Thermally stimulated discharge currents (TSDC) together with the Relaxation Map Analysis (RMA) method is a convenient method to study relaxation processes in complex materials. In the RMA technique, one performs repeated TSDC runs while selecting polarization states through the use of a fixed polarization time schedule and a sucessively higher polarization temperature. Each peak has an associated polarization strength and a pair of activation parameters which can be the Gibbs free activation energy and the activation entropy. In the present paper we propose a different approach to RMA. It consists of a global heating of the sample in an hyperbolic manner to determine the activation parameters as a function of temperature. This determination is done by first selecting a value for the activation entropy for each temperature and then calculating the implied activation enthalpy or Gibbs free energy.

We have reasoned that normal relaxation processes usually proceed with a null or close to zero activation entropy and that only relaxations associated to the glass transitionand/or cooperative phenomena involve large values of the activation entropy. In those cases and based on the curvature of the TSDC curve one can select a minimum activation entropy value which will fit the TSDC curve at a particular temperature. Using this data one determines the polarization strength associated to each pair of activation parameters. Results for the TSDC relaxation parameters spectra of a liquid crystal polymer will be presented together with comparisons between the thermal sampling method and the proposed method.

A thermodynamic model for amorphization is developed which allows analysis of the effect of composition, atomic radii, and elastic constants of constitutive elements on amorphization. The model is based on comparison of the Gibbs free energy and entropy of a defective crystal and undercooled liquid. The glass transition temperatures for thermodynamically unstable amorphous phases are determined as a function of topological parameters of an alloy. It increases with an increase in the atomic size of the solvent element and it has a maximum at a specific alloy composition. Topological criteria for alloy compositions that can produce a thermodynamically stable amorphous state are outlined and discussed.

Plastic yielding in glassy solids has been interpreted as a strain-biased relaxation process, or, equivalently, as a strain-induced glass transition. In the present work, the atomic motions caused by athermal plastic deformation of a binary Lennard-Jones glass are compared to thermal motion in the liquid in terms of the self part of the intermediate structure factor. We find that like at finite temperature, athermal plastic deformation leads to diffusive atomic motion at all length scales beyond about one interatomic distance, effectively promoting structural relaxation. The present approach allows to study the interplay of deformation-induced and thermal relaxation. Preliminary evidence is presented that these two processes occur independently of each other over a wide range of strain rates.

The effect of different Si contents on the glass forming ability (GFA) and the amorphous-to-crystalline transformation has been investigated for the Mg-Cu-Y-Si system. Four Mg60Cu30-yY10Siy (y = 1–5 at.%) alloys were prepared using a relatively simple technique of rapid cooling of the melt in a copper mould. Crystallization was induced by heat treatment of the alloys and the samples were then characterized concerning their microstructure and thermal stability by X-ray diffraction (XRD), optical (OM) and scanning electron microscopy (SEM) and differential scanning calorimetry (DSC) at a constant heating rate. Partial substitution of Cu by Si leads to a transition of the as-cast structure at a constant cooling rate from amorphous to crystalline with increasing Si content. Furthermore, the glass transition temperature (Tg) of the Mg-Cu-Y-Si alloy is lower compared to the Mg-Cu-Y system. The mechanical properties of the bulk Mg-Cu-Y-Si alloys have been investigated and found to vary with the Si content.

Phase evolution during crystallization of an amorphous TiAl film produced by physical vapor deposition (PVD) was studied using differential thermal analysis (DTA), x-ray diffraction (XRD), and transmission electron microscopy (TEM). The following sequence of phase transformations was observed during continuous heating from room temperature: amorphous → body centered cubic (β) → hexagonal close-packed (α) → tetragonal (γ) → ordered α2. The β phase was formed as near-spherical particles that were evenly distributed in the amorphous phase, and the size of these particles was approximately 90 nm. Formation of the α phase by decomposition of β and the remaining amorphous phases led to a very fine feathery-like microstructure arranged in colonies of approximately 100 nm in size. The transformation of the metastable α phase into a mixture of the γ and α2 phases led to formation of an equiaxed γ-grain structure with a grain size of about 150 nm after heating to 850°C.

The thermodynamic properties of various phases of silicon, namely, crystalline, amorphous, and liquid, have been studied using the Reversible Scaling method within Monte Carlo simulations. The recently proposed Environment Dependent Interatomic Potential was employed to model the atomic interactions. The calculated Gibbs free energy and entropy of both crystalline and liquid phases are in good agreement with available experimental data. The glass transition is continuous, taking place at 1150 K. We have also determined the Kauzmann temperature, TK = 955 K, the thermodynamic fragility, F3/4 = 0.64, which indicates a fragile thermodynamic character to l-Si, and the configurational entropy of the amorphous phase, Sconf ≈ 1.2 kB/atom.

The effects of annealing for 30 minutes at temperatures ranging from 123–232°C on the structure evolution, resulting hardness, and tensile properties were determined for amorphous ribbons of Al87Ni7Gd6. Structure evolution was determined using a combination of XRD, DSC, and TEM, while micro-hardness and tensile testing of ribbon specimens was used to evaluate the mechanical properties. Significant effects of annealing on the evolution of structure and properties were obtained, while the superposition of 940 MPa pressure during annealing further enhanced the structure evolution with corresponding changes to the resulting properties.

