Hierarchical Surface Pattern on Ni-Free Ti-Based Bulk Metallic Glass to Control Cell Interactions.

Ni-free Ti-based bulk metallic glasses (BMGs) are exciting materials for biomedical applications because of their outstanding biocompatibility and advantageous mechanical properties. The glassy nature of BMGs allows them to be shaped and patterned via thermoplastic forming (TPF). This work demonstrates the versatility of the TPF technique to create micro- and nano-patterns and hierarchical structures on Ti40 Zr10 Cu34 Pd14 Sn2 BMG. Particularly, a hierarchical structure fabricated by a two-step TPF process integrates 400 nm hexagonal close-packed protrusions on 2.5 µm square protuberances while preserving the advantageous mechanical properties from the as-cast material state. The correlations between thermal history, structure, and mechanical properties are explored. Regarding biocompatibility, Ti40 Zr10 Cu34 Pd14 Sn2 BMGs with four surface topographies (flat, micro-patterned, nano-patterned, and hierarchical-structured surfaces) are investigated using Saos-2 cell lines. Alamar Blue assay and live/dead analysis show that all tested surfaces have good cell proliferation and viability. Patterned surfaces are observed to promote the formation of longer filopodia on the edge of the cytoskeleton, leading to star-shaped and dendritic cell morphologies compared with the flat surface. In addition to potential implant applications, TPF-patterned Ti-BMGs enable a high level of order and design flexibility on the surface topography, expanding the available toolbox for studying cell behavior on rigid and ordered surfaces.

Size-dependent deformation behavior in nanosized amorphous metals suggesting transition from collective to individual atomic transport.

The underlying atomistic mechanism of deformation is a central problem in mechanics and materials science. Whereas deformation of crystalline metals is fundamentally understood, the understanding of deformation of amorphous metals lacks behind, particularly identifying the involved temporal and spatial scales. Here, we reveal that at small scales the size-dependent deformation behavior of amorphous metals significantly deviates from homogeneous flow, exhibiting increasing deformation rate with reducing size and gradually shifted composition. This transition suggests the deformation mechanism changes from collective atomic transport by viscous flow to individual atomic transport through interface diffusion. The critical length scale of the transition is temperature dependent, exhibiting a maximum at the glass transition. While viscous flow does not discriminate among alloy constituents, diffusion does and the constituent element with higher diffusivity deforms faster. Our findings yield insights into nano-mechanics and glass physics and may suggest alternative processing methods to epitaxially grow metallic glasses.

Size-dependent vitrification in metallic glasses.

Reducing the sample size can profoundly impact properties of bulk metallic glasses. Here, we systematically reduce the length scale of Au and Pt-based metallic glasses and study their vitrification behavior and atomic mobility. For this purpose, we exploit fast scanning calorimetry (FSC) allowing to study glassy dynamics in an exceptionally wide range of cooling rates and frequencies. We show that the main α relaxation process remains size independent and bulk-like. In contrast, we observe pronounced size dependent vitrification kinetics in micrometer-sized glasses, which is more evident for the smallest samples and at low cooling rates, resulting in more than 40 K decrease in fictive temperature, Tf, with respect to the bulk. We discuss the deep implications on how this outcome can be used to convey glasses to low energy states.

Compositional dependence of the fragility in metallic glass forming liquids.

The viscosity and its temperature dependence, the fragility, are key properties of a liquid. A low fragility is believed to promote the formation of metallic glasses. Yet, the fragility remains poorly understood, since experimental data of its compositional dependence are scarce. Here, we introduce the film inflation method (FIM), which measures the fragility of metallic glass forming liquids across wide ranges of composition and glass-forming ability. We determine the fragility for 170 alloys ranging over 25 at.% in Mg-Cu-Y. Within this alloy system, large fragility variations are observed. Contrary to the general understanding, a low fragility does not correlate with high glass-forming ability here. We introduce crystallization complexity as an additional contribution, which can potentially become significant when modeling glass forming ability over many orders of magnitude.

