Ke-Fu Yao

Superductile bulk metallic glass

Usually, monolithic bulk metallic glasses undergo inhomogeneous plastic deformation and exhibit poor ductility (<2%) at room temperature. We report a newly developed Pd–Si binary bulk metallic glass, which exhibits a uniform plastic deformation and a large plastic engineering strain of 82% and a plastic true strain of 170%, together with initial strain hardening, slight strain softening and final strain hardening characteristics. The uniform shear deformation and the ultrahigh plasticity are mainly attributed to strain hardening, which results from the nanoscale inhomogeneity due to liquid phase separation. The formed nanoscale inhomogeneity will hinder, deflect, and bifurcate the propagation of shear bands.

Fe-based bulk metallic glass with high plasticity

Fe-based bulk metallic glasses usually exhibit very poor ductility (<0.5%), which has limited their applications. Here the authors report an Fe-based bulk metallic glass which shows a plastic strain of ∼5.2%, together with high strength and distinct strain-hardening characteristics. Its yield strength is ∼2.32GPa, while the ultimate strength is ∼2.80GPa due to the strain-hardening effect. Multiple shear bands and related shear ledges are observed on the deformed specimen. The high plasticity and strain hardening are attributed to the nanoscale inhomogeneity that resulted from liquid phase separation, which can hinder the propagation of shear bands and promote multiple shearing.

Spark plasma sintering and thermal conductivity of carbon nanotube bulk materials

Carbon nanotube (CNT) bulk samples were fabricated by spark plasma sintering (SPS), which, as a rapid consolidation technique, preserved the phase structure and diameter of cylindrical tubules of the CNTs even at high temperatures of up to 2000°C. The thermal conductivity of the resultant bulk samples was measured by the conventional laser-flash method, and the corresponding thermal conductivity was found to be as low as 4.2W∕m∕K at room temperature. This low thermal conductivity of the CNT bulk materials was explained on the basis of multiple physical elements including intensive tube–tube interactions. CNT bulk materials may find potential applications as thermoelectric materials that require low thermal conductivity, but high electrical conductivity.

Highly Uniform and Reproducible Surface Enhanced Raman Scattering on Air-stable Metallic Glassy Nanowire Array

Preparation of surface enhanced Raman scattering (SERS) nanostructures with both high sensitivity as well as high reproducibility has always been difficult and costly for routine SERS detection. Here we demonstrate air-stable metallic glassy nanowire arrays (MGNWAs), which were prepared by a cheap and rapid die nanoimprinting technique, could exhibit high SERS enhancement factor (EF) as well as excellent reproducibility. It shows that Pd40.5Ni40.5P19 MGNWA with nanowires of 55 nm in diameter and 100 nm in pitch possesses high SERS activity with an EF of 1.1 × 105, which is 1–3 orders of magnitudes higher than that of the reported crystal Ni-based nanostructures, and an excellent reproducibility with a relative standard deviation of 9.60% measured by 121 points over an area of 100 μm*100 μm. This method offers an easy, rapid, and low-cost way to prepare highly sensitive and reproducible SERS substrates and makes the SERS more practicable.

Concept of chemical short range order domain and the glass forming ability in multicomponent liquid

Abstract The concept of multicomponent chemical short-range order (MCSRO) domain is systematically developed by the experimental investigation of Zr–Ti–Cu–Ni–Al bulk metallic glass (BMG) and thermodynamic modeling and calculation. The existence of MCSRO domains in Zr-based BMG is verified by the observations of high-resolution transmission electron microscopy (HRTEM) images and the analysis of nano-beam electron diffraction patterns. The size of the nano-beam used in this work is 0.5 nm in diameter. Thermodynamic evaluation of the melt composed of multiple-MCSRO domains and glass-forming ability (GFA) based on the concept of MCSRO domains has also been conducted. It is indicated that the thermodynamic calculation of the GFA based on MCSRO model is consistent with the experimental data of crystallization activation energy and glass transition temperature for Ni-Zr and Zr-Cu binary alloys, and with supercooled liquid region (Δ T x ) for Zr–Ni–Al ternary alloy. The existence of MCSRO domain lowers the free energy of the melt (Δ G MCSRO ), resulting in a large undercooling and a larger energy barrier to the nucleation of a critical crystalline nucleus. Large Δ G MCSRO , low melting point as well as co-existence of multiple MCSRO domains are valid criterion for the valuation of GFA.

