When a glassy material or its liquid state is subjected to sufficiently high pressure, significant changes can take place in the short- and medium-range structure, vibrational density of states, and physical properties. It is crucial to determine and understand the structure-property relations under high pressure from both scientific and technological perspectives, since the glass structures frozen-in under elevated pressure may give rise to properties unattainable under ambient pressure. However, the structural and topological origins of the pressure-induced changes in macroscopic properties are not yet well understood. Here, we address this problem by subjecting various isostatically compressed glasses to isothermal annealing at ambient pressure and monitor the relaxation of glass structure and properties as a function of time and temperature. For different glass systems, density is found to relax in a stretched exponential manner with an exponent close to the Phillips value of 3/5 for relaxation in three dimensions when both short- and long-range interactions are activated [1]. For a compressed soda lime borate glass, we find that upon annealing at 0.9T_g, the pressure-induced increase in boron coordination remains unchanged, while the pressurized values of macroscopic properties such as density, refractive index, and hardness are relaxing [2]. Hence, the pressure-induced changes in macroscopic properties are not necessarily attributed to changes in the short-range order in the glass, but rather to changes in overall atomic packing density and medium-range structures. Moreover, we show that the relaxation mechanism depends on the annealing temperature and different physical properties (e.g., density and hardness) are found to relax on different timescales. [1] M. M. Smedskjaer, S. J. Rzoska, M. Bockowski, J. C. Mauro, Journal of Chemical Physics140, 054511 (2014). [2] M. M. Smedskjaer, R. E. Yougnman, S. Striepe, M. Potuzak, U. Bauer, J. Deubener, H. Behrens, J. C. Mauro, Y. Z. Yue, Scientific Reports4, 3770 (2014).

This talk reports on two interesting new phenomena recently discovered in glasses.
The first part of the talk will discuss the elastic properties of vitreous silica submitted to high pressures in a diamond anvil cell. We show that the compressibility of a silica sample immersed in helium or neon fluid is much smaller than expected from its elastic properties measured by Brillouin light scattering. It results from gas atom penetration into the interstitial free volume of the glass network. This adsorption-induced expansion can be described by a generalized poromechanical model. The second part of the talk will address the long-standing question of the nature of the glassy state. It is commonly believed that far below the glass transition temperature, dynamics in glasses is frozen, i.e. relaxations occur on time scales too large to be observed. X-rays photon correlation spectroscopy is used to reveal the existence of surprisingly fast atomic rearrangements in a sodium silicate glass, even at room temperature. These results challenge our conception of the glassy state and call for a new microscopic theory.
References:
(1) C. Weigel, A. Polian, M. Kint, B. Rufflé, M. Foret, and R. Vacher, Vitreous Silica Distends in Helium Gas: Acoustic Versus Static Compressibilities, Phys. Rev. Lett. 109, 245504 (2012).
(2) B. Coasne, C. Weigel, A. Polian, M. Kint, J. Rouquette, J. Haines, M. Foret, R. Vacher, and B. Rufflé, submitted.
(3) B. Ruta, G. Baldi, Y. Chushkin, B. Rufflé, L. Cristofolini, A. Fontana, M. Zanatta, and F. Nazzani, Revealing the fast atomic motion of network glasses, Nat. Commun. 5, 3939 (2014).

The constituents of any network glass can be broadly classified as either network formers or network modifiers. Network formers, such as SiO₂, Al₂O₃, B₂O₃, P₂O₅, etc., provide the backbone of the glass network and are the primary source of its rigid constraints. Network modifiers play a supporting role, such as charge stabilization of the network formers or alteration of the network topology through rupture of bridging bonds and introduction of floppy modes. The specific role of the modifiers depends on which network formers are present in the glass and the relative free energies of modifier interactions with each type of network former site. This variation of free energy with modifier speciation is responsible for the so-called mixed network former effect, i.e., the nonlinear scaling of property values in glasses having fixed modifier concentration but a varying ratio of network formers. In this talk, a general theoretical framework is presented describing the statistical mechanics of modifier speciation in mixed network glasses. The model provides a natural explanation for the mixed network former effect and also accounts for the impact of thermal history and relaxation on glass network topology.

The development of effective new tools for structural characterization of disordered materials and systems is becoming increasingly important as such tools provide the key to understanding, and ultimately controlling, their properties. The relatively novel technique of correlograph analysis (i.e. the approach calculating angular autocorrelations within diffraction patterns) promises unique advantages for probing the local symmetries of disordered structures. Because correlograph analysis examines a component of the high-order four-body correlation function, it is more sensitive to medium-range orders than conventional diffraction methods. Generally, electron correlograph analysis would be implemented in a transmission electron microscope (TEM) operating in the STEM mode where a nanoscale probe can be controllably created and translated to regions of interest on the specimen. It can be primarily used for characterizing local diffraction symmetry of disordered solid materials, such as amorphous semiconductors, oxides, and metallic glasses. Like fluctuation electron microscopy, electron correlograph analysis is a statistical method, replying on a sufficiently large sampling to distinguish between random and truly persistent structural correlations. The Friedel correlation in the mean electron correlograph unveils the crystallinity of samples with medium-range orders, an important structural parameter that cannot be measured by other techniques. In the presentation, I will describe the practical experimental method and common systematic errors of electron correlograph analysis. First, I will discuss the effects of beamstop, diffraction center locating, and distortions on the electron correlograph analysis. Second, by using both experimental data and numerical simulations, I will elucidate why those seemingly real correlations showing up in one or a few correlographs do not actually reflect the local symmetries of disordered structures, and emphasize that reliable structural information about the sample can only be obtained from the mean correlograph, which is averaged over results from an adequate number of individual diffraction patterns (typically > 50).

I will review our previous work in B₂O₃ glass [1,2] as well as its recent extensions in the liquid phase and in other systems. Using ab-initio simulations, we predicted new low-density crystalline polymorphs [1] which provide a framework to understand the glass properties [2]. Thanks to NMR, the new crystals are shown to be structurally much more similar to the glass than is any of the known polymorphs. Some possible routes to synthetise these crystals will be mentionned. I will also discuss the implications of these findings for the liquid behavior. In particular, the possible existence of a liquid-liquid transition, much similar to that hypothetized in water will be raised.
[1] G. Ferlat et al., Nature Materials, 11, 925 (2012)
[2] G. Ferlat et al., Phys. Rev. Lett., 101, 065504 (2008)

Amorphous transparent conducting oxides (TCO) are of considerable technological interest due to their unusually high electron mobilities, excellent optical properties, wide thermal stability and ease of processing. Significant progress has been made regarding the synthesis and optimization of these thin films (notably Zn and Sn-doped In₂O₃) for a wide range of applications. However, the structural changes upon doping as well as the exact role played by the short and intermediate-range order in defining their valuable optical and electronic properties is still lacking. This knowledge is desirable as it can be leveraged to direct research efforts towards other materials more abundant than Indium, or even point towards compounds hitherto unexplored. Short-range order (typically < 5Å) can be obtained through analysis of extended x-ray absorption fine structure (EXAFS) data which yields a partial structure function centered on the absorber atom. A complementary technique yielding structural information on longer length-scales, i.e. medium-range order, is to analyze the complete pair distribution function (PDF) obtained from total X-ray scattering data. Essentially, total scattering data is the properly corrected and normalized elastic scattering i.e. I(Q) vs. Q, where Q is the scattering vector. The PDF is a real-space distribution of atoms obtained through an appropriately weighted Fourier transform of this function. Taken together, EXAFS and PDF data contain important structural information such as bond distances, bond angles, coordination numbers and structural coherence lengths which are otherwise challenging to access in poorly ordered materials. In this study, we analyze EXAFS and grazing-incidence PDF data on amorphous In-Zn-O films with varying In:Zn ratios. Samples of a given composition but possessing significantly different electronic conductivities displayed only subtle changes in both XAS and PDF data. Further, the PDF data show clear ordering up to almost 3 nm indicating that the films are not truly amorphous. These observations have implications for both theoretical and experimental studies and will be discussed along with other structural insights on the In-Zn-O thin films with the broader aim of arriving at a better understanding of the short and medium-range order in amorphous TCO materials.

Drawing inspiration from nature, where some organisms can control the short-range order of amorphous minerals, we successfully manipulated the short-range order of amorphous alumina by surface and size effects. By utilizing the Atomic Layer Deposition (ALD) method to grow amorphous nanometrically thin films, combined with state-of-the-art electron energy loss spectroscopy (EELS) and X-ray photoelectron spectroscopy (XPS), we show experimentally that the short-range order in such films is strongly influenced by size. This phenomenon is equivalent to the well-known size effect on lattice parameters and on the relative stability of different polymorphs in crystalline materials. Additionally, direct measurements of crystallization temperature have shown a dramatic increase in amorphous to crystalline phase transition temperature for thinner amorphous Al₂O₃ films. Finally, we show that the short-range order changes while still in the amorphous phase, before the amorphous to crystalline transformation takes place.
¹ Leonid Bloch, Yaron Kauffmann, and Boaz Pokroy. Crystal Growth & Design. In press 2014.

Originally, rigidity in glasses was formulated by J.C. Phillips in order to understand the relationship between cooling speed, which measures glass formation ability, and chemical composition. Although in the last years much of this picture has been confirmed by many experiments and simulations, still it is not clear how minimal cooling speed depends on the composition. Here a simple model is provided in which two key ingredients are considered in the glass, metastable states and their multiplicity. Metastable states are considered as in two level system models. However, their multiplicity and topology allows a phase transition in the thermodynamic limit, while a transition to the glass is obtained for fast cooling. By solving the corresponding master equation, the minimal speed of cooling required to produce the glass is obtained as a function of the distribution of metastable and stable states. This allows to understand cooling trends due to rigidity considerations in chalcogenide glasses as in metallic glasses doped with group IV impurities, like the Pd-Ni-P-Si compound.

We report that the properties of metallic glasses can be dramatically tailored by slow deposition at high temperatures without altering chemical composition. The glass transition and crystallization temperatures of the metallic glass can be increased as high as 83 K and 269 K, respectively, compared to the room-temperature deposited one and melt-spun ribbons. The high-temperature deposited glass also shows ultrahigh hardness, over 60 % higher than room-temperature deposited glass. Atomic structure characterization reveals that the exceptional properties of the high-temperature glasses are associated with abundance of medium range order contains mainly distorted icosahedral order with partially cubic symmetry. The finding of the high-temperature deposited glass gives insight on atomic mechanisms of metallic glass formation and has important impact on the technological applications of metallic glasses.

