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Acta materialia 253, 118968 (2023). doi:10.1016/j.actamat.2023.118968, Ni-Nb-based bulk glass-forming alloys are among the most promising amorphous metals for industrial applica- tions due to their incomparable combination of strength, hardness, elasticity and plasticity. However, the main drawback is the limited glass-forming ability, narrowing the field of application to solely small components. In this study, we show that minor additions of P to the binary Ni-Nb system increase the glass-forming ability by 150 % to a record value of 5 mm. P can be easily added by using an industrial Ni-P pre-alloy which is readily available. The partial substitution of Nb by Ta further boosts the glass-forming ability to values 200 % higher than that of the binary base alloy. Besides conventional X-ray diffraction measurements, the amorphous nature of the samples is verified by high-energy synchrotron X-ray diffraction experiments. Moreover, the mechanical properties of the new alloy compositions are characterized in uniaxial compression tests and Vickers hardness measurements, showing a high engineering yield strength of 3 GPa, an extended plastic regime up to 10 % strain to failure and an increase of the hardness to a maximum value of 1000 HV5. Additionally, calorimetric mea- surements reveal that the modified alloys feature an extended supercooled liquid region up to 69 K upon heating, permitting thermoplastic micro molding of amorphous feedstock material., Published by Elsevier Science, Amsterdam [u.a.]

The 18kt gold-based bulk glass-forming composition Au49Ag5.5Pd2.3Cu26.9Si16.3 is modified in order to reduce its fast surface tarnishing caused by internal oxidation of Si, partitioning and fast out-diffusion of Cu and Au. By partially substituting Cu by the species Ga or Sn, the formation of Si as primary crystalline phase is facilitated and a distinct increase of the liquidus temperature is observed. It is found that a reduction of the Si content recovers the glass-forming ability of the novel Ga/Sn-containing alloys, reflected by an increasing reduced glass transition temperature, Trg, and a decreasing driving force for crystallization. This strategy is used to reduce the amount of Cu and Si, which are responsible for the fast surface tarnishing. Corrosion studies of the novel alloys reveal that the Cu release into an artificial saliva test solution is up to eight times lower than that of the mother alloy and the corrosion depth in artificial saliva is drastically lowered by 50% for the Ga-bearing alloys and by 80% for the Sn-bearing alloys. Keywords: Bulk metallic glass, Gold, Corrosion, Alloy development, Jewelry application

Effective strategies to improve the tarnishing resistance of the 18 K (karat) gold-based bulk glass-forming composition Au49Ag5.5Pd2.3Cu26.9Si16.3 were recently found, with the addition of Ga at the expense of Cu and a sufficient reduction of Si. However, the modification of the alloy is accompanied by a reduction of the glass-forming ability (GFA) from 5 to 2 mm in terms of the critical casting thickness, and eventually collapses for Ga contents higher than about 9 at%. In this work, thermodynamic and kinetic studies of the newly discovered 18 K Au-Ag-Pd-Cu-Ga-Si system shed light on the reason for the loss in GFA with increasing Ga content. Investigations of the liquid kinetics in terms of viscosity and transition time, assessed by three-point beam bending and the Tg-shift method, reveal an unexpected change of the fragility to stronger liquid kinetics with increasing Ga concentration, which is usually attributed to an increase in the GFA. Thermodynamic considerations based on specific heat capacity measurements reveal, in contrast, an ascending driving force for crystallization with an increasing Ga-content, accountable for the drop in GFA. Therefore, a Ga-rich melt beyond the threshold concentration of 9 at% is not desirable for the production of bulk metallic glasses (BMGs) from the liquid state. With the intention to outrun this barrier, while preserving the amorphous structure, Ga-ion implantation with a liquid metal focused ion beam source is used as a post-processing routine for a cast Ga-containing 18 K Au-BMG. Hence, the thermodynamic borderline concentration is successfully crossed with additional tremendous improvement of the tarnishing resistance in the Ga-enriched areas.

