Diffusion in metallic glasses

Rutherfordα-particles backscattering technique was employed for measurements of diffusion rates in metallic glasses. Effects of relaxation, crystallization and plastic deformation on diffusion rates were also investigated. It has been observed that the diffusion rates of a metallic solute are of the same orders of magnitude in both metal-metal and metal-metalloid glasses. A higher diffusivity is likely if there is a large difference between melting points of the solute and matrix. Relaxation has no effect on diffusion, however, diffusivity increases on crystallization. An increase in diffusivity is also observed on plastic deformation of metallic glass.     

Crystallization of Zr-Ni metallic glasses

The crystallization of Zr-Ni metallic glasses of composition between 55 and 70 at % Zr has been investigated using differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The samples were prepared by splat quenching in an arc-hammer device. Transformation temperatures, effective activation energies, and enthalpy changes are reported as a function of composition. Results of XRD patterns obtained as a function of annealing temperature in the DSC are presented. A high temperature exothermic DSC peak and XRD patterns indicate the presence of a metastable phase which occurs between 57 and 63.5 at % Zr. The results tend to support suggestions of a connection between the short range structure of the glass and the crystalline phase to which it transforms. It was found that the metastable phase, whose presence is strongest at 57 to 59 at % Zr, and the process of phase separation around the eutectic composition (63.5 at % Zr) play important roles in the crystallization process.     

Corrosion behaviour of metallic glasses

A review is presented concerning the current experimental information on the corrosion behaviour of metallic glasses, particularly of iron-base glasses of metal-metalloid-type obtained by using electrochemical measurements and scanning electron microscopy. The corrosion characteristics of glasses of metal-metal-type, such as Cu-Ti, are also compared with those of known, highly corrosion-resistant metallic glasses. Compositional and structural effects, such as the addition of metallic or metalloid elements, amorphous phase structure effects, passive film formation and stress corrosion cracking, are also discussed.     

Phonon dispersion in metallic glasses

The phonon frequencies of longitudinal and transverse waves for metallic glasses are derived and computed for ternary Pd77.5Si16.5Cu6 glasses. The dispersion of phonon frequency with respect to the wavenumber is found to be influenced by the dielectric screening due to conduction electrons. This revised version was published online in August 2006 with corrections to the Cover Date.     

Thermal rejuvenation in metallic glasses

Abstract

Structural rejuvenation in metallic glasses by a thermal process (i.e. through recovery annealing) was investigated experimentally and theoretically for various alloy compositions. An increase in the potential energy, a decrease in the density, and a change in the local structure as well as mechanical softening were observed after thermal rejuvenation. Two parameters, one related to the annealing temperature, T_a/T_g, and the other related to the cooling rate during the recovery annealing process, V_c/V_i, were proposed to evaluate the rejuvenation phenomena. A rejuvenation map was constructed using these two parameters. Since the thermal history of metallic glasses is reset above 1.2T_g, accompanied by a change in the local structure, it is essential that the condition of T_a/T_g ≥ 1.2 is satisfied during annealing. The glassy structure transforms into a more disordered state with the decomposition of icosahedral short-range order within this temperature range. Therefore, a new glassy structure (rejuvenation) depending on the subsequent quenching rate is generated. Partial rejuvenation also occurs in a Zr₅₅Al₁₀Ni₅Cu₃₀ bulk metallic glass when annealing is performed at a low temperature (T_a/T_g ~ 1.07) followed by rapid cooling. This behavior probably originates from disordering in the weakly bonded (loosely packed) region. This study provides a novel approach to improving the mechanical properties of metallic glasses by controlling their glassy structure.     

Shear bands in metallic glasses

Shear-banding is a ubiquitous plastic-deformation mode in materials. In metallic glasses, shear bands are particularly important as they play the decisive role in controlling plasticity and failure at room temperature. While there have been several reviews on the general mechanical properties of metallic glasses, a pressing need remains for an overview focused exclusively on shear bands, which have received tremendous attention in the past several years. This article attempts to provide a comprehensive and up-to-date review on the rapid progress achieved very recently on this subject. We describe the shear bands from the inside out, and treat key materials-science issues of general interest, including the initiation of shear localization starting from shear transformations, the temperature and velocity reached in the propagating or sliding band, the structural evolution inside the shear-band material, and the parameters that strongly influence shear-banding. Several new discoveries and concepts, such as stick-slip cold shear-banding and strength/plasticity enhancement at sub-micrometer sample sizes, will also be highlighted. The understanding built-up from these accounts will be used to explain the successful control of shear bands achieved so far in the laboratory. The review also identifies a number of key remaining questions to be answered, and presents an outlook for the field.     

Structural investigation of boron-containing metallic glasses

Structural investigations have been made of high boron-containing alloy glasses of compositions Co_100–x B _x withx=34, 36, 38 and 40 at % measured by X-ray diffraction. The characteristic second-peak splitting in the radial distribution function (RDF) is found for all samples presently investigated. The shoulders are also observed near the distances of 0.20 and 0.34 nm. The partial radial distribution functions of Co-Co and Co-B pairs have been derived from the measured total RDF data by applying the concentration method with the anomalous scattering technique, and the contributions from different atomic pairs to total RDF have been discussed. The calculation by the relaxed dense random packing model has been made for Fe-B alloy glasses with three different boron concentrations, i.e., 15, 25, and 40 at % boron and a comparison between the calculated and experimentally determined data is presented. The present model calculation could give the possible representation for the compositional dependence of the X-ray RDF for amorphous Fe-B alloys with boron contents of up to 40 at % as well as the different peak profile observed in the total RDF of X-ray data between Fe-B and Fe-P glasses.     