That shear causes dilatation in densely packed granular systems has been known and studied since the days of Osborne Reynolds (1885). A similar effect occurs in dense packings of hard spheres, which are a useful analogues of metallic glasses. It is now possible to follow this process directly in colloidal hard-sphere glasses, by tracking the invidual particles by confocal microscopy. In dense hard-sphere systems, the moduli are strong functions of the density (they diverge at close packing). These moduli can be measured from the thermal distribution of the local strain energy densities. These measurements show a decrease in the shear modulus (and hence density) upon plastic shear deformation, and a relaxation back to the initial state after the deformation stops. Microscopic mechanisms for these density changes as well as their implications for strain softening and shear instabilities in metallic glasses will be discussed.

Owing to the high cooling rates necessary for glass formation in metallic glasses, parts with a complex geometry have been difficult to cast directly. The metallic glasses' remarkable softening behaviour above the glass transition temperature T_g has led many to suggest, then develop, thermoplastic forming as an alternative manufacturing method for complex parts. Cerium-based BMGs, exhibiting very low T_g as well as a large supercooled liquid region (SCLR), provide an excellent platform on which to study such thermoplastic forming. In order to investigate and develop new thermoplastic forming processes, the SCLR of these alloys must first be appropriately characterised. Heating rate, processing temperature, and strain rate have been examined; their effects on viscosity, flow, and devitrification explored; and the results and conclusions are hereby reported.

Many materials have a high propensity to crystallize as this is the energetically most favorable state; it is thus difficult to make these materials amorphous. However, for certain applications, it would be highly beneficial to make these materials amorphous as many of their physicochemical properties differ from those of their crystalline couterparts. For example, amorphous materials have a much higher solubility than crystalline ones. I will present a microfluidic spray drier, we call it a microfluidic nebulator, that produces very small drops through the use of supersonic air. These small drops serve as containers to form small amorphous nanoparticles; the size of these nanoparticles is determined by the number of solute molecules contained in the drop. The nanoparticle structure is determined by the probability for a crystalline nucleus to form as the drop evaporates; this probability is typically very high in supersaturated solutions as crystalline solids readily form under these conditions. However, the formation of a crystalline nucleus itself entails some time delay. We demonstrate that the nebulator can kinetically suppress the formation of crystalline nuclei; thereby, it produces amorphous nanoparticles from many different materials, even from materials that have a very high propensity to crystallize.

One of the most intriguing issues in supercooled liquids and glassy materials is their dynamic relaxation behaviors, which underlies their physical and mechanical properties. Recently, Yu et al. found that the b-relaxations in metallic glass-forming liquids is sensitive to the chemical interactions among all the constituent atoms. In other words, b-relaxations in metallic glasses are linked to a large mixing enthalpy for the atoms.
We use mechanical spectroscopy to investigate the mechanical relaxation (a and b relaxations) in several typical bulk metallic glasses in isochronal route as well as isothermal mode: Pd-Ni-Cu-P, La-Ni-Cu-Al and Cu-Zr-Al-Dy systems. We investigated the dependence of the mechanical relaxation on the physical aging to study the evolution of the localized atomic mobility with the physical aging. The pronounced JG b relaxations in metallic glasses are associated to large negative values of the enthalpy of mixing among the constituting atoms, these results are in good agreement with the recent report by Yu et al.

Resistive switches offer the prospect of improved performance, efficiency and scalability over current data storage methods. Many device architectures have been proposed, reliant upon a wide variety of materials whose conductance switches in a non-volatile manner with the application of an applied field. Silicon-based switching materials are of particular interest in these devices as they offer the added potential for integration into existing CMOS infrastructures. It is of great importance that the underlying physics of switching is well-understood, such that device optimisation and integration into commercial hardware may be realised. Our device layers are sputter-deposited to create a 37nm thick, amorphous, non-stoichiometric silicon-rich layer of silica sandwiched between conductive electrodes. We report on the material changes leading to reversible resistive switching in silicon suboxide using secondary ion mass spectroscopy (SIMS), x-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and conductive atomic force microscopy (cAFM). Analysis with a range of techniques serves to highlight the broad dynamics of device behaviour, and supports the model of an ionic and defect-dependent switching mechanism which relies upon the presence of nanoscale grain boundaries within the amorphous silica switching layer.

Our recent research accomplishments led us believe that Ga substitution on Al sites need not permit it to remain in trivalent state only. The monovalent Ga atom, has atomic radius is greater than that of trivalent one. In contrast, pentavalent state of Ga has atomic radius smaller than that of trivalent one. The change of valency of Ga in metallic glass composition will lead to change in geometry of main cluster that promotes glass forming ability of the melt. Further, the mixed states of Ga may facilitate formation of clusters of different types leading to phase separation in the melt and thereby in metallic glasses. Keeping these in mind we have recently investigated two such compositions. They refer to Zr_69.5Al_7.5minus;xGa_xCu₁₂Ni₁₁ and Ce₇₅Al_25-xGa_x metallic glass compositions. The purpose of this presentation will be to bring out aforesaid features after Ga substitution on the nature of glass forming ability based on our studies on these alloy systems.
The first part of the presentation deals with the behavior of Zr_69.5Al_7.5minus;xGa_xCu₁₂Ni₁₁ (x = 0, 1.5, 7.5 at.%) alloys. A comparison between their nanohardness and reduced elastic modulus values of the as-synthesized glassy phase with their nanocomposites has been made. The indentation characteristics of a novel Ga substituted glass composition corresponding to x = 7.5 have shown significant improvement in regard to hardness and elastic modulus. The evidence of pile up has been observed in case of as-synthesized glassy ribbons. The load (P) versus depth (h) curves for as-synthesized melt-spun ribbons displayed the presence of displacement burst, which are known as pop-ins. The amount of energy per unit volume required for the shear band formation in glassy state has been estimated based on the pop-ins observed in P-h curve. This seems to decrease with Ga addition. Based on transmission electron microscopic observations of indented glassy specimen, the possibility of nanocrystallization has been ruled out.
The glass forming ability and indentation characteristics of melt-spun Ce₇₅Al_25-xGa_x (x = 0, 2, 4 and 6 at.%) metallic glasses will also be presented. The substitution of Ga decreases the glass transition temperature, while increases the supercooled liquid region. The small amount of Ga substitution has led to appearance of second diffuse halos in the X-ray diffraction (XRD) pattern. The observation of phase separation is thus asserted. The transmission electron microscopic images led us to conclude the formation of nano-amorphous domains after Ga substitution in a glassy matrix. The load dependent hardness behaviour of metallic glasses with and without Ga substitution has been studied. The substitution of Ga improves the microhardness property of Ce-Al alloy. The formation of shear bands around the indentation periphery has been observed. The value of yield strength and Meyer exponent of these alloys has been compared.

We investigate the crystallization of an amorphous eutectic Ni-P alloy, fabricated by electroless deposition as a 10 um thick continuous layer. The purpose of this work is to obtain a detailed understanding of the microscopic mechanisms of crystallization and phase formation upon heating. To this end, we combine a variety of complimentary characterization techniques. DSC (differential scanning calorimetry) during isothermal and isochronal heating reveals the crystallization kinetics. XRD (X-ray diffractometry) provides quantitative information on phase composition as a function of tempering time. Conventional-, high-resolution-, and analytical TEM (transmission electron microscopy) and TEM-based electron diffraction provide high-spatial-resolution information on phase nucleation and spatial distribution of atom species, particularly the crystallography of the nucleating crystalline phases (Ni3P and Ni) and the spatial distribution of phosphorus in the partially and completely crystallized alloy. Our results confirm earlier observations, according to which crystallization proceeds by formation of barrel-shaped grains. Internally, these exhibit a radial microstructure of of slightly misoriented subgrains of Ni3P containing Ni needles with unique ORs (orientation relationships) to Ni3P. However, the preferred Ni-Ni3P ORs we observe differ from those reported in the literature. Combining our observations on the structure and microstructure of partially and completely crystallized Ni-P with the observed crystallization kinetics leads to a new model for the microscopic mechanism of crystallization. This may be useful for understanding the effect of ternary alloying elements on the crystallization behavior and temperature.

V-Zr amorphous thin films potentially have a number of applications as coatings for components subject to aggressive chemical and physical conditions. In this work V-Zr thin films were produced by magnetron sputtering at room temperature and were subsequently characterized and modelled; using atomic scale techniques, to understand the alloys&’ stability and the relationship the amorphous material has to the crystalline Zr, V and V₂Zr phases.
Atomic scale modelling using density functional theory was used to investigate three compositions in the V-Zr system: equimolar VZr, the V rich V₂Zr and Zr rich VZr₂ compositions. Amorphous supercells were created and minimised under constant pressure with a variety of starting densities and supercell sizes. We find that the simulation conditions are pivotal in reproducing the experimentally observed structure. Bulk X-ray diffraction techniques were used as a first check to compare experiment and theory with encouraging results.
Ensembles of electron nanodifraction patterns were collected from the three formulations of amorphous V-Zr films. The electron beam was tuned to the diameter of the nearest-neighbour clusters. In this geometry, and collecting diffraction patterns from sub-4 nm thick regions, angular correlations in the diffraction patterns reflect the symmetries of nearest-neighbour polyhedra. [1] An angular autocorrelation technique was used to quantify the statistical incidence of symmetries in the diffraction patterns of each specimen as a function of the scattering angle. It was found that the symmetries in the diffracted intensities from all the films could not be explained by BCC local ordering alone, but required the presence of more complex intermetallic local ordering, such as the C15 Laves phase (V₂Zr). The experimental data matched simulations from the amorphous supercells.
[1] A. C. Y. Liu, M. J. Neish, G. Stokol, G. A. Buckley, L. A. Smillie, M. D. de Jonge, R. T. Ott, M. J. Kramer, and L. Bourgeois, Phys. Rev. Lett., 110, 205505 (2013)

The stability of undercooled liquids against crystallization is affected by initial viscosity of the stable liquids, viscosity change of the melt due to temperature drop, free energy difference between undercooled liquids and crystalline phases, liquid/solid interface energy, bulk density, non-uniform nucleation, cooling rate of particle and so many other factors. If the cooling rate is fast enough that diffusion among the atoms is limited, the amorphous phase is uniformly solidified. This behavior is called glass transition behavior, and several metallic alloys with such behavior have been discovered, which can be vitrified with only radiative cooling. For example, containerless high-temperature/high-vacuum electrostatic levitation (ESL) technique, which could minimize heterogeneous sites, is useful for investigation of the undercooled liquid behavior. In particular, several high glass-forming alloys, whose cooling rate is fast enough that diffusion among the atoms is limited, can be vitrified with only radiative cooling in these ESL. In this study, we measured the thermo-physical properties and Time-Temperature-Transformation (TTT) diagram for the crystallization of various amorphous metal system from liquid temperature to glass transition temperature, using ESL technique. After that, we analyzed atomic scale structural change of the samples with high energy X-ray scattering experiments to understand the influence of compositional change on glass-forming ability. Indeed, these results could provide a clear understanding of the exceptional stability against crystallization in amorphous metal based on kinetic, thermodynamic and structural principles. Undercooled liquids&’ thermophysical properties and their own structure variation have a pronounced influence on the glass forming ability and knowledge on it is essential in understanding more precisely the crystallization mechanism of amorphous metal.