Laser powder bed fusion (PBF-LB/M) of bulk metallic glasses (BMGs) has experienced growing scientific and industrial interest in the last years, with a special focus on application relevant systems based on zirconium. The high cooling rates and the layer-wise build-up process allow overcoming size and geometry limitations typical for conventional casting routes. Yet, the novel production approach requires different alloy characteristics than casting processes. The present work reports for the first time on the PBF-LB/M-processing of three CuTi-based bulk metallic glass formers in the Vit101 system, allowing to exceed the mechanical performance of most additively formed Zr-based BMGs. Furthermore, the influence of alloy properties like thermal stability and toughness on the PBF-LB/M applicability are systematically studied. Thermal stability plays a minor role to produce amorphous specimen, while notch toughness is found to be a more crucial aspect to achieve parts with low defect density and resulting high mechanical performance. The results suggest fundamentally different alloy development strategies adapted to the needs of the PBF-LB/M-process, leaving classical casting-based optimization of glass forming ability behind and evolving towards a rather toughness-oriented optimization.

Materials and design 209, 109976 (2021). doi:10.1016/j.matdes.2021.109976, Additive manufacturing of Zr-based bulk metallic glasses (BMGs) is subject to growing scientific and industrial attention. Laser-based powder bed fusion of metals (PBF-LB/M) becomes a key technology to overcome current restrictions of size and geometry in the manufacturing of BMGs. For industrial appli- cation, further knowledge about defect formation, such as porosity and crystallization, is mandatory to develop processing strategies and suitable quality assurance. In this context, the influence of the particle size distribution, oxygen contamination, and applied process parameters during the PBF-LB/M of the glass-forming alloy AMZ$_4$ (in at.% Zr$_{59.3}$Cu$_{28.8}$Al$_{10.4}$Nb$_{1.5}$) on the structural and mechanical properties were evaluated. It was found that the addition of SiO$_2$ flow aid to the feedstock is suitable to increase flowability without impeding fabrication of the amorphous material. Furthermore, the processing of par- tially crystalline powder particles into amorphous samples is demonstrated. It indicates that today’s high effort producing amorphous powders and thus the production costs can be reduced. Flexural bending tests and high-energy synchrotron X-ray diffraction reveal that the powder feedstock’s oxygen content is crucial for the amorphization, embrittlement, and flexural strength of PBF-LB/M processed Zr-based BMGs., Published by Elsevier Science, Amsterdam [u.a.]

Acta materialia 220, 117300 (2021). doi:10.1016/j.actamat.2021.117300, Contrary to basic hard sphere structure models, recent studies revealed, significant structural differences between Pt-Cu-Ni-P and Pd-Cu-Ni-P metallic glass-forming liquids with the same stoichiometry. To cover the compositional space between both systems, Platinum is subsequently replaced by Palladium in the composition (Pt/Pd)$_{42.5}$Cu$_{27}$Ni$_{9.5}$P$_{21}$. For this systematic set of alloys, the thermodynamic properties, such as isobaric heat capacity, enthalpy and Gibbs free energy are assessed. A systematic drop of the Gibbs free energy difference between crystal and liquid, providing a lower estimate of the driving force for crystallization was observed, underlining the high glass-forming ability of the Pd-rich systems. Contrary to kinetic fragility data, a change of the thermodynamic fragility can be observed, drawing the picture of an increasing thermodynamically strong behavior with rising Pd-content. Further the temperature induced changes of the total structure factors S(Q) were monitored using high-energy synchrotron X-ray diffraction. Focus was laid on the changes on the medium-range length scale, by analyzing changes of the first sharp diffraction peak. Here a good correlation of the changes in peak-width and the thermodynamic fragility was found. From the determination of the excess enthalpy, large amounts of residual enthalpy in the glassy state were observed for the Pt-rich alloys, supporting the increased ductility of these alloys. The current findings further carve out the different roles of the topologically similar Pt and Pd in the Pt/Pd-Cu-Ni-P alloy system and how the change of the structural motifs on the medium range order is structurally influencing thermal properties such as enthalpy and heat capacity., Published by Elsevier Science, Amsterdam [u.a.]

Physical review letters 125(5), 055701 (2020). doi:10.1103/PhysRevLett.125.055701, We present a detailed investigation of the wave-vector dependence of collective atomic motion in Au$_{49}$Cu$_{26.9}$Si$_{16.3}$Ag$_{5.5}$Pd$_{2.3}$ and Pd$_{42.5}$Cu$_{27}$Ni$_{9.5}$P$_{21}$ supercooled liquids close to the glass transition temperature. Using x-ray photon correlation spectroscopy in a previously uncovered spatial range of only a few interatomic distances, we show that the microscopic structural relaxation process mimics the structure and presents a marked slowing down at the main average interparticle distance. This behavior is accompanied by dramatic changes in the shape of the intermediate scattering functions, which suggest the presence of large dynamical heterogeneities at length scales corresponding to a few particle diameters. A ballisticlike mechanism of particle motion seems to govern the structural relaxation of the two systems in the highly viscous phase, likely associated with hopping of caged particles in agreement with theoretical studies., Published by APS, College Park, Md.

