A. Lindsay Greer

Metallic glasses…on the threshold

Metallic glasses, first discovered nearly 50 years ago, are currently among the most actively studied metallic materials. Available in sections up to several centimeters, with many novel, applicable properties, metallic glasses have also been the focus of research advancing our understanding of liquids and of glasses in general. Here we outline some key features of the glassy state in metals. The next few years will bring exciting advances, as we are on the threshold of exploiting new opportunities for microstructural design, opening up much broader application of the fascinating materials formed from metallic glasses and their derivatives.

Materials science: Changing face of the chameleon

Chalcogenide materials form the basis of CD and DVD technologies. But an identity crisis looms in the wider field: what role do atomic reconfiguration, electronic processes and ionic movement play in these materials?

New horizons for glass formation and stability

It has long been thought impossible for pure metals to form stable glasses. Recent work supports earlier evidence of glass formation in pure metals, shows the potential for devices based on rapid glass–crystal phase change, and highlights the lack of an adequate theory for fast crystal growth.

Condensed matter: Too hot to melt

Melting is a familiar process not expected to show surprises: the melting of ice in a cocktail is expected to produce cooling not heating. Yet just such an effect — inverse melting — has been seen during a study of phase transitions in a polymeric system. As a result the crystalline phase appears to be more disordered than the glassy phase.

Through a glass, lightly

The nature of the glass transition is a long-standing problem in condensed-matter science. New work shows that the motion of a small atomic species through a bulk metallic glass is due to two diffusion processes -- one that dominates below the transition and one that dominates above it.

Metallic glasses: Damage tolerance at a price

Metallic glasses are strong but can be brittle. The discovery of a metallic glass that also shows a high toughness against fracture is remarkable, and establishes metallic glasses, at least those based on noble metals, as materials with the highest known damage tolerance.

Soft imprint lithography of a bulk chalcogenide glass

We report on large-surface-area micro-patterning of a bulk chalcogenide glass by a PDMS soft mould. Micrometre-scale (width ~4μm and depth ~0.8 μm) test patterns such as ribs, channels and a lens array are successfully imprinted into the surface of high refractive index As3S7 bulk glass at 225°C without any applied external pressure. The mean-square roughness of the patterned glass surface is in the range 3 – 10 nm. Soft imprinting of bulk chalcogenide glass is an efficient method for reliable fabrication of optical and photonic devices.

Local microstructure evolution at shear bands in metallic glasses with nanoscale phase separation

At room temperature, plastic flow of metallic glasses (MGs) is sharply localized in shear bands, which are a key feature of the plastic deformation in MGs. Despite their clear importance and decades of study, the conditions for formation of shear bands, their structural evolution and multiplication mechanism are still under debate. In this work, we investigate the local conditions at shear bands in new phase-separated bulk MGs containing glassy nanospheres and exhibiting exceptional plasticity under compression. It is found that the glassy nanospheres within the shear band dissolve through mechanical mixing driven by the sharp strain localization there, while those nearby in the matrix coarsen by Ostwald ripening due to the increased atomic mobility. The experimental evidence demonstrates that there exists an affected zone around the shear band. This zone may arise from low-strain plastic deformation in the matrix between the bands. These results suggest that measured property changes originate not only from the shear bands themselves, but also from the affected zones in the adjacent matrix. This work sheds light on direct visualization of deformation-related effects, in particular increased atomic mobility, in the region around shear bands.

Atomic and vibrational origins of mechanical toughness in bioactive cement during setting

Bioactive glass ionomer cements (GICs) have been in widespread use for ∼40 years in dentistry and medicine. However, these composites fall short of the toughness needed for permanent implants. Significant impediment to improvement has been the requisite use of conventional destructive mechanical testing, which is necessarily retrospective. Here we show quantitatively, through the novel use of calorimetry, terahertz (THz) spectroscopy and neutron scattering, how GICs developing fracture toughness during setting is related to interfacial THz dynamics, changing atomic cohesion and fluctuating interfacial configurations. Contrary to convention, we find setting is non-monotonic, characterized by abrupt features not previously detected, including a glass–polymer coupling point, an early setting point, where decreasing toughness unexpectedly recovers, followed by stress-induced weakening of interfaces. Subsequently, toughness declines asymptotically to long-term fracture test values. We expect the insight afforded by these in situ non-destructive techniques will assist in raising understanding of the setting mechanisms and associated dynamics of cementitious materials.

Sub-micrometer soft lithography of a bulk chalcogenide glass

We demonstrate, for the first time, time- and cost-effective replication of sub-micrometer features from a soft PDMS mold onto a bulk chalcogenide glass over a large surface area. A periodic array of sub-micrometer lines (diffraction grating) with period 625 nm, amplitude 45 nm and surface roughness 3 nm was imprinted onto the surface of the chalcogenide AsSe(2) bulk glass at temperature 225°C, i.e. 5°C below the softening point of the glass. Sub-micrometer soft lithography into chalcogenide bulk glasses shows good reliability, reproducibility and promise for feasible fabrication of various dispersive optical elements, anti-reflection surfaces, 2D photonic structures and nano-structured surfaces for enhanced photonic properties and chemical sensing.