Recently, by using the techniques of HREM, HAADF and EDX line mapping, it [1] was found that Pd-Ni-P bulk metallic glasses (BMGs) undergo amorphous phase separation (APS). Later, by using small angle X-ray scattering method, it [2] was confirmed that the observed APS is consistent with a spinodal mechanism. It is not clear, however, why Pd-Ni-P BMGs, an alloy system of negative heat of mixing, undergo amorphous spinodal decomposition (ASD). In this work, we attempt to measure the metastable liquid miscibility gap by employing the same technique as described in [1].
[1] S. Lan, Y.L. Yip, M.T. Lau, H.W. Kui, J. Non-Crystalline Solids 358 (2012) 1298-1302.
[2] Z.D. Wu, X.H. Lu, Z.H. Wu, H.W. Kui, J. Non-Crystalline Solids 385 (2014) 40-46.

A key challenge for understanding plasticity in amorphous materials is the link between microscopic (i.e., atomistic) and macroscale behaviour (i.e., the scale of the sample). To establish this link, it is useful to coarse-grain the microscopic details and construct a mesoscopic model which tries to capture important aspects from the atomistic scale without fully resolving the atomic motions, being thus computationally efficient. The common approach taken in mesoscale descriptions of amorphous plasticity is to consider plastic activity as a sequence of spatially and temporally localized events, known as shear transformations, which correspond to local rearrangements of atoms within the bulk material in response to the locally acting shear stress. This rearrangement in turn induces an internal (eigen-) stress field which - together with external stresses - determines the spatial and temporal pattern of shear transformation events which characterizes the plastic deformation dynamics of the amorphous structure. We present a mesoscale approach to amorphous plasticity based on a finite element framework with quasistatic, athermal dynamics analogous to lattice based models proposed in the literature, which use the (periodically continued) elastic Green's function of an infinite body to calculate internal stresses in systems with periodic boundary conditions.
The tensorial finite element formulation allows us to analyze a rich variety of collective phenomena such as strain localization and strain avalanches, comparing and benchmarking our results against the established lattice models. The main advantage of the finite element method over these models is that the FEM approach provides natural access to analysing the effects of surfaces and boundary constraints, and hence to physical mechanisms leading to finite size effects which cannot be captured by models which employ periodic boundary conditions. We analyse the results for the deformation behavior of 2D amorphous specimens under non-trivial loadings, such as shear or bending, and analyse the influence of strain gradients, surfaces, and finite size effects. We find that we can conceptually envisage systems with free surfaces as consisting of two distinct regions: an interior “bulk-like” domain where plastic activity evolves essentially as observed in a periodic system, and another “surface-dominated” domain where the interactions between shear transformations are suppressed and deformation behaviour is controlled by surface constraints and external tractions. We quantitatively analyse the deformation behaviour arising from the interplay of surface and bulk (statistical) size effects in 2D and present some preliminary results for 3D specimens.

Deformation of ductile crystalline-amorphous nanolaminates is not well understood due to the complex interplay of interface mechanics, shear banding and deformation-driven chemical mixing. Here we present indentation experiments on 10 nm nanocrystalline Cu / 100 nm amorphous CuZr model multilayers to study these mechanisms down to the atomic scale. By using correlative atom probe tomography and transmission electron microscopy we find that crystallographic slip bands in the Cu layers coincide with non-crystallographic shear bands in the amorphous CuZr layers. Dislocations from the crystalline layers drag Cu atoms across the interface into the CuZr layers. Also, crystalline Cu blocks are sheared into the CuZr layers. In these sheared and thus Cu enriched zones the initially amorphous CuZr layer is rendered into an amorphous plus crystalline nanocomposite.

The alloy system, Pd-Ni-P, has a negative heat of mixing. It is therefore somewhat surprising that Pd-Ni-P bulk metallic glasses (BMGs) undergo amorphous phase separation (ASD) [1, 2]. An attempt has been made to measure the metastable liquid miscibility (MLMG) of undercooled molten Pd-Ni-P. In this work, X-ray diffraction method is employed to characterize undercooled specimens that have entered into the MLMG.
[1] S. Lan, Y.L. Yip, M.T. Lau, H.W. Kui, J. Non-Crystlline Solids 358 (2012) 1298-1302.
[2] Z.D. Wu, X.H. Lu, Z.H. Wu, H.W. Kui, J. Non-Crystalline Solids 385 (2014) 40-46.

A novel method is introduced to determine notch toughness of bulk metallic glasses (BMGs). Through thermoplastic forming of BMGs combined with Si photolithography, unprecedented control in fabricating BMG notch toughness samples can be achieved and many extrinsic influences (e.g., cooling rate, casting defects) can be drastically reduced. Our approach allows us to precisely vary the notch root radius with a high precision of ~ 1 mu;m. In order to systematically study the effect of notch root radius on the notch toughness of BMGs, notches with a broad range of radii from 3 mu;m to 380 mu;m are introduced into the test specimens. Through this method, we have fabricated ~ 100 samples with different notch radii (ρ = 3, 10, 25, 50, 100, 150, 230, 380 mu;m) in two different BMG systems, i.e., Zr₄₄Ti₁₁Cu₁₀Ni₁₀Be₂₅ and Pd₄₃Cu₂₇Ni₁₀P₂₀ BMGs. We found that there exists a critical notch radius, ρ_c, above which the apparent notch toughness scales linearly with ρ^1/2. With decreasing notch radius below ρ_c, the notch toughness retains almost constant rather than degrading. However, this critical notch radius varies in the two different investigated BMGs that exhibit distinct toughness behavior. Such observed phenomenon can be well explained within the framework of characteristic critical distance theory and is similar to previously reported for crystalline metals, ceramics, and polymers, although the mechanistic origins are different. Our finding also suggests that BMGs can be highly tolerant to sharp flaws.

The presence of minute crystalline phases in metallic glasses is unfavorable for the reliable understanding of the intrinsic properties of the bulk metallic glasses(BMG) [1]. However, the detail of the sample characterization has often been omitted in the literature. In this study, the amorphous state of the Zr-Al-Cu-(Ni,Fe) BMG are thoroughly investigated using high intensity 50mu;m X-ray beam with a 2-dimensional detector in order to probe possible existence of minute crystalline phases that are not detectable by conventional X-ray equipments. So far, the results have shown that a small amount of crystalline phases exists in the cross section of most of the Zr-based BMG that were prepared in a conventional method. Experiments are currently being undertaken to fabricate Zr-based BMG without minute crystalline phases to perform further measurements such as mechanical properties. The detail of the results will be presented at the poster.
[1]Q.S Zhang et al. Acta Materialia 58 (2010)904-909

It is generally assumed that the glass transition temperature, T_g, of a bulk metallic glass, BMG, is only a shallow function of the composition. We give evidence for several metallic glasses derived from quarternary (Pt+TM)₆₆B₂₄(Si+Ge)₁₀ systems exhibiting strong changes of T_g (from 570 K to 650K) with varying the type of TM(=transition metal) used, resulting in virtually unchanged reduced glass transition temperature over a wide range of compositions.
The access to a wide range of T_g with minor levels of substitution opens the door to investigate whether the T_g can be shifted with regard to the critical fictive temperature, T_fc, or whether the latter stays put relative to the T_g. As proposed by Kumar et al.¹, the relative position of these two temperatures essentially defines whether a BMG will be ductile, brittle or cooling-rate sensitive in its mechanical behavior.
We further present mechanical properties derived from 3-point bending test, microhardness and elastic constants from elastic wave measurements.
1. Kumar, G., Neibecker, P., Liu, Y. H. & Schroers, J. Critical fictive temperature for plasticity in metallic glasses. Nat. Commun.4, 1536 (2013).

Mechanical properties in nano-sized metallic glasses have been widely investigated to clearly understand the fundamental deformation mechanism of metallic glasses. In particular, the change of yield strength and the occurrence of deformation mode transition (heterogeneous to homogeneous) with sample size reduction have been reported in previous experimental studies through the nano-pillar test results. Research on nano-mechanical responses of metallic glasses could lead to an in-depth understanding of the deformation mechanism related to the nucleation and propagation of shear bands. However, during the pillar fabrication using focused ion beam for nano-pillar test, Ga+ ion beam damage on pillar surface is difficult to avoid and the fabrication takes relatively long for multiple pillar preparations. To avoid these drawbacks of nano-pillar tests, in the present study we performed nano-compression tests of metallic glass nanoparticles prepared by FIB-less method. In our experiments, metallic glass nanoparticles have been fabricated by the selective dissolution technique from phase separated metallic glasses. From the two-amorphous alloys with droplet structure, the reactive matrix phase has been selectively dissolved by chemical process. The shape of the remaining particle phases was clearly spherical, and the sizes of fabricated particles have distribution in 40~500 nm diameter range. The effect of particle size and strain rate on mechanical properties of fabricated amorphous particles has been investigated from the compression test using in-situ compression tester with electron microscope imaging. Due to the distinct morphology of particles compared to that of nano-pillars, the stress-strain relations from particle compression test results are derived based on contact mechanics theory and contact area calculation. From the stress-strain curves and in-situ electron microscope video, we confirmed the metallic glass nanoparticles are not fractured at high strain over 50% and have favorable condition for homogenous-like deformation at strain rate near ~10^-3 s^-1 based on modified Griffith criterion. These results provide us with insights on evaluation of nano-mechanical properties and understanding of fundamental deformation mechanism of metallic glasses.