The effects of minor sulfur addition on the glass-forming ability (GFA), phase formation, thermal stability, and mechanical properties of the Cu47.5Zr45.1Al7.4 bulk metallic glass (BMG) were studied by conventional X-ray diffraction (XRD), in-situ high-energy synchrotron XRD, calorimetry, thermomechanical analysis, and scanning electron microscopy. Results show that sulfur has positive and negative effects on the GFA and thermal stability of Cu-Zr-Al alloys. On the one hand, sulfur can slow down the liquid dynamics, thus inhibiting the precipitation of eutectic phases during casting from the melt and heating from the as-cast glass, and hence favoring the glass formation and thermal stability of the sulfur-bearing alloys. On the other hand, the addition of sulfur promotes the formation of an Al-rich τ3 phase, which easily precipitates and is therefore catastrophic for the GFA and melt stability. The two factors compete with each other and ultimately lead to an optimal sulfur content of 0.75 at.% for the investigated Cu-Zr-Al alloys. Compared with the base alloy, this novel sulfur-containing BMG possesses improved GFA, enlarged supercooled liquid region, and comparable mechanical strength and toughness, making it a promising candidate for industrial applications as structural material. Furthermore, the schematics of the change in time–temperature–transformation diagrams with increasing sulfur concentration were proposed on the basis of experimental results.

The thermodynamic and kinetic properties of Cu45.6Zr45.3Al9.1, Cu47.5Zr45.1Al7.4, and Cu47.3Zr45.8Al6.4Sn0.5 bulk metallic glasses are investigated to help understanding the origin of the glass-forming ability in the Cu–Zr–Al(–Sn) glass-forming system. The thermodynamic driving force for crystallization is determined by using specific heat capacity data and several thermodynamic parameters, e.g., enthalpies and entropies, measured via differential scanning calorimetry and differential thermal analysis. Moreover, a thermomechanical analyzer and a custom-built Couette rheometer were used to measure shear viscosity and to determine the fragilities in the low- and high-temperature regimes, close to the glass transition and in the stable melt respectively. For each alloy, the thermodynamic driving force and low-temperature fragility exhibit good correlation with the critical casting diameter. Melt viscosity was measured up to approximately 150 K above the liquidus temperature, i.e., from 1100 K to 1350 K. At temperature above 1350 K, reactions between the graphite crucible and the melt were observed to occur and may interfere with the viscosity measurement. Considerable differences in fragility between the stable melt and the deeply supercooled liquid were observed and discussed in terms of a fragile-to-strong crossover.

Temperature modulated DSC is used to study the fragility of three different bulk metallic glass forming liquids. Through applying various modulation frequencies, the dynamic glass transition shifts in temperature, allowing to determine the temperature dependence of the average relaxation time for each system. The resulting fragilities are compared with fragility investigations in literature obtained using thermo-mechanical analysis and the heating rate dependence of the calorimetric glass transition. Different methods to compare the data are evaluated and discussed.

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.

Communications Physics 2(1), 83 (2019). doi:10.1038/s42005-019-0180-2, The structural differences between the compositionally related Pt–P- and Pd–P-based bulk glass-forming liquids are investigated in synchrotron X-ray scattering experiments. Although Pt and Pd are considered to be topologically equivalent in structural models, we show that drastic changes in the total structure factor and in the reduced pair distribution function are observed upon gradual substitution. These variations indicate the existence of significant structural differences on the short- (SRO) and medium-range order (MRO) length scale. The structural data suggest that the distribution of the dominant polyhedra and the distribution of their connection schemes gradually change from Pt–P- to Pd–P-based alloys, which is likely connected to the different sensitivities to annealing or cooling rate induced embrittlement. The evolution of the total structure factor and the reduced pair distribution function with increasing temperature indicate the (partial) dissolution of both, the MRO and the SRO, which reflects the thermodynamic properties of the liquids., Published by Springer Nature, London