Bulk metallic glass (BMG) formers are multicomponent alloys that vitrify with remarkable ease during solidification. Technological interest in these materials has been generated by their unique properties, which often surpass those of conventional structural materials. The metastable nature of BMGs, however, has imposed a barrier to broad commercial adoption, particularly where the processing requirements of these alloys conflict with conventional metal processing methods. Research on the crystallization of BMG formers has uncovered novel thermoplastic forming (TPF)-based processing opportunities. Unique among metal processing methods, TPF utilizes the dramatic softening exhibited by a BMG as it approaches its glass-transition temperature and decouples the rapid cooling required to form a glass from the forming step. This article reviews crystallization processes in BMG former and summarizes and compares TPF-based processing methods. Finally, an assessment of scientific and technological advancements required for broader commercial utilization of BMGs will be made.

Processing of Bulk Metallic Glass

This study investigates the aqueous stability of multi-walled carbon nanotubes (MWNTs) in the presence of natural organic matter (NOM). MWNTs were readily dispersed as an aqueous suspension in both model NOM (Suwannee River NOM (SR-NOM)) solutions and natural surface water (actual Suwannee River water with unaltered NOM background), which remained stable for over 1 month. Microscopic analyses suggested that the suspension consisted primarily of individually dispersed MWNTs. Concentrations of MWNTs suspended in the aqueous phase, quantified using thermal optical transmittance analysis (TOT), ranged from 0.6 to 6.9 mg/L as initial concentrations of MWNT and SR-NOM were varied from 50 to 500 mg/L and 10 to 100 mg/L, respectively. Suwannee River water showed a similar MWNT stabilizing capacity as compared to the model SR-NOM solutions. For the same initial MWNT concentrations, the concentrations of suspended MWNT in SR-NOM solutions and Suwannee River water were considerably higher than that in a solution of ...

Natural organic matter stabilizes carbon nanotubes in the aqueous phase

It is known that nanocrystallites can form in shear bands produced during severe bending or high-energy ball milling of thin ribbons of a metallic glass. We present direct experimental evidence that highly confined and controlled local contact at the ultrafine scale in the form of quasi-static nanoindentation of a bulk glassy metal alloy at room temperature can also cause nanocrystallization. Atomic force microscopy and transmission electron microscopy show that nanocrystallites nucleate in and around shear bands produced near indents and that they are the same as crystallites formed during annealing without deformation at 783 kelvin. Analogous to results from recent experiments with glassy polymers, our results are reasoned to be a consequence of flow dilatation inside the bands and of the attendant, radically enhanced, atomic diffusional mobility inside actively deforming shear bands.

http://science.sciencemag.org/content/sci/295/5555/654.full.pdf

Nanocrystallization During Nanoindentation of a Bulk Amorphous Metal Alloy at Room Temperature

Soft magnetic materials are key to the efficient operation of the next generation of power electronics and electrical machines (motors and generators). Many new materials have been introduced since Michael Faraday's discovery of magnetic induction, when iron was the only option. However, as wide bandgap semiconductor devices become more common in both power electronics and motor controllers, there is an urgent need to further improve soft magnetic materials. These improvements will be necessary to realize the full potential in efficiency, size, weight, and power of high-frequency power electronics and high-rotational speed electrical machines. Here we provide an introduction to the field of soft magnetic materials and their implementation in power electronics and electrical machines. Additionally, we review the most promising choices available today and describe emerging approaches to create even better soft magnetic materials.

http://science.sciencemag.org/content/sci/362/6413/eaao0195.full.pdf

Soft magnetic materials for a sustainable and electrified world

Nanoparticles are discrete nanometer (10−9 m)-scale assemblies of atoms. Thus, they have dimensions between those characteristic of ions (10−10 m) and those of macroscopic materials. They are interesting because the number of atoms in the particles is small enough, and a large enough fraction of them are at, or near surfaces, to significantly modify the particle’s atomic, electronic, and magnetic structures, physical and chemical properties, and reactivity relative to the bulk material. Nanoparticle surfaces themselves may be distinctive. Particles may be terminated by atomic planes or clusters that are not common, or not found, at surfaces of the bulk mineral. These, and other size-related effects will lead to modified phase stability and changes in reaction kinetics.

What makes a nanoparticle a nanoparticle? Definitions of the size ranges for molecules, nanoparticles, and macroscopic solids must be compound specific. However, a useful upper limit for nanoparticles is the size at which one of its properties deviates from the value for the equivalent bulk material by an amount that is significantly larger than the error of the method used to make the measurement (a few percent). In practice, some characteristic will probably be different enough to warrant description as a “nanoparticle” if it is less than a few tens of nanometers in diameter, and perhaps less than a fraction of a micron in diameter.

Because of the importance of size-dependent property changes to the materials sciences, size-property relationships have been studied in detail for some systems. For example, for semiconductors, size effects become important when the particle diameter is close to the Bohr diameter of excitons in the bulk phase. Generally, semiconductor size quantization effects (relevant for naturally occurring metal sulfides, for example) appear when particles are less than 10 nm in diameter (Vogel and Urban 1997).

Definition of the …

Nanoparticles in the Environment

a b s t r a c t The rapidly solidified AlCr4.7Fe1.1Si0.3 (at.%) alloy produced at large scale by direct extrusion of gas atomised powders was investigated. Hot-extrusion yielded sound compacts and did not create any major structural changes compared to the as-atomised state. Microstructural characterisation of the as-extruded material revealed submicron scale -Al grains with an embedded Al7Cr crystalline phase and traces of a nano-scale quasicrystalline Al84.6Cr15.4 phase. The long-term thermal stability up to 300 ◦C of compacts was proven. The thermal stability of compacts was accompanied by high mechanical prop- erties, especially high strength and Young's modulus, and creep performance up to 300 ◦C. The same material was prepared by laboratory scale extrusion of cryo-chopped melt-spun ribbons as a reference material. No major differences in the structure or properties of the industrially prepared material were observed compared to the reference material.

Materials Science and Engineering A

In this work the kinetics of the high-temperature oxidation of the powder amorphous carbon and bulk single-wall carbon nanotubes is studied. The thermal degradation of the sample is measured by differential scanning calorimetry using the continuous heating regime up to 1273 K. Also, the oxidation resistance of the samples is evaluated by the mass loss in a thermogravimetric analyzer. Both flowing and static oxygen and dry-air atmospheres are used. The specific role of the external diffusivity of the reagent gas is analyzed. The kinetics of the chemical reaction is specified using the Kissinger, Coats and Redfern methods.

Kinetics of oxidation in various forms of carbon

FINEMET-type nanocrystallization kinetics

The kinetics of crystallization of Fe 80 Si x B 20 − x (with x = 2, 4, 6, 8, 10) and Fe 75 Si 15 B 10 glassy ribbons was examined using differential scanning calorimetry and high precision electrical resistivity measurements. The main crystallization parameters are presented. The corresponding structure of the materials was observed using transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis. The mechanism of crystallization is described in terms of the local ordering (cluster) model.

Emília Illeková

Papers

Materials Science and Engineering A

Kinetics of oxidation in various forms of carbon

FINEMET-type nanocrystallization kinetics

Crystallization characterisics in the FeSiB glassy ribbon system

Kinetics of intermetallic phase formation in the Ti/Al multilayers