It is well known that applications of bulk metallic glasses (BMGs) for structural materials are limited by their brittleness in spite of desirable mechanical properties such as high yield strength, high elastic limit, and relatively low elastic modulus. To overcome this drawback, various composite microstructures with structural heterogeneity of micrometer to nanometer scale in the amorphous matrix have been developed. The heterogeneity can result in nucleation of multiple shear bands and interruption of their propagation, which can lead to enhanced plasticity. In the present study, in order to precisely control the amorphous heterogeneities or amorphous 2nd phases in the amorphous matrix, thermally activated growth behaviors of secondary amorphous phases in Cu-(Zr,Hf)-Al-(Y,Gd) phase separating metallic glasses have been investigated using in-situ heating SAXS experiment and additional intensive structural analysis. From the combination of two ternary systems, Cu-(Zr,Hf)-Al and Cu-(Y,Gd)-Al, phase separating metallic glasses with two distinct amorphous phases were fabricated in Cu-(Zr,Hf)-Al-(Y,Gd) quaternary system. Experimentally, the growth behavior of secondary amorphous phase was investigated by SAXS analysis at elevated temperature near first T_g and T_x. Also, the chemical composition of 2nd phase was evaluated using alloy contrast variation (ACV) method from comparison of SAXS/SANS patterns of various alloy compositions. From the results, the relationship between the composition/size of 2nd phases and the mechanical properties in the phase separating metallic glasses was analyzed. This provides us with a fruitful idea on how to control the properties of metallic glass composites from manipulating the 2nd phases in nanometer scale.

A structural amorphous metal (SAM) containing 0 to 30 vol% of W nanoparticles was tested by several techniques, including micro-bending tests. The density values reached up to 10.98 g/cc with the addition of W nano particles. Nanoindentation tests were performed in order to study the compositional effects on the hardness and modulus. Indentation mapping results show a significant increase in hardness values in the areas surrounding the W particles. Extensive analysis was performed around the nanoparticles in order to study if and how the nanoparticles could aid in crack deflection and improve the fracture toughness.

Rejuvenation is a structural excitation of glassy systems. Rejuvenation increases free volume and potential energy, and is one of the promising approaches to improve the deformability of amorphous metals, which generally exhibit brittle fracture. We here focus on the rejuvenation of ternary amorphous metal induced by thermal loading process. Using molecular dynamics techniques and embedded-atom method interatomic potentials, a ZrCuAl amorphous alloy model was constructed via melt-quenching process. Subsequent thermal loading simulations composed of reheating, isothermal annealing, and requenching were then conducted with various annealing temperatures and final quenching rates. Based on change in potential energy and volume during the thermal loading process, we evaluated level of aging and rejuvenation induced by the thermal loading; aging decreases and rejuvenation increases potential energy and volume. It is observed that the thermal loading induces rejuvenation when an annealing temperature is higher than a critical temperature T_c (T_c is higher than the glass transition temperature T_g) and a cooling rate after isothermal annealing is higher than that of initial melt-quenching process. This is because that erasing of the past aging history by heating at sufficiently high temperature is required to realize thermal rejuvenation. Moreover, a higher quenching suppressing aging during final quenching process is another necessary condition for thermal rejuvenation. We drew a rejuvenation map with respect to both annealing temperature and final cooling rates. The level of rejuvenation increases with increasing quenching rate and annealing temperature up to 1.3T_g. Changes in properties induced by rejuvenation were also investigated; aging increases and rejuvenation decreases topological short-range and medium-range order and elastic constants of amorphous metals. Our results could be useful information when we apply rejuvenation as a practical tool to control properties of amorphous metals.

Today, demand of electric power is expanded with population growth. We must effectively use electric power to build a sustainable society. When electric power comes to user from the generation plant, energy loss occurs in the transformer. Total energy loss is divided into iron loss and copper loss. Amorphous alloy gets much attention because of reduction in this iron loss. Planar Flow Casting (PFC) is well known as an industrial technique for rapid quenching to produce the amorphous ribbon used mainly as soft magnetic materials
However, this amorphous ribbon has defect called air pocket. The air pocket is considered to be formed by air layer induced by wheel revolution. The air pocket is concave defect and roles to make high the iron loss in electric energy loss. So, the purpose of our study is to investigate the influence of process parameters on the formation of the air pocket in order to fabricate more efficient amorphous ribbon in PFC process.
In PFC process, molten alloy is impinged strongly on rotating wheel at small gap between nozzle and wheel surface less than 1mm by high gas pressure to form wide amorphous ribbon. Applying the small gap results in the planar flow of molten alloy ejected from the nozzle, though the molten alloy is impinged strongly onto a wheel surface to get high cooling rate. Main process parameters are ejection gas pressure, gap between nozzle and wheel surface, and wheel surface velocity. We focus on wheel surface velocity, because it is a most convenient process parameter in operating PFC process. Clear differences in the size of the air pockets were apparent in casting under changing only wheel surface velocity. We would like to explain the influence of wheel surface velocity on the formation of air pockets.

In the past decades, there has been a large amount of scientific and industrial interest in metal matrix composites as a way to improve mechanical properties compared to the unreinforced alloys. Recently, new perspectives with 3D network reinforcement have been opened by the availability of ceramic scaffolds produced by freeze-casting of ceramic suspensions. Bulk metallic glasses (BMGs) have high yield strength and elastic strain limit combined with a good corrosion resistance. However, lack of macroscopic ductility could be an important drawback in many applications. The introduction of secondary phase into the glassy matrix encourages the formation of multiple shear bands and interferes with the propagating shear band, which delay failure and improve the toughness of the BMGs. In this study, Innovative BMG/ceramic composites produced by reactive infiltration of the melt into ceramic scaffolds prepared by freeze-casting technique. The ZrCuAgAlBe alloy system with the highest GFA in Zr-based BMGs was selected as a matrix because an interfacial reaction can deteriorate thermal stability of glassy matrix. The microstructure, including interfacial wetting behavior and interfacial adhesion between the scaffolds and the matrix, and mechanical properties of the BMG composites were systematically investigated.

Mechanical treatments, such as hot-and cold-working, are commonly used to optimize mechanical properties for polycrystalline metals, and there has long been interest in exploring similar possibilities for metallic glasses. For example, various mechanical treatments such as cold rolling or shot peening have been introduced to alter the glassy structure including structural anisotropy, dilatation and formation of free volume, which cause to improve plasticity of monolithic metallic glasses. High pressure torsion (HPT), in which a disk-shaped sample is deformed by torsional straining under pressure, has shown to be an efficient technique for grain refinement in the nanometer regime in various materials thereby significantly enhancing mechanical properties such as micro-hardness, tensile strength and ductility. Contrary to crystalline materials, it is recently proposed that HPT process can cause structural rejuvenation and mechanical softening of bulk metallic glasses, but there are still limited approaches to understand the effect of HPT on BMGs. In this study, spark plasma sintered BMG and their composites were subjected to a large amount of deformation by HPT process which enables us to apply severe deformation for brittle materials. As a result, HPT process increases the structural disordering and amount of excess free volume in the amorphous structure which suggests structural rejuvenation by heavy deformation. Interestingly, the tendency of rejuvenation changes by depending on the characteristics of secondary phases in BMG composites. Furthermore, the variations of thermal property and mechanical behavior of BMG and their composites after HPT process were systematically investigated. The understanding built-up from this study will be used to explain the successful control of following properties as well as microstructure in various BMG and their composites by severe plastic deformation technique.

Hot Isostatic Pressing (HIPping) was employed in an attempt to reduce the micro-porosity in bulk metallic glass (BMG) castings. A zirconium/copper-based alloy, shown to be very suitable for multi-scale tooling applications, was studied. Specimens in the form of rods were produced using arc melting and a drop casting technique, and their amorphous nature confirmed using X-ray diffraction. Isothermal and non-isothermal analyses were carried out using a differential scanning calorimeter to characterize the super cooled liquid region of the alloy. The resultant data were used to select the range of process parameters for use in the HIP treatment of the cast alloy. Porosity in the BMG samples was characterized by metallography, optical microscopy and quantitative image analysis. The area fraction of porosity in the as-cast BMG was found to be around 0.01%. Samples in the form of discs were cut from the cast BMG rods and subjected to HIPing at temperatures within their supercooled liquid range. The HIP experiments were dedicated to study the individual effects of pressure, temperature and time on porosity of BMG samples, in terms of the spatial and size distribution of pores. Post-HIP samples were found to be still amorphous, as confirmed by X-ray diffraction. Significant reduction in area fraction (by an order of magnitude) and number density of pores was achieved as the result of treating the BMG samples with HIP. Our findings confirm that HIP is an effective treatment to reduce significantly porosity levels in BMGs while also keeping the alloy amorphous.

Nanometer-scale structure in the solid state and dynamics in the supercooled liquid state are expected to have major effects on bulk metallic glass properties and process of the glass transition. We have measured aspects of both structure and dynamics using coherent electron nanodiffraction. We have developed the electron scattering equivalent of photon correlation spectroscopy, a technique we call electron correlation microscopy, which uses time-resolved nanodiffraction to measure the a relaxation time of glass-forming alloys heated into the supercooled liquid above T_g in the electron microscope. Experiments on Pd₄₀Ni₄₀P₂₀ show an average tau;_α varying from 25 s to 10 s between T_g and T_g + 25 K, consistent with previous measurements. Analysis of single coherent speckles suggests a broad distribution of relaxation times from different nanoscale regions of the glass.
We have used fluctuation electron microscopy, another coherent nanodiffraction technique, to study the static structure of Zr-Cu-Al BMGs. Experiments on Zr₅₀Cu₄₅Al₅ and Zr₅₀Cu₃₅Al₁₅ show a correlation between the thermal stability of nanoscale structure in the as-quenched state and glass-forming ability. Zr₅₀Cu₃₅Al₁₅ is the poorer glass former, and it shows weaker icosahedral-like nanoscale order and more stable crystal-like nanoscale order than Zr₅₀Cu₄₅Al₅, the better glass former.