Insight into the high reactivity of commercial Fe–Si–B amorphous zero-valent iron in degrading azo dye solutions

Improving intrinsic reactivity is one of the key requirements in applying zero-valent iron in the field. As a new kind of zero-valent iron, iron based amorphous alloys were recently found to be capable of rapidly remediating wastewater. However, the mechanisms for the rapid degradation have not yet been fully understood. In this study, commercial Fe–Si–B amorphous alloy ribbons (Fe–Si–BAR) were used to degrade azo dyes (Direct Blue 6 and Orange II) to study the reaction kinetics, pathway and mechanism behind the high reactivity of these iron based amorphous alloys. The results show that, under the same conditions, the surface normalized reaction rate constants for the decomposition of Orange II and Direct Blue 6 by Fe–Si–BAR could be 1300 and 60 times larger respectively than those obtained by using 300 mesh iron powders. Through UV-vis spectrophotometry and mass spectrometry, it is found that the intermediate products of the azo dyes degraded by Fe–Si–BAR are similar to those produced in degradation by iron powders. However, the controlling step of the degradation reaction by Fe–Si–BAR turns out to be the diffusion process rather than the surface chemical reaction found in the reaction by iron powders. Further analysis indicates that the high degradation efficiency of Fe–Si–BAR results from its amorphous structure and the metalloid additions, which could enhance the catalytic effect and promote the formation of a non-compact and easily detached oxide layer on the surface. The experiments under different environmental conditions show that the factors that influence the degradation efficiency of crystalline iron powders affect that of Fe–Si–BAR in a similar way, but Fe–Si–BAR is capable of efficiently degrading wastewater under broader conditions than the crystalline iron powders. The results indicate that Fe–Si–BAR is a promising environmental catalyst for wastewater treatment.

Fabrication and microwave absorption properties of carbon-coated cementite nanocapsules

By utilizing a simple and low-cost arc-discharge method in either liquid nitrogen or ethanol at ambient temperature and pressure, carbon-coated cementite (Fe3C) nanocapsules, with size ranges of 10–60 nm and 10–20 nm, respectively, have been synthesized on a large scale. The Fe3C/C nanocapsules synthesized in different media possess similar permeability but different permittivity, which results from the different defect amounts within the carbon shell. It has been found that the as-prepared products exhibit different electromagnetic wave absorption abilities: for the ones prepared in liquid nitrogen, the optimal reflection loss is above -10 dB in the range of 1-18 GHz with the thickness ranging from 1 to 10 mm; meanwhile, for those fabricated in ethanol, the reflection loss could be below -20 dB within the thickness range of 1.5-2.4 mm in the frequency range of 10-15 GHz, and reach -38 dB at a thickness of 1.9 mm with a matching frequency of 12.9 GHz. This indicates that the nanocapsules prepared in ethanol exhibit good electromagnetic wave absorption properties. These results provide a new way to fabricate carbon-coated Fe3C nanocapsules with the ability of electromagnetic wave absorption.

Two-zone heterogeneous structure within shear bands of a bulk metallic glass

Shear bands, the main plastic strain carrier in metallic glasses, are severely deformed regions often considered as disordered and featureless. Here we report the observations of a sandwich-like heterogeneous structure inside shear bands in Pd40.5Ni40.5P19 metallic glass sample after plastic deformation by high-resolution transmission electron microscopy. The experimental results suggest a two-step plastic deformation mechanism with corresponding microstructure evolution at atomic scale, which may intimately connected to the stability of the shear band propagation and the overall plastic deformability.

Direct experimental evidence of nano-voids formation and coalescence within shear bands

Fracture mechanisms of metallic glasses are fundamentally different from that of crystalline alloys. Nano-voids formation and coalescence inside shear bands were believed to be one of the reasons causing the final failures. Although molecule dynamic simulations have successfully simulated cavitation in shear bands of brittle metallic glasses, direct experimental evidences are still rare. By carefully examining the shear bands of Pd40.5Ni40.5P19, nano-voids and their coalescence have been observed in the center-diffused region of shear bands. It is experimentally confirmed that nano-voids formation and their coalescence into large voids within shear bands is one of fracture mechanisms of metallic glasses.

Molecular dynamic simulations and atomic structures of amorphous materials

A long-standing issue of using molecular dynamics (MD) to simulate local atomic structures in nonequilibrium metals and alloys is the huge difference in cooling rates used in experimental studies and theoretical calculations. In this study, a unique approach was introduced to correct the fast time steps involved in the MD calculations. This approach has demonstrated various medium-range ordered zones with imperfect ordered packing, which are verified experimentally by high-resolution transmission electron microscopy and its selected simulation imaging of Zr2Ni glass.