Mechanical properties of bulk metallic glasses

The mechanical properties of bulk metallic glasses, including their superior strength and hardness, and excellent corrosion and wear resistance, combined with their general inability to undergo homogeneous plastic deformation have been a subject of fascination for scientists and engineers. The scientific interest stems from the unconventional deformation and failure initiation mechanisms in this class of materials in which the typical carriers of plastic flow (dislocations) are absent. Metallic glasses undergo highly localized, heterogeneous deformation by formation of shear bands, a particular mode of deformation of interest for certain applications, but which also causes them to fail catastrophically due to uninhibited shear band propagation. Varying degrees of brittle and plastic failure creating intricate fracture patterns are observed in metallic glasses, quite different from those observed in crystalline solids. The tension–compression anisotropy, strain-rate sensitivity, thermal stability, stress-induced crystallization and polyamorphism transformations, are some of the attributes that have sparked engineering studies on bulk metallic glasses. Understanding of the glass-forming ability and the deformation and failure mechanisms of bulk metallic glasses, has given insight into alloy compositions and intrinsically-forming or extrinsically-added reinforcement phases for creating composite structures, to attain the combination of high strength, tensile ductility, and fracture toughness needed for use in advanced structural applications. The relative ease of fabricating metallic glasses into bulk forms, combined with their unique mechanical properties, has made these materials attractive options for possible applications in aerospace, naval, sports equipment, luxury goods, armor and anti-armor systems, electronic packaging, and biomedical devices.     

The fracture topography of metallic glasses

This paper discusses the way in which the features found on some tensile fracture surfaces of amorphous metals are formed. The relevance of the strongly inhomogeneous plastic flow present in these solids to the formation of the particular fracture topography discussed is pointed out, as well as features common to the well-known fracture micromechanisms, cleavage and dimpled rupture.     

The fracture of bulk metallic glasses

The fracture of metallic glasses has received relatively little attention until recently. The development of bulk metallic glasses (BMGs) with more compositions, large sample sizes and diverse fracture behaviors provides a series of ideal model systems for the study of fracture in glassy materials. The fracture toughness of different BMGs varies significantly from approaching ideally brittle to the highest known damage tolerance. Diverse fracture patterns on the fracture surface, fracture modes and dynamic propagation of cracks have been observed in different BMGs. In this review paper, we present a comprehensive view of the state-of-the-art research on various aspects of the fracture of BMGs, including fracture behavior and characteristics, fracture mode, fracture criterion, fracture toughness, and fracture morphology. Accumulated experimental data on BMG fracture are presented and their possible theoretical connections with continuum fracture mechanics and the atomic-scale process are introduced and discussed. Modeling studies of the fracture of BMGs by various computational methods are also reviewed. The review also presents a number of perspectives, including the relation of BMG fracture study to other topics, and unsolved issues for future investigation.     

Fracture toughness of some metallic glasses

The critical stress intensity factor of Fe₄₀Ni₄₀B₂₀, Fe₃₀Cr₁₀Ni₄₀B₂₀, Ni₈₀Si₁₀B₁₀ and Ni₈₀Si₅B₁₅ metallic glass ribbons was measured. Stressing by ultrasound, a new method for preparation a sharp crack in a metallic glass, was used. Only shear rupture failures was investigated for all alloys.     

Fracture of Ni-Fe base metallic glasses

The fracture toughnesses of specimens of three transition metal base metallic glasses, Ni₄₈Fe₂₉P₁₄B₆Al₃, Ni₃₉Fe₃₈P₁₄B₆Al₃ and Ni₄₉Fe₂₉P₁₄B₆Si₂ are reported. Each alloy was tested in a characteristic thickness, i.e., 25m (Ni₄₈), 43m (Ni₃₉) and 72 m (Ni₄₉) andK _C values of 120, 62 and 30 kg mm^–3/2, respectively, were observed. It is suggested that this variation is associated primarily with a transition from plane strain (K _IC 30 kg mm^–3/2) toward plane stress conditions as sample thickness is decreased. The fatigue crack propagation rate in the Ni₃₉ alloy is also reported;da/dn (mm/cycle) 2×10^–8 K ^2.25, whereK has units of kg mm^–3/2. When the respective data are plotted in terms of (K/E), whereE is Young's modulus, the crack growth behaviour for the Ni-Fe glasses approximates that for crystalline ferrous alloys. A classical chevron pattern, macroscopically at 90° to the tensile axis, is observed when amorphous metallic alloy strips fracture under plane strain conditions. On a finer scale, the chevrons exhibit a sawtooth structure, and the sawtooth surfaces show a fine scale, equi-axed vein pattern. This indicates that local failure occurs by shear rupture.