The effects of thermo-mechanical treatments on the short- and medium-range order in Cu₆₄Zr₃₆ bulk metallic glasses were investigated by Molecular Dynamics simulations. In the first part of this talk, we will present investigations of structure-specific nearest-neighbor histograms and bond-angle distributions to highlight the unique role of Cu-centered icosahedra as compared to other types of Voronoi polyhedra. We find that in both, the glassy and supercooled liquid temperature regime, only icosahedra reveal a distinct tendency to form superclusters. The spatial distribution and interconnectivity of the icosahedra were also studied as a function of cooling rate and compared to a homogeneous icosahedral distribution. In the second part, we will present our studies on shear-band structure upon uniaxial deformation of the sample. During deformation the icosahedral fraction reduces and the superclusters become partially destructed. Via determination of an effective disorder temperature and investigations of the corresponding mean-square displacements, we are able to characterize the liquid-like structure of the shear band. This structural information is then used to discuss the enhancement of the self-diffusion coefficient in the shear band in comparison to that of the remaining matrix.

The dynamic-mechanical response of glasses provides important insights into atomic transport mechanisms. Relaxation processes involve a range of relaxation times, but their spectrum cannot be obtained explicitly from the data. Most previous studies assumed relaxation kinetics to be described by a stretched exponent, which amounts to an a priori assumption about the spectrum shape. We have recently been able to compute spectra from quasi-static anelastic relaxation data for a metallic glass, using nonlinear least-squares fits [1]. These spectra exhibited distinct peaks corresponding to an atomically-quantized hierarchy of shear transformation zones (STZs). We use these spectra to simulate the frequency-dependent loss modulus [2]. We show that the high-frequency (or low-temperature) tail observed in experiment corresponds to the same STZ mechanism as the rest of the curve: both a and b relaxations are due to STZs -- the tail is due to small, and therefore fast, STZs, which are reversible due to the small volume fraction they occupy. Many authors have shown that the time-temperature superposition principle was obeyed by loss-modulus curves measured at varying temperatures, yielding a single apparent activation energy. We show that this method yields Arrhenius behavior even for a broad range of STZ activation energies, and is thus not a reliable indicator. These results likely apply to any glass that does not exhibit intramolecular relaxations. Finally, we computed relaxation-time spectra from published frequency-dependent loss moduli for a metallic glass [3]. Using simulated data with noise, we validated the method by showing that the assumed input spectrum can be recovered. Arrhenius behavior is obtained for each STZ size, further confirming the STZ model. Above T_g, the typically high apparent activation energy is shown to be an artifact of the temperature dependence of the high-frequency shear modulus.
This work has been funded by NSF Grant DMR-1307884.
-------------
1) J.D. Ju, D. Jang, A. Nwankpa, and M. Atzmon: J. Appl. Phys. 109, 053522 (2011).
2) J. D. Ju and M. Atzmon, MRS Comm., doi:10.1557/mrc.2014.12, Published online 12 May 2014.
3) J. D. Ju and M. Atzmon, Acta Mater.74, 183 (2014).

There is a clear interest, for bulk metallic glasses, in the atomic mobility is very sensitive to structural changes occurring during annealing or deformation. It is well known that amorphous materials are in out of equilibrium state below the glass transition temperature T_g. Modifications of enthalpy and entropy are induced by various treatments. For instance, annealing can induce physical aging, i.e. structural relaxation or even crystallization (when annealing time is long enough or annealing temperature high enough) and then an increase of short range order (SRL) or long range order (LRO). On the other hand, it is remarkable that plastic deformation (irreversible deformation) can increase the disorder in amorphous materials.
In the current work, the dynamic mechanical properties of Zr₅₆Co₂₈Al₁₆ bulk metallic glass have been investigated by dynamic mechanical analysis (DMA), high-resolution transmission electron microscopy (HRTEM) and nanoindentation technique. The influence of structural relaxation, plastic deformation and crystalline phases on atomic mobility in Zr₅₆Co₂₈Al₁₆ bulk metallic glass has been analyzed. In the amorphous state, dynamic mechanical properties of Zr₅₆Co₂₈Al₁₆ amorphous alloy are similar to that observed in other based bulk metallic glasses. At low temperature, the material is mainly elastic and driving frequency has no influence. In contrast, in the glass transition region, the visco-elastic component becomes very large. Then frequency has a major effect on the mechanical response. It has also been shown that the atomic mobility can be evaluated through the value of the loss factor. The atomic mobility increases by increasing the loss factor. Results are interpreted using a physical model, based on the existence of defects in the material, called quasi-point defects. Atomic mobility is reduced by structural relaxation or formation of quasi-crystalline or crystalline phases. In contrast, plastic deformation, introduced by cold rolling, increases the concentration of defects and therefore enhances the atomic mobility. The concentration of defects is evaluated by DMA and nanoindentation tests in the framework of the quasi-point defects theory.

Great progress has been made in understanding the atomic structure of metallic glasses, but there is still no clear connection between atomic structure and glass-forming ability. By adding three new concepts to the efficient cluster packing model, we show that glass-forming ability can be predicted from simple topology - the relative size and concentrations of atoms that make up a glass structure. This model distinguishes between topologies that cannot form glasses, topologies where marginal glasses can form, and specific topologies where bulk metallic glasses (BMGs) are most likely. The new concepts providing this predictive capability will be described and validated by comparing with previously known BMGs, and by predicting new BMGs. This model is not fully predictive, and still lacks a description of the chemical contribution to glass formation.

Using molecular dynamics (MD) simulations, we show that glass transition in a model phase separating amorphous metal alloy—Cu₅₀Nb₅₀—occurs by gelation. Below the glass transition temperature , a percolating network of icosahedral short-range order (ISRO) forms along transitional zones between regions of compositional enrichment in the material. This ISRO network is mechanically stiff, halts coarsening of compositional medium range order (CMRO), and constrains the dynamics of surrounding atoms, leading to anomalous diffusion. This ISRO network and its influence on the physical properties of the system bears striking resemblance to gelation in colloidal systems, in which a system-spanning, dynamically arrested network of locally preferred structures imparts stiffness. Gelation may be relevant to glass transition and microalloying in more conventional bulk metallic glasses.

In a three-dimensional model mimicking realistic Cu₆₄Zr₃₆ metallic glass, we demonstrate that quasilocalized low-frequency vibrational modes correlate strongly with fertile sites for shear transformations. We have also uncovered a direct link between these soft modes/spots and the local atomic packing structure: geometrically unfavored motifs (GUMs) constitute the most flexible local environments that encourage soft modes and high propensity for shear transformations, while local configurations preferred in this alloy, i.e., the full icosahedra (around Cu) and Z16 Kasper polyhedra (around Zr), contribute the least.

Metallic glasses (MGs) have attracted vast interest due to their extraordinary properties. Despite the bulk of theoretical and experimental efforts, their atomic structure and its relation to their properties are only vaguely known and a general model to this end is still missing. In order to describe the arrangement of atoms in MGs, various cluster-based structural models have been suggested (e.g. “efficient cluster packing” and “cluster-plus-glue-atom”). Many research groups have demonstrated the appositeness of these models through theoretical simulations in combination with experimental structure analysis. The decisive step i.e. the determination of the link between the structure and properties of MGs is, however, still missing. Recently a bottom-up approach to the fabrication of MGs by deposition of alloy metal clusters from the gas phase was proposed. Cluster-assembled metallic glasses (CAMGs) could play a key role in in determination of the structure-property relation in MGs since the structures and properties of their building blocks (alloy metal clusters) can be determined in state-of-the-art cluster science labs. Although the way is stony and steep, the first attempt to tackle this challenge (performed on CuZr binary alloy) has proven very promising.

An universal equation has been derived for the variable activation energy of viscous flow Q(T) of the generic Frenkel (or Eyring) equation of viscosity eta;(T)=A×exp(Q/RT) which has two constant asymptotes - high Q_H at low temperatures and low Q_L at high temperatures. Among different types of atomic mechanisms of viscous flow recent data on a viscous flow model based on network defects - broken bonds termed configurons - were analysed. The defect model used by many researchers states that higher the concentration of defects (e.g. configurons) the lower the viscosity. Additionally the defect model results in temperature relationship for viscosity eta;(T) which is a continuous function of temperature T both for glassy and liquid amorphous materials. The glass-liquid transition within configuron percolation theory (CPT) is treated as a percolation-type phase transition. It is accompanied by formation of percolation macroscopic clusters made up of configurons which are dynamic in nature. The characteristic linear size of dynamic clusters formed is given by correlation length which universally depends on formation Gibbs free energy of configurons G_d=H_dminus; TS_d ., where H_d and S_d are the enthalpy and the entropy of formation. It becomes macroscopic large at glass transition temperature T_g. Fractal-type medium range order is characteristic for correlation length sizes and homogeneous and isotropic disordered state is characteristic for macroscopic sizes. A reduction of topological signature (Hausdorff dimensionality) occurs at glass transition for the disordered bonding lattice: the dimensionality reduces from 3 for glass to fractal 2.4minus; 2.8 for melt. This is the main signature change which differentiates a glass from a liquid. Namely the signature change is responsible for drastic changes of material behaviour at glass transition (see e.g. J. Non-Cryst. Solids, 382, 79 (2013)). The CPT gives in an universal viscosity equation eta;(T)=A₁T[1+A₂exp(B/RT)][1+Cexp(D/RT)] valid at all temperatures. We show that a particular result from CPT is the universal temperature relationship for the activation energy of viscous flow:
Q(T)=Q_L+RT×ln[1+exp(-S_d/R) exp((Q_H-Q_L)/RT)]
Thus Q(T) depends on asymptotic energies Q_L at high temperatures (for the liquid phase), Q_H at low temperatures (for the glassy phase) and entropy of configurons S_d. This equation has two asymptotes, namely Q(TH, and Q(T>>T_g) = Q_L. The equation for Q(T) practically coincides in the transition range of temperatures with Sanditov&’s equation (JETP, 110, 675 (2010)). In addition we propose a simple analytical approach which provides numerical values of thermodynamic parameters of bonds (e.g. H_d and S_d) in glass forming materials from four shear viscosity coefficient data at four temperatures, two of which should be below and other two above glass transition temperature. This method aims to replace earlier used complex fitting procedures for the nonlinear viscosity equations to experimental data.