Contamination with oxygen is known to reduce the processability and otherwise affect the properties of Zr-based bulk metallic glasses (BMGs). Due to their affinity for oxygen, however, it is difficult and/or expensive to reduce the oxygen level in these materials. As a result, the glass-forming ability (GFA) of commercial-grade BMGs is significantly lower than that of research-grade materials. We present the application of a novel electrochemical method to this problem. The general principle is to remove oxygen from the material by direct electrochemical reduction in a molten-salt electrolyte. A cathode consisting of a porous metallic phase containing dissolved oxygen is submerged in a molten mixture of chlorides. When a current is applied, oxygen is ionised at the cathode, dissolved in the molten salt and discharged at the anode. We investigate thermodynamic and kinetic limitations to the reduction in oxygen content and the resulting improvement in GFA that can be achieved when this method is applied to glass-forming alloys such as Zr55Cu30Al10Ni5.

The microstructures exhibited by gas atomized powders of two alloys, Al-6Gd-6Ni-1Fe and Al-5Y-10Ni (at. %), have been analyzed using electron microscopy. It was found that the microstructure depends critically on the particle size obtained during atomization. Small particles (<1μm) tend to be completely amorphous. Larger particles (1–10μm) can also be amorphous, but often contain a mixture of amorphous material and crystalline Al. Three distinct morphologies are observed: nanocrystalline particles, fractal and dendritic growths embedded in an amorphous matrix. The yet larger particles (>30μm) are often fully crystalline, consisting of fine interspersed Al and intermetallic grains.

The effects of changes in notch radius on the toughness of two different Zr-based bulk metallic glasses have been determined. It is shown that increases in notch radius produce large increases to the toughness, accompanied by extensive shear banding and crack bifurcation. The fracture toughness of twenty (20) fatigue precracked specimens exhibiting planar crack growth were in the range 20.3 ± 6.7 MPa√m for the two Zr-based glasses. Increasing the notch radius to 110 μm produced notch toughness in the range 95.3 ± 8.3 MPa√m for nine (9) tests on Vitreloy I, well in excess of that typically observed in most structural materials. Toughness tests conducted on two fatigue precracked specimens of Vitreloy I at 77 K produced values for fracture toughness that were in the range 17.9 ± 2.7 MPa√m, similar to that obtained at 298 K. The fatigue precracked fracture toughness of metallic glass composites containing large crystalline regions of a body centered cubic Zr-Ti-Nb alloy were in the range 29–42 MPa√m, but the values were temperature dependent over the range 148 K to 500 K. Fracture surfaces were analyzed via Scanning Electron Microscopy (SEM).

The fatigue behaviour of melt-spun Fe78-xCrxSi10B12 and Fe77.5-xCrxSi7.5B15 (0≤×≤8) amorphous alloy wires is compared with those of HT steel wires. The wires were tested in a novel bend fatigue testing machine that imparts compressive to tensile stresses. Tensile strength and microhardness were also measured; these and the fatigue performance improved with the substitution of Fe by Cr. The fatigue performance of the glassy alloy wires was generally similar to that of HT steel wires when expressed on the basis of bend strain but inferior when expressed on the basis of bend stress. Fractography studies revealed differences in the fracture surface features for the amorphous wires, depending on the stress range amplitude. It was also observed that corrosion pits nucleated on the surfaces of glassy wires with zero or low Cr content and resulted in crack initiation at these points.

Tension and compression tests have been performed with high alignment fixtures on two different zirconium-based bulk metallic glass specimens at atmospheric pressure as well as with levels of superimposed hydrostatic pressure up to 700 MPa. The results show essentially no difference in applied flow stress or fracture stress between compression and tension specimens over the range of pressures tested. However, a difference in fracture plane angle between compression and tension specimens was observed, suggesting a normal stress effect, as in granular solids, on the flow and fracture behavior of the bulk metallic glass specimens. The data are compared to various flow and fracture theories over a very wide range of normal stresses. Over the range tested, a Mohr-Coulomb flow theory appears to best describe the data.

The effect of electron irradiation on the microstructural change and phase stability of melt-spun Fe71.0Zr9.0B20.0 metallic glass having a wide supercooled liquid region of 71 K was examined. Crystallization from the amorphous phase was accelerated by electron irradiation, and this irradiation was effective in producing a nanocomposite microstructure.

