, is a promising reinforcer for polymer composites. Here, we find that DNT can significantly improve the frictional resistance of polymethyl methacrylate (PMMA) composites via atomistic simulations and density functional theory (DFT) calculations. Results show that the friction coefficient of PMMA composite is reduced by 25.98% with the incorporation of DNT due to the excellent interfacial interactions including vdW interaction and mechanical interlocking. These lead to reduced cohesive energy at the Fe-PMMA interface and lower polymer mobility. The improvement of frictional resistance is more significant by nitrogen-doped DNT (by 43.72%) due to the rougher landscape of molecular electronic potential and larger binding energy, resulting in better interfacial interactions. Besides, it is found that the improvement of frictional resistance by DNT is less significant at elevated temperatures. The degrading mechanisms are attributed to the generation of free volume at the DNT-PMMA interface, which decreases the interfacial shear strength and weakens the interfacial interactions. These findings could make advances towards the understanding of physical mechanisms governing the frictional properties of polymer composites, and provides useful guidance for the design of advanced polymer composites with good service life.

Controlled gel expansion through colloid oscillation.

We model the behavior of a single colloid embedded in a cross-linked polymer gel, immersed in a viscous background fluid. External fields actuate the particle into a periodic motion, which deforms the embedding matrix and creates a local microcavity, containing the particle and any free volume created by its motion. This cavity exists only as long as the particle is actuated and, when present, reduces the local density of the material, leading to swelling. We show that the model exhibits rich resonance features, but is overall characterized by clear scaling laws at low and high driving frequencies, and a pronounced resonance at intermediate frequencies. Our model predictions suggest that both the magnitude and position of the resonance can be varied by varying the material's elastic modulus or cross-linking density, whereas the local viscosity primarily has a dampening effect. Our work implies appreciable free-volume generation is possible by dispersing a collection of colloids in the medium, even at the level of a simple superposition approximation.

Deformation-induced medium-range order changes in bulk metallic glasses

Bulk metallic glasses (BMGs) naturally have excellent strength and elasticity while structural rejuvenation into higher energy glassy states is often required to improve ductility. However, our understanding of the detailed atomic ordering changes that occur during rejuvenation processes, such as plastic deformation, remains limited. This study utilizes nanobeam electron diffraction in a transmission electron microscope as an effective method to reveal the structural changes that occur after deformation in two Zr-based BMGs. Our findings indicate that heavy deformation from indentation or fracture causes an increase in the size of fcc-like medium-range order (MRO) clusters in a harder icosahedral dominated matrix, which corresponds to local softening of the BMGs. By examining the structure evolution at different points in the fracture process, we reveal that the mechanism of growth of MRO clusters is likely driven by enhanced diffusion from local temperature rise and/or free volume generation rather than deformation-induced nucleation and growth of new MRO sites.

Deformation and fracture behaviors of Zr-based metallic glasses under different constraint shear angles

The deformation and fracture behaviors of Zr35Ti30Be27.5Cu7.5 metallic glasses (MGs) under different shear angles were systematically investigated via shear tests with different constraint shear angles of 30°, 45° and 60°. The fracture strength of the MGs subjected to constraint shear tests increases gradually with increasing shear angles. This positive relationship between the fracture strength and the applied shear angles was verified by the Mohr-Coulomb criterion, suggesting a good agreement between experimental results and theoretical predictions. Due to the large shear stress, the content of free volume generation is greater than the content of free volume annihilation in MGs with the small constraint shear angles and then, a large amount of free volume aggregates to form multiple shear bands, manifesting as a large number of serration events. The formation and interaction of multiple shear bands can prevent the shear bands from rapidly expanding into cracks, significantly improves the plasticity of MGs.