Amorphous polymers have structure on a range of length scales. At the smallest scales the packing of monomers affects the friction between them. At intermediate scales, the constraint that polymers cannot pass through each other leads to topological constraints called entanglements. Orientation of chains can be important on scales from bonds to the end-end separation. The talk will start by discussing the relation between monomer structure, packing and the yield stress. Coarse-grained potentials that remove atomic detail reproduce melt structure but dramatically suppress the yield stress, indicating the close connection between monomer friction and yield stress. The functional form of strain hardening is shown to be less sensitive to monomer friction and correlated instead with entanglements and the persistence length. Results for different strain histories can be collapsed when plotted against orientational order on large and short scales. Next the connection between entanglements and mechanical strength is revealed by studies of the evolution of interface strength with interdiffusion time. These are correlated with the real space distribution of entanglements as evaluated with the Primitive Path Algorithm. Both the maximum shear strength and tensile fracture energy scale linearly with the density of interfacial entanglements before saturating at bulk values. Entanglements are also critical to the formation and stability of bulk crazes, which increase the fracture energy by orders of magnitude.

The nature of the amorphous network in glass and highly crosslinked glassy polymers strongly controls thermo-mechanical properties. In silicate glasses, this network structure is modified by the inclusion of additional compounds, such as soda and lime, leading to changes in ductility. In highly crosslinked polymers, topology is controlled by the overall degree of reaction and processing conditions. Atomistic simulations of soda lime silicate and dynamically reacted epoxy network structures allow examination of defects and topology, i.e., sterically hindered polymerization vs. network scission via modifier species. This information is used to gain insight into the structure-property relationship of highly connected amorphous network materials. Networks in both systems are allowed to evolve over the course of the simulations, avoiding over-constrained structures. Analysis of our results in the framework of network theories for both polymers and inorganic glasses reveals commonalities between these systems.

Silicon-based polymer-derived ceramics (PDCs) are X-ray amorphous materials resistant to crystallization beyond 1400 ^oC. These ceramics exhibit many unusual properties, such as oxidation and corrosion resistance, as well as a resistance to creep at elevated temperatures. Amorphous PDCs are synthesized by careful pyrolysis of Si-containing preceramic polymers. The structure after pyrolysis has generally been described to contain nanodomains of amorphous carbon and amorphous Si-networks, where the Si-networks are comprised of SiO_xC_4-x and SiC_xN_4-x tetrahedral units in the SiOC and Si(B)CN PDC systems, respectively. The morphology and chemistry of the nanodomains can be fine-tuned so that the properties can be adjusted, therefore PDCs have been hypothesized for use in a variety of technological applications, such as semiconductors, barrier coatings, sensors, construction materials, and Li-ion battery anode materials.
SiOC and Si(B)CN PDCs were synthesized by pyrolysis of a series of preceramic polymers with systematically modified organic and/or functional groups to determine the influence on the final ceramic structure. Solid-state ²⁹Si nuclear magnetic resonance (NMR) spectroscopy was carried out to identify distinct ²⁹Si molecular species and to correlate the molecular species to the intermediate-range structure. The data reveal for the first time that preceramic polymers with different functional groups result in dramatically different intermediate-range structures after pyrolysis. In the case of SiBCN PDCs synthesized using polysilylcarbodiimide as the preceramic polymer, it was found that the structure contains dense nanodomains of amorphous silicon nitride. This is in contrast to the structure found in the SiBCN PDCs synthesized using polysilazanes that contain an amorphous Si-C-N percolated network constructed of SiC_xN_4-x tetrahedral units, where silicon can be tetrahedrally bonded to either nitrogen, carbon, or a combination of the two. The detailed analyses of the short- and intermediate-range structures of PDCs synthesized with various preceramic polymers can help to explain the interesting properties that have been reported, such as thermal stability, electrical conductivity, and chemical durability.

We have used fluctuation electron microscopy (FEM) to measure medium range order / molecular packing in 40-85 nm thick indomethacin thin films. Vapor-deposited ultrastable indomethacin exhibits extraordinary thermodynamic and kinetic stability, with fictive temperature ~ 30 K below the glass transition temperature of a liquid-cooled indomethacin ordinary glass [1]. Periodic packing of molecules parallel to the film surface has been discovered in the ultrastable indomethacin glass, but not in an ordinary glass with x-ray diffraction [2]. We measured molecular packing perpendicular to film surface with FEM. Extremely low beam current of 1.5 pA at 200 kV was used to minimize beam damage, and the average electron diffraction from the sample is unchanged at total electron doses four times larger than required for an FEM experiment. A FEM variance peak centered near 0.1 Å^-1 was found in ultrastable indomethacin. It represents medium range order packing of molecules with interplanar distance ~1 nm, approximately the size of one indomethacin molecule. The variance from an ordinary indomethacin glass is significantly smaller, suggesting that ordinary glass is less heterogeneous. This is the first study of medium-range order in molecular glasses and may provide insight into the structural differences between the remarkable ultrastable thin films and ordinary glasses.
[1] Swallen, S. F., Kearns, K. L., Mapes, M. K., Kim, Y. S., McMahon, R. J., Ediger, M. D., Wu, T., Yu, L., Satija, S. Science 2007, 315, 353.
[2] Dawson, K. J., Zhu L., Yu, L., Ediger, M. D. J. Phys. Chem. B 2011, 115, 455.

The surface structure, mechanical properties, and alignment capabilities of oriented polymer substrates are of interest for nanomanufacturing and device applications. We have used atomic force microscopy (AFM) and the surface force apparatus (SFA) to study the effects of uniaxial stretching on chain orientation and nanoscale tribology of glassy polymer substrates. Examples will be shown of the different friction responses of semi-crystalline and amorphous polymers along and across the stretching direction, and how this friction response is altered as the strength of adhesion between the polymer and the countersurface is deliberately changed.

Aluminosilicate glasses have wide technological applications as well as importance in geoscience. The atomic structures of the glasses are strongly influenced by the composition and chemistry such as modifier to alumina ratio and modifier cation field strength, which in term impact the properties such as chemical durability, mechanical strength, and ionic diffusion. The correlation of composition, structure and properties is otherwise complicated although significant progress has been made by characterizations such as solid state NMR. In this paper, we report the study of structure and structure-property correlations of sodium aluminosilicate glasses from peralumina to peralkali compositions by using molecular dynamics simulations with a set of highly effective partial charge pairwise potentials. Structure features such as aluminum coordination, oxygen tricluster, and silicon Q_n distributions as a function of composition were studied and correlated to the calculated mechanical properties. In addition, the field strength effect of modifier cations on aluminum coordination speciation was investigated. It was found that higher field strength cations facilitate higher coordinated Al species, in excellent agreement with experimental results. Implification of these structure changes on mechanical and thermodynamic behaviors will discussed

The mechanisms of density-driven structural collapse in prototypical network-forming glasses like B₂O₃, SiO₂, GeO₂ and GeSe₂ will be considered, where the debate is informed by the results obtained from in situ high-pressure neutron diffraction experiments made with a Paris-Edinburgh press [1-5]. The diffraction data are compared to new molecular dynamics simulations, made using schemes that suit the materials under investigation. In the case of oxide materials, we will show that the coordination number of network-forming structural motifs, which play a key role in defining the topological ordering, can be rationalized in terms of the oxygen packing-fraction over an extensive pressure and temperature range [6].
[1] A. Zeidler et al. J. Phys.: Condens. Matter 21 (2009) 474217.
[2] J. W. E. Drewitt et al. Phys. Rev. B 81 (2010) 014202.
[3] P. S. Salmon et al. J. Phys.: Condens. Matter 24 (2012) 415102.
[4] K. Wezka et al. J. Phys.: Condens. Matter 24 (2012) 502101.
[5] P. S. Salmon and A. Zeidler Phys. Chem. Chem. Phys. 15 (2013) 15286.
[6] A. Zeidler, P. S. Salmon, and L. B. Skinner, Proc. Natl. Acad. Sci. USA (in press).

Thermoset polymers have been extensively used as the matrix of fiber composites because of their high stiffness, high creep resistance and high thermal resistance. These favorable thermal and mechanical properties result from the three-dimensional amorphous network structure created by crosslinking chemical reactions during a carefully designed curing process. A fundamental understanding of the structural evolution and the relationship between material properties and molecular network structures is of great interests to the material community as well as aerospace industry.
Crosslinking degree and crosslink density are two important characteristics for amorphous thermoset polymers and they are related but not identical. Although great attentions have paid on their measurement by various methods, accurate determination of these characteristics is still a challenge for experimentalists. We have recently developed a procedure to mimic chemical reactions during crosslinking process of thermosets at atomistic scale employing molecular dynamics. The predicted density, coefficient of thermal expansion, glass transition temperature and elastic constants of the resulting polymer are in excellent agreement with experiments. However, the detailed topology of network structure has not been examined and structure-property relationship needs to be established.
In this talk, we will present our most recent progress in computational characterization of polymer network. The algorithm in our computational characterization will be described in detail. Our calculations indicate the network structures formed at different conditions could be very different even with the same crosslinking degree. There is a strong relationship between material properties and crosslink density. Some material properties are more sensitive to the structure details.

Random packing structure of colloidal system has been attracting attention for modeling amorphous materials because of its similarity to amorphous structure. Recently, vibration in the colloidal system is investigated, and the relationship between the structure of colloidal system and the density of state is discussed. In the previous studies, spontaneous motions of particles are generally observed. However, the motion is fast and the displacement is small, and direct monitoring of the each particle is not straightforward especially for three-dimensional colloidal system. In this study, we excite vibration using an external source, and observe a structural evolution caused by the vibration. By this method, contribution of a specific vibration to structural evolution is made measurable without monitoring the motions. We finally observe that there is a specific frequency that crystallizes a colloidal system.

