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A high-temperature Couette rotating cylinder rheometer has been developed for shear viscosity investigations of metallic glass forming liquids under precisely controlled flow and environmental conditions. These materials generally exhibit viscosities in the range of 10−2 to 10−1 Pa s above their liquidus temperature. While knowledge of the viscosity is crucial for estimating the glass forming ability and processing capabilities of these liquids, accurate rheological measurements are challenging due to their vulnerability to oxidation and contamination from the crucible and processing environment. Thus, the present device was constructed to measure the shear viscosities of these reactive melts in an inert atmosphere using shear cells manufactured from isostatically pressed graphite. A custom suite of LabVIEW programs provides all the necessary data acquisition tools and controls for the motor, inductive generator, sample temperature, and torque sensor. The setup includes a proportional-integral-derivative controller that allows for both isothermal and continuous heating/cooling experiments from room temperature up to 1400 K. The system is calibrated using NIST reference oils at room temperature. To demonstrate the functionality of the apparatus at high temperatures, the viscosity of the metallic glass forming alloy Zr59.3Cu28.8Al10.4Nb1.5 (AMZ4) was measured above the melting point and was found to be in excellent agreement with previously reported reference values from levitation-based measurement techniques. [ABSTRACT FROM AUTHOR]

Viscosity of the Fe-based bulk metallic glass-forming liquid Fe43Cr16Mo16C15B10 is measured around the glass transition and in the stable liquid. Low-temperature measurements are conducted using three-point beam bending in a thermomechanical analyzer, and high-temperature data are obtained from the damping behavior of an oscillating droplet which is electromagnetically levitated in microgravity on a reduced-gravity aircraft. The alloy displays an intermediately strong liquid behavior (D*=15.1) at low temperatures and a fragile behavior (D*=7.9) at high temperatures. Hence, the temperature dependence of viscosity changes drastically between the high- and the lowtemperature regime, which suggests the existence of a fragile-to-strong liquid-liquid transition in the supercooled liquid. Furthermore, viscosity and fragility data are discussed with respect to the glass-forming ability of the alloy. [ABSTRACT FROM AUTHOR]

The thermodynamic and kinetic properties of Cu 45.6 Zr 45.3 Al 9.1 , Cu 47.5 Zr 45.1 Al 7.4 , and Cu 47.3 Zr 45.8 Al 6.4 Sn 0.5 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. • The thermodynamic and kinetic origins of glass-forming ability of Cu–Zr–Al(-Sn) bulk metallic glasses are investigated. • Low- and high-temperature viscosities and kinetic fragilities are experimentally determined. • Thermodynamic driving force for crystallization is estimated using specific heat capacity data and enthalpies. • In our custom-built Couette rheometer, chemical reactions are observed at temperature above 1350 K. [ABSTRACT FROM AUTHOR]

Ni-Nb-based bulk glass-forming alloys are among the most promising amorphous metals for industrial applications 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 measurements 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. [Display omitted] [ABSTRACT FROM AUTHOR]