Jun-Qiang Wang

Excellent capability in degrading azo dyes by MgZn-based metallic glass powders

The lack of new functional applications for metallic glasses hampers further development of these fascinating materials. In this letter, we report for the first time that the MgZn-based metallic glass powders have excellent functional ability in degrading azo dyes which are typical organic water pollutants. Their azo dye degradation efficiency is about 1000 times higher than that of commercial crystalline Fe powders, and 20 times higher than the Mg-Zn alloy crystalline counterparts. The high Zn content in the amorphous Mg-based alloy enables a greater corrosion resistance in water and higher reaction efficiency with azo dye compared to crystalline Mg. Even under complex environmental conditions, the MgZn-based metallic glass powders retain high reaction efficiency. Our work opens up a new opportunity for functional applications of metallic glasses.

Nucleation of shear bands in amorphous alloys

Significance A shear band is a region of highly localized plastic flow that develops during loading in both crystalline and amorphous materials. Shear bands directly determine the ductility of an amorphous phase, but relatively little is known about their nucleation. We use nanoindentation to probe shear band nucleation by measuring the first pop-in event during loading which is associated with shear band formation. We analyze a large number of independent measurements on four different metallic glasses and reveal a bimodal distribution of nucleation events that operate at different stress levels. The discovery of multiple shear band nucleation sites challenges the current view of a single type of site and offers opportunities for controlling the ductility of amorphous alloys. The initiation and propagation of shear bands is an important mode of localized inhomogeneous deformation that occurs in a wide range of materials. In metallic glasses, shear band development is considered to center on a structural heterogeneity, a shear transformation zone that evolves into a rapidly propagating shear band under a shear stress above a threshold. Deformation by shear bands is a nucleation-controlled process, but the initiation process is unclear. Here we use nanoindentation to probe shear band nucleation during loading by measuring the first pop-in event in the load–depth curve which is demonstrated to be associated with shear band formation. We analyze a large number of independent measurements on four different bulk metallic glasses (BMGs) alloys and reveal the operation of a bimodal distribution of the first pop-in loads that are associated with different shear band nucleation sites that operate at different stress levels below the glass transition temperature, Tg. The nucleation kinetics, the nucleation barriers, and the density for each site type have been determined. The discovery of multiple shear band nucleation sites challenges the current view of nucleation at a single type of site and offers opportunities for controlling the ductility of BMG alloys.

Soft ytterbium-based bulk metallic glasses with strong liquid characteristic by design

A family of Yb-based bulk metallic glasses with excellent glass-forming ability has been fabricated based on the elastic moduli correlations. The YbZnMg(Cu) glasses exhibit very strong liquid characteristic in fragility (m=26±5), while soft mechanical characteristics, such as low bulk elastic modulus (e.g., Young’s modulus is about 26.5 GPa), small Poisson’s ratio (0.276), low Vickers hardness (1.52 GPa) and Debye temperature, and exceptionally low glass transition temperature (Tg∼347 K). The soft bulk metallic glasses with exceptional values of Tg, fragility, Debye temperature, and elastic moduli confirm some found correlations in metallic glasses.

Correlations between elastic moduli and molar volume in metallic glasses

We report clear correlations between bulk modulus (K) and average molar volume Vm, and between Poisson’s ratio ν and Vm for various bulk metallic glasses. The origin for the correlations between elastic moduli and Vm are discussed. The established correlation, associated with Poisson’s ratio ν, and since the ν correlates with plasticity of metallic glasses, indicates that the average molar volume is important factor to be considered for plastic metallic glasses searching. The found correlations also suggest a close relation between the mechanical properties and the short-range atomic bonding, and assist in understanding deformation behavior in metallic glasses.

Core–shell-structured nanoporous PtCu with high Cu content and enhanced catalytic performance

A core–shell-structured bimetallic nanoporous PtCu catalyst with a high non-noble metal content (Cu: ∼55 at%) and uniformly distributed ultrafine ligaments (∼3 nm) is fabricated by one-step dealloying a well-designed Pt4Cu21Mn75 single-phase ternary precursor in 1 M (NH4)2SO4 aqueous solution. The one-step dealloying involves a two-step corrosion process: one is fast dealloying the most active Mn from the ternary alloy to form nanoporous PtCu and the next step is a slow dealloying process which would slowly dissolve Cu from the PtCu alloy ligament surface forming a core–shell-structured nanoporous PtCu alloy with a Pt shell and a PtCu alloy core. Electrochemical measurements manifest that the core–shell-structured nanoporous PtCu exhibits greatly enhanced catalytic activity towards the electro-oxidation of methanol and formic acid compared with both nanoporous Pt and the state-of-the-art Pt/C catalyst. With evident advantages of facile preparation and enhanced catalytic performance, the nanoporous core–shell-structured PtCu catalyst is very promising as an anode catalyst in fuel cells. Moreover, this strategy (i.e., dealloying well-designed Mn-based ternary alloys) can also be used to fabricate other uniform nanoporous core–shell-structured alloys such as the nanoporous NiCu alloy.