Nonaffine Strains Control Ductility of Metallic Glasses.

The origin of limited plasticity in metallic glasses is elusive, with no apparent link to their atomic structure. We propose that the response of the glassy structure to applied stress, not the original structure itself, provides a gauge to predict the degree of plasticity. We carried out high-energy x-ray diffraction on various bulk metallic glasses (BMGs) under uniaxial compression within the elastic limit and evaluated the anisotropic pair distribution function. We show that the extent of local deviation from the affine (uniform) deformation in the elastic regime is strongly correlated with the plastic behavior of BMGs beyond yield, across chemical compositions and sample history. The results suggest that the propensity for collective local atomic rearrangements under stress promotes plasticity.

Data-driven discovery of a universal indicator for metallic glass forming ability.

Despite the importance of glass forming ability as a major alloy characteristic, it is poorly understood and its quantification has been experimentally laborious and computationally challenging. Here, we uncover that the glass forming ability of an alloy is represented in its amorphous structure far away from equilibrium, which can be exposed by conventional X-ray diffraction. Specifically, we fabricated roughly 5,700 alloys from 12 alloy systems and characterized the full-width at half-maximum, Δq, of the first diffraction peak in the X-ray diffraction pattern. A strong correlation between high glass forming ability and a large Δq was found. This correlation indicates that a large dispersion of structural units comprising the amorphous structure is the universal indicator for high metallic glass formation. When paired with combinatorial synthesis, the correlation enhances throughput by up to 100 times compared to today's state-of-the-art combinatorial methods and will facilitate the discovery of bulk metallic glasses.

Correlation symmetry analysis of electron nanodiffraction from amorphous materials.

Angular symmetry in diffraction reflects rotational symmetry in the sample. We introduce the angular symmetry coefficient as a method to extract local symmetry information from electron nanodiffraction patterns of amorphous materials. Symmetry coefficients are the average of the angular autocorrelation function at the characteristic angles of a particular rotational symmetry. The symmetry coefficients avoid non-structural features arising from Fourier transformation and Friedel symmetry breakdown that affect the angular power spectrum approach to determining angular symmetries in amorphous nanodiffraction. Both methods require thin samples to avoid overlapping diffraction from clusters of atoms separated in the thickness of the sample, but symmetry coefficients are more forgiving. Electron nanodiffraction experiments on a Pd-based metallic glass sample demonstrate both potentially misleading information in angular power spectrum and the utility of symmetry coefficients.

Unleashing nanofabrication through thermomechanical nanomolding.

Advancements in nanotechnology require the development of nanofabrication methods for a wide range of materials, length scales, and elemental distributions. Today's nanofabrication methods are typically missing at least one demanded characteristic. Hence, a general method enabling versatile nanofabrication remains elusive. Here, we show that, when revealing and using the underlying mechanisms of thermomechanical nanomolding, a highly versatile nanofabrication toolbox is the result. Specifically, we reveal interface diffusion and dislocation slip as the controlling mechanisms and use their transition to control, combine, and predict the ability to fabricate general materials, material combinations, and length scales. Designing specific elemental distributions is based on the relative diffusivities, the transition temperature, and the distribution of the materials in the feedstock. The mechanistic origins of thermomechanical nanomolding and their homologous temperature-dependent transition suggest a versatile toolbox capable of combining many materials in nanostructures and potentially producing any material in moldable shapes on the nanoscale.

Single-Crystal Nanostructure Arrays Forming Epitaxially through Thermomechanical Nanomolding.

For nanostructures in advanced electronic and plasmonic systems, a single-crystal structure with controlled orientation is essential. However, the fabrication of such devices has remained challenging, as current nanofabrication methods often suffer from either polycrystalline growth or the difficulty of integrating single crystals with substrates in desired orientations and locations to create functional devices. Here we report a thermomechanical method for the controlled growth of single-crystal nanowire arrays, which enables the simultaneous synthesis, alignment, and patterning of nanowires. Within such diffusion-based thermomechanical nanomolding (TMNM), the substrate material diffuses into nanosized cavities under an applied pressure gradient at a molding temperature of ∼0.4 times the material's melting temperature. Vertically grown face-centered cubic (fcc) nanowires with the [110] direction in an epitaxial relationship with the (110) substrate are demonstrated. The ability to control the crystal structure through the substrate takes TMNM a major step further, potentially allowing all fcc and body-centered cubic (bcc) materials to be integrated as single crystals into devices.

