Diffusion experiments in bulk metallic glasses are reviewed. Measurements carried out so far are focused on Zirconium based alloys and cover the amorphous and deeply undercooled liquid states as well as the equilibrium and slightly undercooled melts. Experimental findings are compared with theoretical predictions for atomic transport in glass forming systems.

Zr-Cu-Ni-Al belongs to the best glass forming systems known. Hydrogen charging was performed electrochemically in a 2:1 glycerin-phosphoric acid electrolyte. In comparison to binary Zr-Ni glasses the absorption kinetics in amorphous Zr-Cu-Ni-Al were found to be slower, but the storage capacity is similar. Desorption is hindered by the formation of thin ZrO2 layers. Partial replacement of Ni by Pd in an amorphous Zr68 5Cu13Ni11Al7.5 alloy was found to change the absorption behavior to a faster kinetic and to improve the desorption. The influence of hydrogen on the thermal stability was studied by DSC as well as by xray diffraction and TEM. Hydrogen was observed to play an important role. With increasing H/M ratio amorphous Zr69.5Cu12Ni11Al7.5 was found to transform during annealing above the glass transition temperature into a quasicrystalline structure with decreasing grain size until a nanocrystalline microstructure is achieved. Above a hydrogen content of H/M = 0.05 instead of quasicrystals a tetragonal phase with lattice parameters close to those of Zr2Ni is formed. At very high hydrogen contents phase separation is assumed to take place followed by the formation of nanocrystalline ZrH2 and other phases with reduced Zr content.

The internal friction and Young's modulus of amorphous Mg84Ni12.5Y3.5 alloy obtained by the melt spinning technique have been measured by a vibrating reed method at frequency of 250 Hz at heating and cooling runs in the temperature range from 300K to 625K.. The crystallization kinetics of the alloy was studied by the calorimetric methods (DSC and DTA). The structure of the samples was determined by the x-ray diffraction technique. The Young modulus measurements have revealed the irreversible multi-step changes (up to 50%) accompanied by the irreversible internal friction peaks. These changes were observed in the same temperature intervals where the anomalies of thermal properties were found out. The results obtained are explained by the structural rearrangement from amorphous to nanocrystalline state during the annealing.

It is often said that the fragility and specific heat discontinuity at the glass transition are related characteristic features of glasses. We give a relation between these quantities for simple liquids, and also find that the temperature dependence of the shear modulus should be closely related to these quantities. According to the Interstitialcy Theory, the liquid state shear modulus G = Go, exp (-βc) = Go exp {-γ[(T/To)-l ]}, where Gois the shear modulus at a reference temperature To, which can be taken to be the glass temperature. In these relations, βis the diaelastic shear susceptibility, c is the interstitialcy concentration, T is the temperature, and dc/dT = γ/βTo. It has been proposed by Dyre, Olsen and Christensen that U in the viscosity η = ηo exp (U/kT) be given as the work done in shoving aside particles during a diffusion step, and therefore be proportional to the (liquid state) shear modulus. If so, then combining the above relations, the fragility F = [d log η/d (Tg/T]T=Tg becomes F=(1+γ) log [η (Tg)/ηo], where γ = - (Tg/G)(dG/dT). Then, since δCv, = UF(dc/dT), where UF is the interstitialcy formation energy, one obtains further with reasonable simplifying approximations δCv/Cv∼γG(T)/Goo, where Goo is the zero temperature crystalline shear modulus. For a typical fragile glass γ ∼ 2 (F ∼ 50) with G(Tg)/Goo ∼ 1/2, then δCv/Cv, ⊼ 1, ut close to zero for strong glasses, in fair agreement with available experimental results. The perspective given by the Interstititialcy Theory is then that the fragility is given phenomenologically by the temperature dependence of the shear modulus, or microscopically by the rate of increase of the equilibrium interstitialcy concentration with temperature.

The effect of B and C additions up to 0.4 at. % on glass formation and mechanical properties of a Zr-base alloy Vitreloy 105 was studied using various techniques. All alloys were prepared by arc melting and drop casting. Boron additions increase the glass forming ability by lowering Tm and increasing Tg. Carbon additions only lower Tm but not affect Tg. B and C additions occupy free space and do not harden the glass phase.

The paper begins with some brief remarks about the history and background of the field of bulk glass-forming metallic alloys. This is followed by a discussion of multicomponent glass-forming alloys and deep eutectics, the chemical constitution of these new alloys, and how they differ from metallic glasses of a decade ago or earlier. The development of bulk glass forming alloys has led to interesting studies of the deeply undercooled liquid alloys, which are made possible by the exceptional stability with respect to crystallization. Recent advances made in this area will be illustrated by several examples. The paper continues with a discussion of some of the physical properties of bulk metallic glasses. Mechanical properties are specifically discussed. Some interesting potential applications of bulk metallic glasses will be mentioned.