Equal channel angular extrusion (ECAE) is used to consolidate amorphous Vitreloy 106a (Zr58.5Nb2.8Cu15.6Ni12.8Al10.3) powder into bulk metallic glass. Consolidations are performed on gas atomized powder containing 1280 ppmw oxygen encapsulated in copper cans and 780 ppmw oxygen powder encapsulated in nickel cans in a 90° die-angle tool at temperatures above the glass transition temperature (Tg) but below the rapid crystallization temperature (Tx), in the supercooled liquid region (ΔT). Results show that V106a is successfully consolidated to nominal full density after one extrusion pass. Good particle-to-particle bonding and significant particle deformation are observed in the consolidated alloy. A lower oxygen content is beneficial due to the larger undercooled region and the additional time for processing. The low oxygen content material shows little change in thermal stability after being consolidated when compared to the original powder but fine interparticle cracks are observed in some of the consolidates. ECAE appears to be a viable method for consolidating amorphous metal powders into bulk amorphous metal.

Tungsten wire or particle reinforced metallic glass matrix composites are produced by infiltrating liquid Zr57Nb5Al10Cu15.4Ni12.6 (Vit106) into tungsten reinforcements at 1150 K and at 1425 K. X-ray diffraction, differential scanning calorimetry, and scanning electron microscopy are carried out to characterize the composite. The matrix of the composite processed at 1150 K is mostly amorphous, with some embedded crystals. During processing, tungsten dissolves in the glass-forming melt and upon quenching precipitates over a relatively narrow zone near the interface between the tungsten and matrix. In the composites processed at 1425 K, tungsten dissolves in the melt and diffuses through the liquid medium, and then reprecipitates upon quenching. The faster kinetics at this high temperature results uniform distribution of the crystals throughout the matrix. Mechanical properties of the differently processed composites containing wires and particles are compared and discussed. The composites exhibit a plasticity of up to 16 % without sacrificing the high strength to failure that is comparable to monolithic Vit 106.

In this work we present a theoretical scheme to study the thermodynamic and transport properties of simple fluids at the liquid-glass transition line. This scheme makes use of a recent reformulation of the classical perturbation theory of liquids [M. Robles and M. López de Haro, Phys. Chem. Chem. Phys. 3, 5528 (2001)]. Using the hard-sphere fluid as a reference system our approach requires the choice of an equation of state for the hard-sphere system and a criterion to determine an effective (density and temperature dependent) diameter. Selecting the diameter in the same way as in the Mansoori-Canfield /Rasaiah-Stell variational perturbation theory and two different equations of state for the hard-sphere system, the liquid-glass transition line in the density vs. temperature plane for a Lennard-Jones fluid derived with our approach is shown to be in very good agreement with recent numerical simulations. The transition line in the pressure vs density plane and the value of the transport coefficients in the vicinity of the liquid-glass transition for the Lennard-Jones fluid are also examined.

How the spatial heterogeneities play an important role in the dynamics of density fluctuations near the colloidal glass transition is discussed from a new viewpoint. A nonlinear stochastic diffusion equation for the density fluctuations recently proposed by one of the present authors (MT) is numerically solved in two different initial states, an equilibrium state and a nonequilibrium state. Depending on initial states, the dynamics of the spatial heterogeneities are shown to be quite different from each other. Such differences are thus shown to influence the slow relaxations of density fluctuations, leading to different types of two-step relaxations.

Maximum undercooling results for the icosahedral phase (i-phase), polytetrahedral C14 Laves phase, and solid solution phases are presented as a function of composition in Ti-Zr-Ni liquids. Containerless processing was achieved using the electrostatic levitation facility located at NASA/Marshall Space Flight Center. The maximum reduced undercooling decreases with increasing icosahedral short-range order in the ordered phase. The first synchrotron x-ray diffraction data from aerodynamically levitated liquids of Ti-Zr-Ni alloys suggest an icosahedral short-range order in the liquids, supporting Frank's hypothesis, correlating icosahedral order in the liquid with the nucleation barrier to the crystal phase. The strong negative heats of mixing between Ti/Zr and Ni and their relative atomic sizes likely favor the formation of this local icosahedral order.

Using molecular dynamics and a modified embedded atom potential developed by our group we studied the diffusivity and viscosity of molten Ni1-xZrx alloys as a function of composition, temperature, and cooling rate. Previous results indicate that these potentials represent the Ni-Zr system quite well. Liquid alloys were quenched at rates of 5 × 1011 and 1012 K/s. For x > 0.04 the solidified alloys were crystalline. For higher x values, the solidified alloys were amorphous. For the amorphous alloys, the composition dependence of the calculated glass transition temperature Tg follows the general trend of experimental Tg values. The calculated viscosity and diffusivity show systematic variation with composition. For the undercooled Ni-6 at.% Zr melt the calculated viscosity shows the Vogel-Fulcher-Tammann (VFT) behavior characteristic of a “fragile” glass.

Molecular dynamics simulations of structure, thermodynamic and kinetic properties of model metallic AgCu alloy are performed to elucidate its behavior at glass transition. In spite of small variations of inherent structure of the alloy the relaxation kinetics undergo dramatic changes at the glass transition. The time dependences of the mean square displacements and the non-Gaussianity parameter show the signatures of anomalous diffusion in an intermediate time region. Analysis of time evolution of van Hove correlation function indicates the existence both jump displacements and short-range cooperative atomic rearrangements. Below Tg these cooperative rearrangements do not contribute to long-range diffusion but they still dominate the relaxation at short time.