Nanomolding of Gold and Gold-Silicon Heterostructures at Room Temperature.

Nanofabrication techniques are limited by at least one of the required characteristics such as choice of material, control over geometry, fabrication requirements, yield, cost, and scalability. Our previously developed method of thermomechanical nanomolding fulfills these requirements, although it requires high processing temperatures. Here, we demonstrate low-temperature molding where we utilize the enhanced diffusivity on "eutectic interfaces". Gold nanorods are molded at room temperature using Au-Si alloy as feedstock. Instead of using alloy feedstock, these "eutectic interfaces" can also be established through a feedstock-mold combination. We demonstrate this by using pure Au as feedstock, which is molded into Si molds at room temperature, and also the reverse, Si feedstock is molded into Au molds forming high aspect ratio Au-Si core-shell nanorods. We discuss the mechanism of this low-temperature nanomolding in terms of lower homologous temperature at the eutectic interface. This technique, based on enhanced eutectic interface diffusion, provides a practical nanofabrication method that eliminates the previous high-temperature requirements, thereby expanding the range of the materials that can be practically nanofabricated.

Fast Surface Dynamics on a Metallic Glass Nanowire.

The dynamics near the surface of glasses can be much faster than in the bulk. We studied the surface dynamics of a Pt-based metallic glass using electron correlation microscopy with sub-nanometer resolution. Our studies show an ∼20 K suppression of the glass transition temperature at the surface. The enhancement in surface dynamics is suppressed by coating the metallic glass with a thin layer of amorphous carbon. Parallel molecular dynamics simulations on Ni80P20 show a similar temperature suppression of the surface glass transition temperature and that the enhanced surface dynamics are arrested by a capping layer that chemically binds to the glass surface. Mobility in the near-surface region occurs via atomic caging and hopping, with a strong correlation between slow dynamics and high cage-breaking barriers and stringlike cooperative motion. Surface and bulk dynamics collapse together as a function of temperature rescaled by their respective glass transition temperatures.

Static-state particle fabrication via rapid vitrification of a thixotropic medium.

Functional particles that respond to external stimuli are spurring technological evolution across various disciplines. While large-scale production of functional particles is needed for their use in real-life applications, precise control over particle shapes and directional properties has remained elusive for high-throughput processes. We developed a high-throughput emulsion-based process that exploits rapid vitrification of a thixotropic medium to manufacture diverse functional particles in large quantities. The vitrified medium renders stationary emulsion droplets that preserve their shape and size during solidification, and energetic fields can be applied to build programmed anisotropy into the particles. We showcase mass-production of several functional particles, including low-melting point metallic particles, self-propelling Janus particles, and unidirectionally-magnetized robotic particles, via this static-state particle fabrication process.

Biocompatibility of platinum-based bulk metallic glass in orthopedic applications.

Bulk metallic glasses (BMGs) are a class of amorphous metals that exhibit high strength, ductility paired with wear and corrosion resistance. These properties suggest that they could serve as an alternative to conventional metallic implants that suffer wear and failure. In the present study, we investigated Platinum (Pt)-BMG biocompatibility in bone applications. Specifically, we investigated osteoclast formation on flat and nanopatterned Pt57.5Cu14.7Ni5.3P22.5(atomic percent) as well as titanium (control). Specifically, receptor activator of NF-κB (RANK) ligand-induced murine bone marrow derived mononuclear cell fusion was measured on multiple nanopatterns and was found to be reduced on nanorods (80 and 200 nm in diameter) and was associated with reduced tartrate-resistant acid phosphatase (TRAP) and matrix metalloproteinase (MMP9) expression. Evaluation of mesenchymal stem cell (MSC) to osteoblast differentiation on nanopatterned Pt-BMG showed significant reduction in comparison to flat, suggesting that further exploration of nanopatterns is required to have simultaneous induction of osteoblasts and inhibition of osteoclasts.Invivo studies were also pursued to evaluate the biocompatibility of Pt-BMG in comparison to titanium. Rods of each material were implanted in the femurs of mice and evaluated by x-ray, mechanical testing, micro-computed tomography (micro-CT), and histological analysis. Overall, Pt-BMG showed similar biocompatibility with titanium suggesting that it has the potential to improve outcomes by further processing at the nanoscale.