Liquids and glasses are known to have “medium-range order (MRO)” beyond the nearest neighbors. However, the origin of the order and how to characterize it properly has not been well established. In oxides and polymers covalent bonds are considered to be the origin, but even metallic glasses, many of which do not have covalent bonds, still show strong MRO. In this case “caging” and “jamming” are considered to cause MRO, but the details are unclear. In this talk we discuss the role of the elastic field in establishing the MRO, particularly in the context of rapid increase in viscosity in the supercooled liquid with decreasing temperature. Viscosity can be expressed in terms of the shear stress correlation function in the Green-Kubo equation. If we express the macroscopic shear stress in terms of the atomic-level shear stress, we recognize that the increase in viscosity occurs through both spatial and temporal development of correlations among the local stresses [1]. We calculated the two-dimensional local stress-stress correlations for 2D liquids, and found that surprisingly there are strong anisotropic stress correlations up to high temperatures. The pattern of the stress correlations follows the Eshelby theory, which is a continuum theory of elastic inclusion, fairly well, as we predicted earlier [2]. However, it is modulated by the derivative of the pair-density function (PDF) and the calculated correlation is substantially weaker than the theory at short-range. The second point is the evidence that the elastic Eshelby field induces structural relaxation to increase stress correlation. The correlation at the nearest neighbor due to the Elshelby field is negative, but relaxation results in positive correlation, leading to frustrated but weakly biased local shear domain, similar to the polar nano-regions (PNR) in relaxor ferroelectrics. We discuss the similarities and differences among the spin-glass, relaxor ferroelectrics, strain glass and real glass, and the possibility of creating a unified theory of glasses as a class.
1. V. A. Levashov, J. R. Morris and T. Egami, Phys. Rev. Lett.106, 115703 (2011).
2. T. Egami and D. Srolovitz, J. Phys. F12, 2141 (1982).

Fragility is a fundamental concept that is often argued to be correlated with glass formability. It is defined from dynamical aspects of the liquid, generally based on the rate of increase of the viscosity as a function of the inverse temperature scaled to the glass transition temperature, T_g. Here a more fundamental scaling temperature, T_coop, is identified, which corresponds to the onset of cooperative motion in the liquid at high temperature. A universal high temperature limiting viscosity is also found, which is equal to the number density multiplied by Planck&’s constant. Scaling by T_Coop and the limiting viscosity gives a universal viscosity curve for all metallic liquids studied, independent of their fragility. The glass transition temperature can be accurately predicted from T_coop, indicating a deep connection between the onset of cooperative dynamics and the dynamical processes of the glass transition. Additionally, experimental studies of the temperature dependence of the liquid structure factor, S(q), show a correlation between the rate of structural ordering and the kinetic fragility. The magnitudes of the height of the first peaks in S(q) and the pair correlation function, g(r), show abrupt changes on approaching the glass transition temperature in fragile liquids. They change more continuously with decreasing temperature in strong liquids. The connection between the rate of structural ordering and fragility is confirmed by measurements of the high-temperature viscosities of these metallic liquids. This demonstrates a link between structure and dynamics in the liquids that is often assumed, but not previously demonstrated experimentally. Taken together, these results suggest that liquid metals are ideal for investigations of fundamental issues related to fragility and glass formation. (Partially Supported by the National Science Foundation (DMR-12-06707) and NASA (NNX10AU19G)).

Given its “off-equilibrium” nature, the liquid to glass transition is by essence a kinetic phenomenon which manifests by large thermodynamic variations during cooling/heatings cycles across the glass transition.
Here we will review the notion of reversibility in glasses, and present Molecular Dynamics simulations showing that certain glass-forming liquids are found to exhibit minuscule thermodynamic changes during such cycles, and therefore define glassy materials that can be viewed as “thermally reversing” given the obtained optimal volumetric or enthalpic recovery.
When the topology of the corresponding network structure is analyzed, it is found that such “optimal” liquids actually adapt under stress by experiencing larger bond-angle excursions indicative of a softening of underlying bond-bending interactions, and exhibit stress-free (isostatic) character. Additional anomalous behaviors are also found in dynamic and structural properties. Ultimately, these results show close connections with experiments on network glasses, and thermally reversing compositional windows, widely observed in chalcogenide, modified oxides and solid electrolyte glasses which are signatures of rigid but unstressed networks that form a so-called “intermediate phase”.These findings substantiate the notion of rigidity in disordered molecular systems, while also revealing new implications for the topological engineering of glasses.

Understanding the relaxational behavior of multi-component glass-forming metallic liquids is of critical importance to the development of novel alloy systems such as bulk metallic glasses (BMG). However, such relaxations are highly activated and complicated because of both quenching and chemical disorders, and thus remain to be investigated. Herein, we report observations of an unusual non-monotonic temperature dependence of the collective fast structural relaxation time beyond the melting temperature in a strong BMG (LM601 Zr_50.75Cu_36.23Ni_4.03Al_9.0) measured by quasi-elastic neutron scattering. Consequently, the slope of the Arrhenius plot of the relaxation time, the activation energy, shows a negative slope, which signals an apparent unrealistic negative activation energy. Hence, this could be a characteristic of some form of microscopic instability in the system. Heat capacity measurements at the same temperature range also show presence of an auxiliary peak. Classical Molecular Dynamics simulations of Cu-Zr-Al system, using Embedded Atom Potential (EAM), reveal a crossover from uncorrelated dynamics to correlated dynamics in α-relaxations above the melting temperature. These mechanisms in tandem are key dynamical features of this glass-forming metallic liquid system and can provide better insight into improving glass forming ability by understanding liquid state of these systems.

In this talk, we will discuss recent structural measurements of the effects of a change in the local coordination on doping with magnetic impurities based upon recent x-ray absorption spectroscopy measurements and well as density-functional simulations. Based upon the work of Skelton et al [1] which indicated a (different) magnetic ground states in the amorphous and crystalline states of GST, we have investigated the local structure about Fe and Mn atoms in as-deposited and crystalline GST. We report on the results of this modelling. We also discuss some recent results on the structural basis of resistance drift in the amorphous state of GST. PC-RAM device evaluation tests have shown that upon switching to the amorphous state, the measured sample resistivity drifts upwards with time following a power law. This phenomena is problematic for device performance especially for multilevel storage in a single bit as the resistance values are unstable with time. There has been much speculation in the literature as to the fundamental mechanisms with some speculating that mechanical stress plays a role while others attribute drift to structural relaxation although in the usual picture of structural relaxation, the annihilation of point defects would be expect to decrease, not increase resistivity. To address this question, we have measured both the resistivity of as-deposited amorphous GST samples under a drift-inducing annealing schedule as well as Ge L3-edge XANES. These measurements show a clear change in structure with time that correlates well with the observed resistivity change. These structural changes are interpreted as being due to the annihilation of tetrahedrally coordinated Ge-Ge bonds which lead to charge trapping a decrease in charge carriers for transport. Subsequent lattice relaxation towards an octahedral environment then leads to localisation of a lone-pair of electrons on the s-orbitals of Ge atoms. This process also suggests that pre-annealing the amorphous phase leads to faster crystallisation rates, namely that annihilation of tetrahedral sites occurs, ordering of pyramidal sites into the octahedral structure of the crystalline phase can proceed more quickly.
[1] J. M. Skelton and S. R. Elliott. In silico optimization of phase-change materials for digital memories: a survey of first-row transition-metal dopants for Ge2Sb2Te5. J Phys. C Solid State, 25(20):205801, 2013.

Amorphous or disordered materials are intrinsically defined by a lack of order, which typically refers to an idealized description of a corresponding crystalline phase that seldom exists in real materials. Starting from well-defined local structures - local order - at the scale of the coordination sphere of the constituent atoms, amorphous materials only appear as disordered because they lack of long-range order. The qualification of long-range ordering is itself strongly dependent upon the characterization techniques (e.g., cross section contrast in diffraction experiments). Many material properties (mechanical, optical, etc.) often depend on the controlled introduction of either disorder or order within the structure - e.g., solid solutions, defects, phase separation, nucleation & growth - that allow control of chemical and structural order in glasses, ceramics or glass ceramics.
We have recently introduced cutting-edge solid-state NMR methods involving the manipulation of multi-spin systems, which in conjunction with DFT-based computations, allow the qualification and quantification of local order in disordered system from the atomic to the nanometer scale in terms of geometrical and chemical order [1]. From these experiments and simulations, we show that amorphous solids often exhibit significant ordering that is a clue to understanding their macroscopic properties [2].
1. D. Massiot, R.J. Messinger, S. Cadars, M. Deschamps, V. Montouillout, N. Pellerin, E. Veron, M. Allix, P. Florian, F. Fayon, "Topological, Geometric, and Chemical Order in Materials: Insights from Solid-State NMR" Accounts Chem. Res. 46 1975-1984 2013
2. F. Fayon, C. Duée, T. Poumeyrol, M. Allix, D. Massiot, "Evidence of Nanometric-Sized Phosphate Clusters in Bioactive Glasses as Revealed by Solid-State 31P NMR" J. Phys. Chem. 117 2283-2288 2013

Observables of primary importance to most experiments derive from the electronic and vibrational properties, and charge carrier transport depends on these plus the electron-phonon coupling. In this talk, I review the electronic and vibrational consequences of topological structural disorder, describe universal features of the localized to extended (Anderson) transition for both electrons and phonons, and via ab initio computations of deformation potentials, describe electron-phonon couplings in amorphous Si and amorphous Se. The nature of generalized Wannier functions is reported for a-Si, and shown to be similar to the decay of analogously computed functions in diamond, a result that is also obtained for the single-particle density matrix using Kohn-Sham orbitals. These notions lead to a simplified model of photo-response that is helpful in explaining recent experiments in amorphous Si and amorphous Se.

