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.

Model Metallic Glasses for Superior Electrocatalytic Performance in a Hydrogen Oxidation Reaction.

Metallic glasses or amorphous alloys, with their excellent chemical stability, disordered atomic arrangement, and ability for thermoplastic nanostructuring, show promising performance toward a range of electrocatalytic reactions in proton-exchange membrane fuel cells. However, there are knowledge gaps and a distinct lack of understanding of the role of amorphous alloy chemistry in determining their catalytic activity. Here, we demonstrate the influence of alloy chemistry and the associated electronic structure on the hydrogen oxidation reaction (HOR) activity of a systematic series of Pt42.5-xPdxCu27Ni9.5P21 bulk metallic glasses (BMGs) with x = 0 to 42.5 at%. The HOR activity and electrochemical active surface area as a function of composition were in the form of volcano plots, with a peak around equal proportion of Pt and Pd. The lower relative electron work function and higher binding energy of the Pt core level explain the reduced charge-transfer resistance and improved electrocatalytic activity due to weakened chemisorption of protons in the mid-range composition. Density functional theory calculations show the lower free energy change and higher hydrogen adsorption density for these Pt42.5-xPdxCu27Ni9.5P21 BMGs, suggesting a synergistic effect from the presence of both noble metals, Pt and Pd.

Solid State Joining of a Cold Rolled Zr-Based Bulk Metallic Glass to a Wrought Aluminum Alloy by Power Ultrasonics.

Ultrasonic metal welding (UMW) enables joining in the solid state at relative low temperatures with short cycle times. This technique is of particular interest for joining metallic glasses to each other or to other materials, because crystallization of the amorphous structure can be prevented due to the low thermal loading and the rapidity of the process. In this work, UMW is applied to join one 1 mm thick sheet of a commercial wrought aluminum alloy (AA5754) and one 0.4 mm thick strip of a commercial Zr-based bulk metallic glass (AMZ4). The introduced heat of the welding process is detected with thermocouples and thermal imaging. To investigate the strength of the joint and the influence on the microstructure, mechanical tensile tests are carried out in combination with scanning electron microscopy and differential scanning calorimetry. The results show that ultrasonic metal welding is a suitable technique to join amorphous bulk metallic glasses to crystalline aluminum alloys. The metallic glass component retains its amorphous structure in the joint, and the joint strength is higher than the strength of the Al sheet. These findings will help to develop future applications of BMG-based multi-material components, including medical tools.

Effect of composition and thermal history on deformation behavior and cluster connections in model bulk metallic glasses.

The compositional dependence and influence of relaxation state on the deformation behavior of a Pt-Pd-based bulk metallic glasses model system was investigated, where platinum is systematically replaced by topologically equivalent palladium atoms. The hardness and modulus increased with rising Pd content as well as by annealing below the glass transition temperature. Decreasing strain-rate sensitivity and increasing serration length are observed in nano indentation with increase in Pd content as well as thermal relaxation. Micro-pillar compression for alloys with different Pt/Pd ratios validated the greater tendency for shear localization and brittle behavior of the Pd-rich alloys. Based on total scattering experiments with synchrotron X-ray radiation, a correlation between the increase in stiffer 3-atom cluster connections and reduction in strain-rate sensitivity, as a measure of ductility, with Pd content and thermal history is suggested.

Influence of Initial Temperature and Convective Heat Loss on the Self-Propagating Reaction in Al/Ni Multilayer Foils.

A two-dimensional numerical model for self-propagating reactions in Al/Ni multilayer foils was developed. It was used to study thermal properties, convective heat loss, and the effect of initial temperature on the self-propagating reaction in Al/Ni multilayer foils. For model adjustments by experimental results, these Al/Ni multilayer foils were fabricated by the magnetron sputtering technique with a 1:1 atomic ratio. Heat of reaction of the fabricated foils was determined employing Differential Scanning Calorimetry (DSC). Self-propagating reaction was initiated by an electrical spark on the surface of the foils. The movement of the reaction front was recorded with a high-speed camera. Activation energy is fitted with these velocity data from the high-speed camera to adjust the numerical model. Calculated reaction front temperature of the self-propagating reaction was compared with the temperature obtained by time-resolved pyrometer measurements. X-ray diffraction results confirmed that all reactants reacted and formed a B2 NiAl phase. Finally, it is predicted that (1) increasing thermal conductivity of the final product increases the reaction front velocity; (2) effect of heat convection losses on reaction characteristics is insignificant, e.g., the foils can maintain their characteristics in water; and (3) with increasing initial temperature of the foils, the reaction front velocity and the reaction temperature increased.