Boosting cyclability performance of GeP anode via in-situ generation of free expansion volume

Among the germanium-based composites, germanium phosphide (GeP) is a promising anode material for lithium-ion batteries (LiBs) because of its good electronic conductivity, low binding energy and high theoretical energy density. Nevertheless, the severe volume variation and sluggish reaction kinetics hindered its applications. To solve the above issues, this work come up with a killer idea that inspired by the free volume theory of polymers, which was mainly involved creation of free volume into GeP matrix. Specifically, structural design that elaborately deals with severe aggregation while helps alleviating volume expansion and contraction of GeP have being made by a facile ball milling-assisted in-situ generation of free expansion volume. Compared with pristine GeP and direct ball milled GeP, the addition of NaCl as grinding agent plays a very positive role on generating free volume space in flake-like GeP particles, which was assembled by numerous amorphous GeP granules. In this way, the as obtained GeP particles exhibited enhanced reversible capacity, improved cyclability and rate performance (883mAhg−1 at 0.2 C after 100th cycle, and 680mAhg−1 at 2 C) when served as anode material in LiBs. This study highlights a highly efficient and cost effective approach in advancing the high performance GeP towards practical applications for energy storage.

Binary mixtures of ionic liquids: Ideal, non-ideal, or quasi-ideal?

The mixing of ILs provides an opportunity for fine tuning the physiochemical properties of ILs for various applications. However, a suitable mixture having desired properties can only be designed when the physiochemical properties of the mixtures of ILs along with their spectroscopic properties are well understood. With an aim to achieve this objective, three different mixtures with a common anion, namely, [C2C1im][C4C1im][NTf2], [C3C1pyr][C4C1pyr][NTf2], and [C3C1im][C3C1pyr][NTf2], have been investigated in the current study. Investigations have been carried out at the macroscopic level by observing the thermophysical properties, such as molar volume and thermal expansion coefficient, and at the microscopic level with time-resolved fluorescence measurements and the pulse field gradient nuclear magnetic resonance (NMR) technique. The results obtained from the thermophysical study have indicated that excess molar volume for imidazolium-based IL-IL mixtures may be linked to the free volume created by the alkyl chain of the imidazolium cation whereas for the mixture of pyrrolidinium ILs, lowering of density can give rise to free volume. Analysis of time-resolved fluorescence anisotropy data has provided clear evidence in favor of the presence of free volume in the binary mixture of ILs. NMR studies have also supported the fluorescence anisotropy data. The outcome of the present investigation reveals that the mixtures show appreciable deviation from ideal behavior and the deviation from the ideal behavior is caused due to the generation of free volume in the resultant mixture, describing these IL mixtures as quasi-ideal rather than ideal or non-ideal.

Role of elastostatic loading and cyclic cryogenic treatment on relaxation behavior of Ce-based amorphous alloy

In this work, Ce65Cu15Al10Ni8Co2 bulk metallic glass was annealed at sub-Tg and then elastostatically and cryogenically was loaded and treated, respectively. X-ray Diffraction (XRD) analysis was carried out to show amorphousness of treated alloys, and enthalpy variations and specific heat capacity were evaluated using differential scanning calorimetry (DSC). The results showed that the designed treatment procedure leads to the recovery of relaxation, i.e. rejuvenation, from the relaxed state to the as-cast condition. It is suggested that the generation of free volume and the structural rearrangements, appeared in the abnormal behavior of DSC curves, are responsible for recovery of relaxation. It is also found that the primary elastostatic loading facilitates the subsequent rejuvenation evolution caused by the cryogenic cycling process.

How does the structural inhomogeneity influence the shear band behaviours of metallic glasses

ABSTRACTHere we propose a model to understand the influence of structural inhomogeneity on the shear band behaviours of metallic glasses. By considering the inhomogeneous structure and stress concentration, the model predicts that the strain for shear band nucleation in metallic glasses can be variable and far below the theoretical elastic limit. During sliding, the shear band will approach a dynamic equilibrium state of balanced free volume generation and annihilation. By considering the accumulation of irreversible structure change, the shear band will finally develop into fracture. Under fluctuating load, the shear band shows an ‘activate-arrest’ behaviour, which results from a delayed response to the external load change. These results reasonably explain and correlate the physics behind the elastic limit, stick-slip shear band behaviour, implicit shear events, and shear band fracture in metallic glasses. The study can provide another perspective and platform to understand the correlations between structural inhomogeneity and shear band behaviours in metallic glasses, and further explore other shear band related phenomena not only in metallic glasses but also in the class of shear-softened materials.