Possible existence of two amorphous phases of D-mannitol related by a first-order transition.

We report that the common polyalcohol D-mannitol may have two amorphous phases related by a first-order transition. Slightly above its glass transition temperature Tg (284 K), the supercooled liquid (SCL) of D-mannitol transforms to a low-energy, apparently amorphous phase with stronger hydrogen bonds. The enthalpy of this so-called Phase X is approximately halfway between those of the known amorphous and crystalline phases, a position low for glass aging and high for crystal polymorphs. Similar to the SCL, Phase X is transparent with broad X-ray diffraction and Raman scattering; upon temperature cycling, it exhibits a glass-transition-like change of heat capacity. On fast heating, Phase X transforms back to the SCL near Tg + 50 K, enabling a determination of their equilibrium temperature. The presence of D-sorbitol as a plasticizer enables observation of a first-order transition from the SCL to Phase X entirely in the liquid state (liquid-liquid transition). The transition from D-mannitols SCL to Phase X has intriguing similarities with the formation of the glacial phase of triphenyl phosphite (TPP) and the conversion from high-density to low-density amorphous ice, both studied intensely in the context of polyamorphism. All three processes occur near Tg with substantial enthalpy decrease toward the crystalline phases; the processes in water and D-mannitol both strengthen the hydrogen bonds. In contrast to TPP, D-mannitols Phase X forms more rapidly and can transform back to the SCL. These features make D-mannitol a valuable new model for understanding polyamorphism.

High-entropy bulk metallic glasses as promising magnetic refrigerants

In this paper, the Ho20Er20Co20Al20RE20 (RE = Gd, Dy, and Tm) high-entropy bulk metallic glasses (HE-BMGs) with good magnetocaloric properties are fabricated successfully. The HE-BMGs exhibit a second-order magnetic phase transition. The peak of magnetic entropy change ( ΔSMpk) and refrigerant capacity (RC) reaches 15.0 J kg−1 K−1 and 627 J kg−1 at 5 T, respectively, which is larger than most rare earth based BMGs. The heterogeneous nature of glasses also contributes to the large ΔSMpk and RC. In addition, the magnetic ordering temperature, ΔSMpk and RC can be widely tuned by alloying different rare earth elements. These results suggest that the HE-BMGs are promising magnetic refrigerant at low temperatures.

Characterization of activation energy for flow in metallic glasses

The molar volume (V{sub m}) scaled flow activation energy ({Delta}E), namely as the activation energy density {rho}{sub E}={Delta}E/V{sub m}, is proposed to describe the flow of metallic glasses. Based on the energy landscape, both the shear and bulk moduli are critical parameters accounting for the {rho}{sub E} of both homogeneous and inhomogeneous flows in metallic glasses. The expression of {rho}{sub E} is determined experimentally to be a simple expression of {rho}{sub E}=(10/11)G+(1/11)K. The energy density perspective depicts a realistic picture for the flow in metallic glasses and is suggestive for understanding the glass transition and deformation in metallic glasses.

Extended elastic model for flow in metallic glasses

Vertically aligned Ni nanowires and nanotubes have been electrodeposited in alumina templates at room temperature. The detailed study of angular dependent coercivity and squareness demonstrates that the magnetic easy axis of Ni nanowires is perpendicular to that of Ni nanotubes axis. The mechanisms of magnetization reversal in Ni nanowires and Ni nanotubes are found to occur through the nucleation mode with the propagation of transverse domain wall and curling mode, respectively. Field dependant magnetization results at different temperatures have depicted that the magnetocrystalline anisotropy might cause a crossover of easy axis at room temperature to that of low temperature in both Ni nanowires and nanotubes. Furthermore, the variation in temperature dependent coercivity illustrates that the magnetoelastic anisotropy induced by the alumina matrix plays a dominant role in the magnetization reversal of the nanowires and nanotubes at low temperature.

Interactions of Shear Bands in a Ductile Metallic Glass

Shear bands play a key role in the plastic deformation of metallic glasses (MGs). Even though there are extensive studies on the initiation and propagation of shear bands, the interactions among them have not been systematically studied yet. The interactions between the primary shear bands (PSBs) and secondary shear bands (SSBs) in a ductile Zr-based MG were studied. The residual stress near PSBs can deflect the propagation direction and reduce the propagation velocity of SSBs, which contributes to the plasticity and toughness of the MG. It was demonstrated that the probability and strength of the interactions between PSBs and SSBs would become stronger for MGs with larger Young s modulus and smaller shear modulus, i. e., larger Poisson s ratio. These results are valuable in understanding the plastic deformation of MGs and may be helpful in designing new MGs with desirable mechanical properties.