The fracture and fatigue properties of the Zr41.2Ti13.8Cu12.5Ni10Be22.5 (at.%) bulk metallic glass alloy have been examined. The plane-strain fracture toughness of the fully amorphous alloy was found to exceed 50 MPa√m, although results were sensitive to strain rate, showed significant variability and were influenced by the presence of residual stresses following processing. Fracture surfaces exhibited a characteristic vein morphology, consistent with micromechanical models for meniscus instabilities. Local melting was evident, consistent with the emission of light during rupture and very high local temperatures (>1000 K) measured during fracture. Upon partial or complete crystallization, the alloy was severely embrittled, with toughnesses dropping to ∼1 MPa√m and the hardness increasing by ∼10%. Under cyclic loading, crack-propagation behavior in the amorphous structure was similar to that observed in polycrystalline metals; the crack-advance mechanism was associated with alternating crack-tip blunting and resharpening, as evidenced by presence of fatigue striations. Conversely, the (unnotched) stress-life (S/N) properties were markedly different. Crack initiation and subsequent growth occurred quite readily due to the lack of microstructural barriers that would normally provide local crack-arrest points. This resulted in a very low fatigue limit of∼4% the ultimate tensile strength.

Our work has focused on the fracture and fatigue crack-growth mechanisms of a Zr41.25Ti13.75Ni10Cu12.5Be22.5 bulk metallic glass. These alloys exhibit failure strengths on the order of 2 GPa and toughnesses of 10–30 MPa√m with remarkably little plastic deformation. The effect of stress state on strength properties was studied in an effort to model fracture mechanisms. While fracture surfaces suggest significant plastic flow, failure strains are quite small and appear to be independent of stress state. Other methods of energy dissipation, including adiabatic heating, are discussed. Temperature increases of more than 20°C have been noted on the side face of the fracture sample at the crack tip during fracture. The micromechanics of fatigue crack growth are also considered and related to the batch chemistry, environment, and microstructure.

A bulk amorphous Zr55Al10Ni5Cu30 alloy prepared by squeeze casting was found to exhibit high mechanical strength values, i.e., uniaxial tensile fracture strength (σt) of 1850 MPa, three-point bending strength (σb) of 3200 MPa, bending fatigue strength (σf) of 1100 MPa, Charpy impact fracture energy (Ef) of 135 kJ/m2 and fracture toughness of 68 MPa√m. The σb, σf and Ef are about two times higher than those for the corresponding bulk amorphous alloys prepared by unidirectional solidification and powder consolidation techniques, though the σt of the squeeze cast sample is higher by about 15 % than those for the other samples. The remarkable increases in the σb, σf and EF are presumably due to the introduction of high compressive residual stress of about 1240 MPa in the outer surface region only for the squeeze cast sample. The finding of the effectiveness of the compressive residual stress on the increase in the mechanical strength under the bending stress mode is important and expected to be widely used as a new strengthening mechanism for bulk amorphous alloys, as is the case for reinforced oxide glasses subjected to strengthening treatment.

We used molecular dynamics (MD) methods to study the deformation behavior of metallic alloy crystal nanowires of pure Cu, NiCu alloy and NiAu alloy, under high rates of uniaxial tensile strain, ranging from 5* 108/s to 5* 10/s. These nanowires are just about 2 nm thick and hence cannot sustain dislocations, instead we find that deformation proceeds through twinning and coherent slipping mechanisms. NiAu has a 13% size mismatch whereas NiCu only 2.5%. As a result the critical strain rate at which the “nanowire crystals” flow like a “liquid” is 100 times smaller for NiAu. We also calculated the elastic constants at each strain state for all strain rates to identify the relation between mechanical “shear” instability and deformation process.

We investigated the deformation behavior of the supercooled liquid in a La55Al25Ni20 (at.%) metallic glass ribbon that has a wide supercooled liquid region of 65 K before crystallization. The deformation of the metallic glass was divided into three types, namely, homogeneous deformations with and without a stress overshoot and an inhomogeneous deformation. The supercooled liquid above the glass transition temperature (Tg) exhibited a Newtonian viscosity and superplastic-like behavior during isothermal tensile deformation. The metallic glass was elongated as much as 1000 % at high strain rates ranging from 10−2 s−1 to 10o s−1 without embrittlement. The maximum elongation to failure was in excess of 1800 % at a strain rate of 1.7×10−1 s−1 and at 503 K (Tg+20 K, 0.71Tm) under about 40 MPa.

The thermal properties of an amorphous alloy (composition in at.%: Zr-10Al-5Ti-17.9Cu-14.6Ni), and particularly the glass transition and crystallization temperature as a function of heating rate, were characterized using Differential Scanning Calorimetry (DSC). X-ray diffraction analyses and Transmission Electron Microscopy were also conducted on samples heat-treated at different temperatures for comparison with the DSC results. Superplasticity in the alloy was studied at 410°C, a temperature within the supercooled liquid region. Both single strain rate and strain rate cycling tests in tension were carried out to investigate the deformation behavior of the alloy in the supercooled liquid region. The experimental results indicated that the alloy did not behave like a Newtonian fluid.

Microformability of new amorphous alloys in the supercooled liquid state and microforming techniques for the materials are shown. In the supercooled liquid state, the materials reveal perfect Newtonian viscous flow characteristics and furthermore exhibit an excellent property of microformability on a submicron scale. As for microforming techniques, microforging and micro extrusion of amorphous alloys are introduced in addition to the fabrication method of micro dies of photochemically machinable glass. As a result, amorphous alloys are expected as one of the most useful materials to fabricate micromachines.