Effect of compositional elements and processing routes on structural and thermal response in Fe-based metallic glasses

The effect of varying thickness on structure and thermal properties is investigated in Fe–B–Nb and Fe–B–Si–Nb glassy alloy systems with the addition of different Nb and Co content, respectively. Accordingly, four alloys with the nominal compositions of Fe76-xB18Nb6+x (x = 0, 2) and (Fe1-yCoy)72B12Si12Nb4 (y = 0, 0.5) are processed in the forms of ribbon and rod. All alloys of ribbons represent amorphous structure, while those of rods explain the crystalline structure with the exception of heteromorphous structure for Co added alloy. The Nb addition causes about same glass transition temperature (Tg) but increases crystallization onset (Tx), resulting in a wider supercooled liquid region (ΔTx = Tx-Tg). Moreover, Nb also improves glass forming ability (GFA) with more free volume generation and higher structural relaxation exothermic heat (ΔH0). The Co addition also improves free volume generation and higher ΔH0 for ribbon, but lower amount free volume generation in bulk alloys (rod) than ribbon.

Experimental studies of shear bands in Zr-Cu metallic glass

Shear bands are the key feature that control deformation in bulk metallic glasses (BMGs). This study provides a comprehensive analysis of plastic deformation of a Zr-Cu-based BMG at room temperature. Experiments were conducted to observe the evolution of shear bands in the material. It was shown that shear bands formed discretely in the material which allows for material deformation to occur across it. Additionally, individual shear bands were characterised to obtain a better understanding of shear-band-induced plasticity. Assessment of mechanical properties, such as hardness and elastic modulus, indicate the deformed regions of the material were weaker than undeformed regions. No compositional or structural changes were found in shear-band of the studied BMG suggesting generation of local free volume in the deformed region.

Hydrogen-induced strain localisation in oxygen-free copper in the initial stage of plastic deformation

Single crystals of oxygen-free copper oriented to easy glide of dislocations were tensile tested in order to study the hydrogen effects on the strain localisation in the form of slip bands appearing on the polished specimen surface under tensile straining. It was found that hydrogen increases the plastic flow stress in Stage I of deformation. The dislocation slip localisation in the form of slip bands was observed and analysed using an online optical monitoring system and atomic force microscopy. The fine structure of the slip bands observed with AFM shows that they consist of a number of dislocation slip offsets which spacing in the presence of hydrogen is markedly reduced as compared to that in the hydrogen-free specimens. The tensile tests and AFM observations were accompanied with positron annihilation lifetime measurements showing that straining of pure copper in the presence of hydrogen results in free volume generation in the form of vacancy complexes. Hydrogen-enhanced free-volume generation is discussed in terms of hydrogen interactions with edge dislocation dipoles forming in double cross-slip of screw dislocations in the initial stage of plastic deformation of pure copper.

A size-dependent free volume prediction model of Zr55Cu30Al10Ni5 bulk metallic glass in the supercooled liquid region

The specimen size has a significant influence on the flow behavior of Zr55Cu30Al10Ni5 bulk metallic glass throughout thermoplastic compression processes in the supercooled liquid region. Due to enhanced structural relaxation, the recovery enthalpy increases with specimen size decreasing. Based on combined differential scanning calorimetry experiments and finite element modeling simulations, the different free volume annihilations caused by the specific surface area discrepancy are considered as the main cause of size-dependent free volume variation during the pre-heating process. Thermodynamics and structural relaxation theory are introduced to provide a reasonable explanation of the size-dependent flow behavior at elevated temperatures. A novel prediction model involving size-induced free volume variations has been established by considering the influences of compression deformation variables. The quantitative theoretical prediction agrees well with experimental data, which indicates that the developed prediction model can accurately describe the size-dependent free volume annihilation and generation during the thermoplastic forming process. Furthermore, the reasonable theoretical analysis and size-dependent free volume prediction model provide a profound understanding of the size-dependent flow behavior and its relation to internal structural evolution behaviors of metallic glasses.