The effect of shear on the liquid-liquid transition and crystallization of the undercooled Zr41.2Ti13.8Cu12.5Ni10.0Be22.5(Vit1) bulk metallic glass forming alloy.

An entropy driven liquid-liquid transition (LLT) from a fragile (less ordered) to a strong (highly ordered) liquid occurs in the phase during undercooling. In this work, we show that this ordering transition as well as the applied shear rate affect the onset of crystallization. By recording simultaneously melt viscosity and temperature profiles, we quantitatively determine the shift in the upper part of the time-temperature-transformation diagram of Vit1 to shorter times with increasing shear rate. This acceleration in nucleation rate can be explained by the classical nucleation theory of crystals only if we take into consideration the effect of both shear flow and equilibrium viscosity. A critical assessment of the results concludes that shearing must first trigger the nucleation of the strong liquid from the fragile liquid and that the crystallization proceeds in a second step from the strong liquid. The fragile-to-strong transition decreases the configurational entropy of the liquid leading to a smaller interfacial energy between liquid and crystal, thus reducing the activation barrier for crystallization.

Vitrification decoupling from α-relaxation in a metallic glass.

Understanding how glasses form, the so-called vitrification, remains a major challenge in materials science. Here, we study vitrification kinetics, in terms of the limiting fictive temperature, and atomic mobility related to the α-relaxation of an Au-based bulk metallic glass former by fast scanning calorimetry. We show that the time scale of the α-relaxation exhibits super-Arrhenius temperature dependence typical of fragile liquids. In contrast, vitrification kinetics displays milder temperature dependence at moderate undercooling, and thereby, vitrification takes place at temperatures lower than those associated to the α-relaxation. This finding challenges the paradigmatic view based on a one-to-one correlation between vitrification, leading to the glass transition, and the α-relaxation. We provide arguments that at moderate to deep undercooling, other atomic motions, which are not involved in the α-relaxation and that originate from the heterogeneous dynamics in metallic glasses, contribute to vitrification. Implications from the viewpoint of glasses fundamental properties are discussed.

Ultrafast scanning calorimetry of newly developed Au-Ga bulk metallic glasses.

The isothermal crystallization times and critical cooling rates of the liquid phase are determined for the two bulk metallic glass forming alloys Au49Ag5.5Pd2.3Cu26.9Si16.3 and Au51.6Ag5.8Pd2.4Cu20.2Ga6.7Si13.3 by using fast differential scanning calorimetry, covering the whole timescale of the crystallization event of the metallic melt. In the case of Au49Ag5.5Pd2.3Cu26.9Si16.3, a typical crystallization nose was observed, whereas for the Au51.6Ag5.8Pd2.4Cu20.2Ga6.7Si13.3, a more complex crystallization behavior with two distinct crystallization noses was found. Even for the complex crystallization behavior of the Au51.6Ag5.8Pd2.4Cu20.2Ga6.7Si13.3 alloy it is shown that the minimal isothermal nose time [Formula: see text] does allow for a quantification of the macroscopic critical thickness. It is discussed in contrast to the critical cooling rate, which is found to allow less exact calculations of the critical thickness and thus does not correlate well with the critical cooling rate from macroscopic experiments. Additionally the crystallization data of Au49Ag5.5Pd2.3Cu26.9Si16.3 was modeled using classical nucleation theory with the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation, enabling a determination of the interfacial energy.

X-Ray Photon Correlation Spectroscopy Reveals Intermittent Aging Dynamics in a Metallic Glass.

We use coherent x rays to probe the aging dynamics of a metallic glass directly on the atomic level. Contrary to the common assumption of a steady slowing down of the dynamics usually observed in macroscopic studies, we show that the structural relaxation processes underlying aging in this metallic glass are intermittent and highly heterogeneous at the atomic scale. Moreover, physical aging is triggered by cooperative atomic rearrangements, driven by the relaxation of internal stresses. The rich diversity of this behavior reflects a complex energy landscape, giving rise to a unique type of glassy-state dynamics.