Combinatorial measurement of critical cooling rates in aluminum-base metallic glass forming alloys.

Direct measurement of critical cooling rates has been challenging and only determined for a minute fraction of the reported metallic glass forming alloys. Here, we report a method that directly measures critical cooling rate of thin film metallic glass forming alloys in a combinatorial fashion. Based on a universal heating architecture using indirect laser heating and a microstructure analysis this method offers itself as a rapid screening technique to quantify glass forming ability. We use this method to identify glass forming alloys and study the composition effect on the critical cooling rate in the Al-Ni-Ge system where we identified Al51Ge35Ni14 as the best glass forming composition with a critical cooling rate of 104 K/s.

Atomic-Scale Imprinting by Sputter Deposition of Amorphous Metallic Films.

With its ease of implementation, low cost, high throughput, and excellent feature replication accuracy, nanoimprinting is used to fabricate structures for electrical, optical, and biological applications or to modify surface properties. If ultraprecise and/or subnanometer-sized patterns are desired, nanoimprinting has shown only limited success with polymers, silica glasses, or crystalline materials. In contrast, the absence of an intrinsic length scale that would interfere with imprinting resolution enables bulk metallic glasses (BMGs) to replicate structures down to the atomic scale through thermoplastic forming (TPF). However, only a small number of BMG-forming alloys can be used for TPF-based atomic-scale imprinting. Here, we demonstrate an alternative sputter deposition-based approach for the replication of atomic-scale features that is suited for a very broad range of amorphous alloys, thereby dramatically extending the available chemistries. Additional advantages are the method's scalability, its ability to replicate a wide range of molds, its low material consumption, and the fact that the films can readily be applied onto almost any workpiece, which together open up new avenues to atomically defined surface structuring and functionalization. Our method constitutes the advancement from proof of concept to a practical and highly versatile toolbox of atomic-scale imprinting to be explored for the science and technology of atomic-scale imprinting.

Semiconductor-to-conductor transition in 2D copper(ii) oxide nanosheets through surface sulfur-functionalization.

Functionalization is a widely-used strategy to modulate and optimize the properties of materials towards various applications, including sensing, catalysis, and energy generation. While the influence of sulfur-functionalization of carbon materials and oxides like ZnO and TiO2 has been studied, far less research has been devoted to analyzing sulfur-functionalization of CuO and other transition metal oxide nanomaterials. Here, we report sulfur-functionalization of copper(ii) oxide nanosheets synthesized by using a soft-templating procedure, with sulfur-addition based on hydrogen sulfide gas as a source. The resulting sulfur-functionalization does not change the overall crystal structure and morphology of the CuO nanosheets, but leads to a decrease in surface hydroxyl groups. Sulfur induces a semiconductor-to-conductor state transition of the CuO nanosheets, which is supported by computational modeling. The metallic transition results from shifting of the Fermi level into the valence band due to formation of Cu-S bonds on the surface of the CuO nanosheets.

General Nanomolding of Ordered Phases.

Large-scale, controlled fabrication of ordered phases is challenging at the nanoscale, yet highly demanded as their well-ordered structure and chemistry is the key for advanced functionality. Here, we demonstrate a general nanomolding process of ordered phases based on atomic diffusion. Resulting nanowires are single crystals and maintain their composition and structure throughout their length, which we explain by a self-ordering process originating from their narrow Gibbs free energy. The versatility, control, and precision of this thermomechanical nanomolding method of ordered phases provides new insights into single crystal growth and suggest itself as a technology to enable wide spread usage for nanoscale and quantum devices.