Odd-even effects, the non-monotonic dependency of physical properties on odd/even number of structural units, are widely observed in homologous series of crystalline materials. However, such alternation is not expected for amorphous molecular materials because of absence of periodic packing. Herein, we report an unusual odd-even effect of glass transition temperature and relaxation time in a network-forming ionic glass homologue. To understand this phenomenon&’s molecular origin, we performed incoherent elastic neutron scattering measurements of the nanosecond atomic dynamics. The results indicated that the even-numbered cations showed much slower dynamics than neighboring odd-membered cations. The observed difference in mobility exists both around T_g and extends to the liquid temperature regime well above T_g. Further quasi-elastic neutron scattering measurements confirmed that the one-methylene unit structural difference causes up to 7 times difference in backbone dynamics. Pair distribution function (PDF) measured using neutron diffractometry indicates very small difference in structure of these odd/even ionic glasses. Our results suggest that odd-even effect, one of the most well-known structure- property relationships, exists in liquid and glassy state and has a dynamical origin.

Stress driven nucleation of nanocrystals in amorphous alloys has been a subject of intensive debate in the past decade. It has long been postulated that nanocrystals formed succeeding the occurrence of shear bands in deformed amorphous alloys. In this study, we show, via in situ nanoindentation of amorphous Cu₄₄Zr₄₄Al₁₂ alloy in a transmission electron microscope, that the formation of nanocrystals occurred at a ultra-low stress of 0.25 GPa in the elastic deformation regime, accompanied by load drops without evidence of shear bands [Y. Liu et al, Materials Research Letters, 2014. http://dx.doi.org/10.1080/21663831.2014.911778]. Furthermore, during successive loading, repetitive nanocrystal nucleation events were observed, and the stress required for nucleation kept on increasing to ~ 0.54 GPa, implying the occurrence of a ‘hardening&’ effect in amorphous alloy. This study provides direct evidence to advance our understanding on deformation induced nanocrystallization of amorphous alloys.

Size induced change in mechanical property is often reasoned based on defects and their reaction to external loading in small and confined samples, at least for crystalline materials. Different from crystalline materials, amorphous solids do not possess obvious extended structural defects, which in principle should make the disordered material insensitive to size effect. Nevertheless, there have been a slew of reports from experiments showing size effects on mechanical properties, namely, strengthening and enhanced plasticity in metallic glasses, while other show no or marginal effect. In this talk, I will go over an analysis to show how sample dimension could influence mechanical properties. Our emphasis is on surface induced state and its impact on stress-strain responses in nanoscale metallic glasses. We show that it is the complex interplay between the internal and external stresses that dominate the overall mechanical response in amorphous solids; and there exist a critical length scale at which the material should respond differently in two regimes: one dominated by a surface state and other by shear band.

It has long been conjectured that any metallic liquid can be vitrified into a glassy state provided that the cooling rate is sufficiently high. Experimentally, however, only metallic liquids within a finite compositional range can be successfully vitrified, due to the limited attainable cooling rates. Here, we report an experimental approach to achieve an ultrafast cooling rate, which enables formation of metallic glasses with compositions far beyond the glass-formation zone identified by conventional vitrification techniques, offering unique possibilities to study their structure-property relationships. Combining in-situ transmission electron microscopy observation and atoms-to-continuum modeling, we investigated the formation condition and the thermal stability of these novel metallic glasses. Our technique also exhibits great control over the reversible vitrification-crystallization processes, suggesting its potential in micro-electro-mechanical applications. The ultra-high cooling rate obtained in the present study makes it possible to explore the fast kinetics and structural behavior of supercooled metallic liquids.

Recent advances in metallic glass research demonstrated that the sample size is a tuning parameter to toughen metallic glasses, in addition to chemistry and processing. Shorter sample size not only delays fracture but also enhances the yield stress. The yielding of MG is likely to be controlled by the shear band propagation, with its size dependency not quantitatively understood. Due to the extreme localization of shear band in space and time, shear band propagation is difficult to study in experiments. Here, we employ molecular dynamics (MD) simulations to provide quantitative insights on how sample size will affect shear band propagation, without referring to any continuum level assumptions. We then discuss the asymptotic behavior of the stress/strain field during the propagation of the shear band and compare it to continuum level theory. Insights gained here will improve the understanding of the fundamental mechanics of shear band propagation.

Glass materials, including metallic glasses, typically fracture in tension at room temperature in a globally elastic manner. Although homogeneous tensile plasticity and necking of nanoscale metallic glasses have been reported, controversy exists regarding possible contributions from specimen preparation and testing techniques. Here, we show the separate effects of sample size reduction and extrinsic effects on the homogeneous tensile plasticity and necking of Pd₄₀Cu₃₀Ni₁₀P₂₀ glassy wires tested at room temperature. An intrinsic transition from catastrophic shear fracture to plasticity and necking was obtained in this glass when its diameter approaches the length scale of the shear-band nucleus size (i.e. 500 nm). Further reduction of the wire diameter to 267 nm produces complete ductile necking. Extrinsic effects introduced during sample preparation and/or testing produce entirely different results.

Thin film metallic glasses (TFMGs) have recently attracted attention with a potential to mitigate the brittle-like mechanical behavior of the bulk counterparts, leading to superior mechanical properties among which a rupture strength close to the theoretical limit and large elastic deformation [1]. Moreover, TFMGs have shown to sustain outstanding plastic deformation above 10% involving homogeneous deformation, instead of shear bands-induced severe localization [2]. Most studies deal with FIB-milled micro-scale specimens involving the potential risk of defect generation. The on-going debate on length-scale effects on shear bands formation is not fully settled.
Here, we investigate the mechanical properties of submicron Zr₆₅Ni₃₅ TFMGs by using and innovative method based on an on-chip internal stress actuated microtensile testing set-up [3]. The on-chip test structures are produced using microfabrication techniques involving cleaning, deposition, lithography, and etching. After release from the substrate, the test specimens are subjected to uniaxial tension. Tests are simultaneously performed on thousands of specimens, deformed up to different strain levels to obtain the stress-strain curve up to fracture, while avoiding FIB-milling.
Zr₆₅Ni₃₅ (% at.) TFMGs have been deposited by DC-magnetron sputtering with different thicknesses smaller than 400 nm. The amorphous structure was confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The elastic properties were investigated by Brillouin spectroscopy, while nano-indentation has been used to extract the hardness and activation volume.
Then tensile tests show very large ductility up to 12% involving elastic deformation up to 4% and up to 8% of homogenous plastic deformation before failure. The measured elastic modulus is in agreement with the Brillouin spectroscopy data. Besides the TFMGs thickness, the effect of different actuator-specimen sizes on film mechanical properties has also been tested. It is worth noting that large ductility is obtained for specimens up to 6 micrometers wide and up to 100 micrometers long which is significantly more than earlier results reported in the literature [2]. On this basis, we present an effective model to describe the mechanical properties of TFMGs and their changes with respect to bulk counterparts.
References
[1] L. Tian et al., Nat. Commun. 3:609 (2012) pp. 1-6.
[2] Q. Deng et al., Acta Mater. 59 (2011) 6511.
[3] M. Coulombier et al., Rev. Sci. Instrum. 83 (2012) 105004.

Introducing a soft crystalline phase into an amorphous alloy can promote the compound's ductility. Here we synthesized multilayered nanolaminates consisting of alternating amorphous Cu₅₄Zr₄₆ and nanocrystalline Cu layers. The Cu layer thickness was systematically varied in different samples. Mechanical loading was imposed by nanoindentation and micropillar compression. Increasing the Cu layer thickness from 10 nm to 100 nm led to a transition from sharp, cross-phase shear banding to gradual bending and co-deformation of the two layer types (amorphous/nanocrystalline). Specimens with a sequence of 100 nm amorphous Cu₅₄Zr₄₆ and 50 nm Cu layers show a compressive flow stress of ~2.57 GPa, matching the strength of pure CuZr metallic glass, hence exceeding the linear rule of mixtures. In pillar compression, 40% strain without fracture was achieved by the suppression of preliminary shear band propagation. The results show that inserting a ductile nanocrystalline phase into a metallic glass prevents catastrophic shear banding. The mechanical response of such nanolaminates can be tuned by adjusting the layer thickness.

Metallic glasses typically fail via shear band formation and exhibit minimal macroscopic plasticity. However, shear band formation can be suppressed when nm-scale layers of metallic glass are embedded within alternating layers of crystalline metals. The resulting nanolaminate composites exhibit both high strength and significantly larger strains to failure compared to larger scale metallic glass samples. In principle, the high density of interfaces in the nanolaminate can serve as nucleation sites for both dislocations and shear bands, but they also can suppress macroscopic failure by mitigating the propagation of these defects across layers. Presently, little is known about these effects on the strength of metallic glasses; rather, only the bulk strength or hardness of such nanolaminates is reported.
This work makes use of a new method to determine the individual layer flow strengths in nanolayered composites—not just bulk composite strength—by coupling finite element simulations of nanoindentation with experimental nanohardness and micropillar compression data. In particular, nanohardness measurements are found to be sensitive to the mismatch in strength between the constituents whereas micropillar compressive strength is not. For Cu/PdSi nanolaminates, the method reveals that 20 nm thick PdSi layers have ~4-5 times the flow strength of neighboring 20 nm Cu layers. Further, the 20 nm thick PdSi layers are ~1.5x the strength of monolithic PdSi pillars. This contrasts with prior work on monolithic PdSi pillars showing little sample size dependence on flow strength. The findings suggest that size-dependent flow strength in metallic glass may be dependent on whether the material is bounded by free surface (e.g., micropillars) or crystalline material (e.g., nanolaminates).

Metallic glass is generally considered as isotropic in terms of structure and mechanical properties. But this perception is only valid in samples free of stress and without initial structural inhomogeneity introduced in processing. We have shown that under external loading, structural symmetry is generally broken in the initially isotropic metallic glass samples and subsequently the mechanical properties become anisotropic. This finding connects the early experimental observation of structural anisotropy to the underlying mechanical anisotropy. But this phenomenon is explored only in the continuum regime. In this work, we present new results from extensive atomistic simulation that show the mechanical anisotropy varies in metallic glass samples with the smallest length scale down to nanometers. We characterize the variations of the mechanical anisotropy and its effects on overall mechanical properties.

Chalcogenide phase change alloys (Ge₂Sb₂Te₅, GeTe and related materials) are the subject of extensive experimental and theoretical research because of their use in optical (DVD) and electronic (phase change memories, PCM) storage devices [1]. Both applications rest on the fast (10-100 ns) and reversible transformation between the crystalline and amorphous phases induced by heating either vi