Melt fluxing to elevate the forming ability of Al-based bulk metallic glasses

Breaking the vitrification limitation of monatomic metals

The question of whether all materials can solidify into the glassy form proposed by Turnbull half a century ago remains unsolved. Some of the simplest systems of monatomic metals have not been vitrified, especially the close-packed face-centred cubic metals. Here we report the vitrification of gold, which is notoriously difficult to be vitrified, and several similar close-packed face-centred cubic and hexagonal metals using a method of picosecond pulsed laser ablation in a liquid medium. The vitrification occurs through the rapid cooling during laser ablation and the inhibition of nucleation by the liquid medium. Using this method, a large number of atomic configurations, including glassy configurations, can be generated simultaneously, from which a stable glass state can be sampled. Simulations demonstrate that the favourable stability of monatomic metals stems from the strong topological frustration of icosahedra-like clusters. Our work breaks the limitation of the glass-forming ability of matter, indicating that vitrification is an intrinsic property of matter and providing a strategy for the preparation and design of metallic glasses from an atomic configuration perspective.

Centrifugal Atomization of Glass-Forming Alloy Al86Ni8Y4.5La1.5

Centrifugal atomization is a rapid solidification technique for producing metal powders. However, its wide application has been limited to the production of common metal powders and their corresponding alloys. Therefore, there is a lack of research on the production of novel materials such as metallic glasses using this technology. In this paper, aluminum-based glassy powders (Al₈₆Ni₈Y_4.5La_1.5) were produced by centrifugal atomization. The effects of disk speed, atomization gas, and particle size on the cooling rate and the final microstructure of the resulting powder were investigated. The powders were characterized using SEM and XRD, and the amorphous fractions of the atomized powder samples were quantified through DSC analysis. A theoretical model was developed to evaluate the thermal evolution of the atomized droplets and to calculate their cooling rate. The average cooling rate experienced by the centrifugally atomized powder was calculated to be approximately 7 × 10⁵ Ks^-1 for particle sizes of 32.5 μm atomized at 40,000 rpm in a helium atmosphere. Amorphous fractions from 60% to 70% were obtained in particles with sizes of up to 125 μm in the most favorable atomization conditions.

Glass-Forming Ability and Thermal Properties of Al70Fe12.5V12.5X5(X = Zr, Nb, Ni) Amorphous Alloys via Minor Alloying Additions

The Al₇₀Fe_12.5V_12.5Ni₅, Al₇₀Fe_12.5V_12.5Zr₅ and Al₇₀Fe_12.5V_12.5Nb₅ alloys were prepared via mechanical alloying. The influence of Zr, Nb or Ni addition on the glass-forming ability of Al-Fe-V amorphous alloys have been investigated. The structure of Al₇₀Fe_12.5V_12.5Ni₅ was amorphous and Al₇₀Fe_12.5V_12.5Zr₅ was not completely amorphous by transmission electron microscopy, selected area electron diffraction and differential scanning calorimetry. Different criteria were used to evaluate the influence of the addition of alloy elements on the Glass-forming ability. The Al₇₀Fe_12.5V_12.5Ni₅ amorphous alloys exhibits higher glass-forming ability and activation energies of crystallization. Comparison of the effective atomic size ratio and mixture enthalpy on the glass-forming ability of these amorphous alloys demonstrates that the effective atomic size ratio value becomes more significant than the values of mixture enthalpy.

Fe3C cluster-promoted single-atom Fe, N doped carbon for oxygen-reduction reaction

A key challenge in carrying out an efficient oxygen reduction reaction (ORR) is the design of a highly efficient electrocatalyst that must have fast kinetics, low cost and high stability for use in an energy-conversion device (e.g. metal-air batteries). Herein, we developed a platinum-free ORR electrocatalyst with a high surface area and pore volume via a molten salt method along with subsequent KOH activation. The activation treatment not only increases the surface area to 940.8 m2 g-1 by generating lots of pores, but also promotes the formation of uniform Fe3C nanoclusters within the atomic dispersed Fe-Nx carbon matrix in the final material (A-FeNC). A-FeNC displays excellent activity and long-term stability for the ORR in alkaline media, and shows a greater half-wave potential (0.85 V) and faster kinetics toward four-electron ORR as compared to those of 20 wt% Pt/C (0.83 V). As a cathode catalyst for the Zn-air battery, A-FeNC presents a peak power density of 102.2 mW cm-2, higher than that of the Pt/C constructed Zn-air battery (57.2 mW cm-2). The superior ORR catalytic performance of A-FeNC is ascribed to the increased exposure of active sites, active single-atom Fe-N-C centers, and enhancement by Fe3C nanoclusters.

Preparation, Characterization, and Properties of Novel Ti-Zr-Be-Co Bulk Metallic Glasses

We developed novel Ti-Zr-Be-Co bulk metallic glasses through Co addition based on a ternary Ti45Zr20Be35 alloy. By altering the alloying routes and alloying contents, the influence of Co alloying on glass-forming ability, thermal stability, thermoplastic formability, crystallization behavior, and corrosion resistance has been investigated systematically. It was found that the best alloying route for enhancing the glass-forming ability, thermoplastic formability, compressive plasticity, and corrosion resistance is to replace Be by Co. Ti45Zr20Be23Co12 possesses the largest critical diameter of 15 mm for glass formation. Ti45Zr20Be27Co8 possesses the highest thermoplastic formability which is comparable to that of Vitreloy alloys. Ti45Zr20Be25Co10 exhibits the largest room temperature plasticity of 15.7% together with a high specific strength of 3.90 × 105 Nm/kg. The addition of Co also strongly affects the crystallization behavior of the base alloy, resulting in a more complex crystallization process. The corrosion resistance of Ti-Zr-Be alloy in 1 mol/L HCl solution can also be enhanced by Co alloying. The related mechanisms have been explained in detail, which provide guidance for the composition design of Ti-based metallic glasses with improved properties.

Crystallization Behavior of Al70Fe12.5V12.5Nb₅ Amorphous Alloy Formed by Mechanical Alloying

In this study, the formation and crystallization of the Al₇₀Fe_12.5V_12.5Nb₅ amorphous alloys has been investigated. The addition of Nb enhances the supercooled liquid region and glass forming ability of the Al-Fe-V amorphous alloys. The Al₇₀Fe_12.5V_12.5Nb₅ amorphous alloy exhibits two distinct crystallization steps and a large supercooled liquid region at more than 100 K. Kissinger and Ozawa analyses showed that the two activation energies for crystallization (E_x) were estimated to be 366.3 ± 23.9 and 380.5 ± 23.9 kJ/mol. Large supercooled liquid regions are expected to gain an application field of Al-based amorphous alloys.