Intrinsic dissipation mechanisms in metallic glass resonators.

Micro- and nanoresonators have important applications including sensing, navigation, and biochemical detection. Their performance is quantified using the quality factor Q, which gives the ratio of the energy stored to the energy dissipated per cycle. Metallic glasses are a promising material class for micro- and nanoscale resonators since they are amorphous and can be fabricated precisely into complex shapes on these length scales. To understand the intrinsic dissipation mechanisms that ultimately limit large Q-values in metallic glasses, we perform molecular dynamics simulations to model metallic glass resonators subjected to bending vibrations at low temperatures. We calculate the power spectrum of the kinetic energy, redistribution of energy from the fundamental mode of vibration, and Q vs the kinetic energy per atom K of the excitation. In the harmonic and anharmonic response regimes where there are no atomic rearrangements, we find that Q → ∞ over the time periods we consider (since we do not consider coupling to the environment). We identify a characteristic Kr above which atomic rearrangements occur, and there is significant energy leakage from the fundamental mode to higher frequencies, causing finite Q. Thus, Kr is a critical parameter determining resonator performance. We show that Kr decreases as a power-law, Kr ∼ N-k, with increasing system size N, where k ≈ 1.3. We estimate the critical strain ⟨γr⟩∼ 10-8 for micrometer-sized resonators below which atomic rearrangements do not occur in the millikelvin temperature range, and thus, large Q-values can be obtained when they are operated below γr. We also find that Kr for amorphous resonators is comparable to that for resonators with crystalline order.

Fast Screening of Corrosion Trends in Metallic Glasses.

Corrosion trends in the bulk metallic glass forming alloy system Zr-Cu-Al are studied through a fast screening visual characterization method of thin film alloy libraries prepared by magnetron co-sputtering. Significant distinct brightness changes are present within the Zr-Cu-Al system when the thin film library is immersed in 3.5 wt % NaCl. Through additional quantification of corrosion current density, a correlation between change in brightness and corrosion current density is revealed, suggesting an effective rapid screening of corrosion simply by a visual method. For materials discovery with optimized multiproperties, we utilize the corrosion fast screening results and superimpose them on the composition dependence of the glass forming ability. This allows us to rapidly identify alloys with the best combination of glass forming ability and corrosion resistance, which we then confirm in bulk form.

Accelerated discovery and mechanical property characterization of bioresorbable amorphous alloys in the Mg-Zn-Ca and the Fe-Mg-Zn systems using high-throughput methods.

Ternary amorphous alloys in the magnesium (Mg)-zinc (Zn)-calcium (Ca) and the iron (Fe)-Mg-Zn systems are promising candidates for use in bioresorbable implants and devices. The optimal alloy compositions for biomedical applications should be chosen from a large variety of available alloys with best combination of mechanical properties (modulus, strength, hardness) and biological response (in situ degradation rates, cell adhesion and proliferation). As a first step towards establishing a database designed to enable such targeted material selection, amorphous alloy composition libraries were fabricated employing a combinatorial magnetron sputtering approach where Mg, Zn, and Ca/Fe are co-deposited from separate sources onto a silicon wafer substrate. Composition analysis using energy dispersive X-ray spectroscopy documented a composition range of ∼15-85 at% Mg, ∼6-55 at% Zn, and ∼5-60 at% Ca for the Mg-Zn-Ca library and ∼26-84 at% Mg, ∼10-61 at% Zn, and ∼7-55 at% Fe for the Fe-Mg-Zn library. X-ray diffraction measurements established that amorphous alloys (i.e., glasses) form in almost the entire range of composition at the high cooling rates during sputtering for both alloy libraries. Finally, the effective material modulus, the Oliver-Pharr hardness, and the yield strength values obtained using nanoindentation reveal a wide range of mechanical properties within both systems.