Bulk metallic glass matrix composites are processed and investigated by X-ray diffraction, DSC, optical microscopy, SEM, microprobe, TEM, and mechanical testing. Ceramics such as SiC, WC, or TiC, and the metals W or Ta are introduced as reinforcements into the metallic melt. The metallic glass matrix remains amorphous after adding up to 30 vol% of particles. The thermal stability of the matrix does not deteriorate after adding the particles. ZrC layers form at the interfaces between the bulk metallic glasses and the WC or SiC particles. Si and W are released into the matrix in which Si enhanced the glass forming ability. The composites are tested in compression and tension experiments. Compressive strain to failure increases by over 300% compared to the unreinforced Zr57Nb5Al10Cu15.4Ni12.6 and the energy to break of the tensile samples increases by over 50% adding 15 vol. % W.

As in the past, materials will shape the new century. Dramatic changes are taking place in the fields of energy and information based on new synthetic materials. In energy, the generation of electricity by amorphous silicon alloy thin film photovoltaics; the storage of electricity in nickel metal hydride batteries which are the batteries of choice for electric and hybrid vehicles. In the information field, phase change memories based on a reversible amorphous to crystalline transformation are widely used as optical memories and are the choice for the new rewritable CDs and DVDs. The scientific and technological bases for these three fields that have become the enabling technologies are amorphous and disordered materials. We will discuss how disordered, multielemental, multiphase materials can throw new light upon metallic conductivity in both bulk and thin film materials. We will demonstrate new types of amorphous devices that have the ability to learn and adapt, making possible new concepts for computers.

Two Zr-base bulk metallic glasses were investigated in the present work. DSC experiments were performed at different heating rates (dT/dt). Evolution of the characteristic temperatures, glass transition and onset of crystallisation, were determined as a function of dT/dt. Evolution of shear elastic modulus and internal friction are measured as a function of temperature and resulting microstructural evolution; these evolutions are related to variation of the atomic mobility.

The understanding of dynamic failure mechanisms in bulk metallic glasses is important for the application of this class of materials to a variety of engineering problems. This is true not only for design environments in which components are subject to high loading rates, but also when components are subjected to quasi-static loading conditions where observations have been made of damage propagation occurring in an unstable, highly dynamic manner. This paper presents preliminary results of a study of the phenomena of dynamic crack initiation and growth as well as the phenomenon of dynamic localization (shear band formation) in a beryllium-bearing bulk metallic glass, Zr41.25Ti13.75Ni10Cu12.75Be22.5. Pre-notched and prefatigued plate specimens were subjected to quasi-static and dynamic three-point bend loading to investigate crack initiation and propagation. Asymmetric impact loading with a gas gun was used to induce dynamic shear band growth. The mechanical fields in the vicinity of the dynamically loaded crack or notch tip were characterized using high-speed optical diagnostic techniques. The results demonstrated a dramatic increase in the crack initiation toughness with loading rate and subsequent crack tip speeds approaching 1000 m s−1. Dynamic crack tip branching was also observed under certain conditions. Shear bands formed readily under asymmetric impact loading. The shear bands traveled at speeds of approximately 1300 m s−1 and were accompanied by intense localized heating measured using high-speed full-field infrared imaging. The maximum temperatures recorded across the shear bands were in excess of 1500 K.

Due to their very low thermal conductivities and large thermal expansion values, bulk metallic glasses (BMGs) undergo differential cooling during processing. Large thermal gradients are generated across a specimen leading to residual stress buildup. A thin surface layer contains compressive stresses balanced by tension in the middle. Such stresses can not only influence the mechanical behavior of BMGs, but they can also lead to problems during manufacturing of large or intricate components. Analytical and finite element modeling was used to predict the values and distribution of such stresses as a function of processing conditions. Neutron diffraction measurements were then performed on model specimens which included crystalline phases as “strain gages”. It was shown that significant stresses, on the order of several hundred MPa, can be generated in BMGs. Modeling and diffraction results are presented and their implications discussed.

The sliding characteristics of Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 bulk metallic glass have been examined in vacuum and in air using a pin/disk geometry without lubrication. The counterface material was either the same metallic glass (self-mated) or 52100 steel. Normal load was 0.1–1 kgf. The test system was equipped for continuous measurement of friction force. In addition, a Kelvin probe allowed continuous monitoring of changes in the structure and chemistry of the disk surface. Post-test characterization included optical and electron microscopy, X-ray diffraction and EDS of worn surfaces and debris. Friction coefficients in both vacuum and air were typical of those expected for ductile materials, with values ranging from 0.4 to 0.9 (higher values with lower load). Wear rates and average debris size increased with load. Wear rates were larger in air than in vacuum. Wear surface appearance and chemical composition were influenced by plastic deformation, material transfer and environmental interactions. After tests in air, the wear tracks and debris had a granular appearance and oxygen concentrations were high. Changes in the Kelvin probe signal correlated well with visual observations and with concentrations of oxygen detected by EDS.