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Abstract Effective strategies to improve the tarnishing resistance of the 18 K (karat) gold-based bulk glass-forming composition Au 49 Ag 5.5 Pd 2.3 Cu 26.9 Si 16.3 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 T g -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. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

The crystallization route and kinetics of a Cu-Zr-Al bulk metallic glass (BMG), based on Cu 50 Zr 50 , were investigated using in-situ synchrotron small- and wide-angle X-ray scattering (SAXS/WAXS), in-situ high-energy X-ray diffraction (HEXRD), and differential scanning calorimetry (DSC). The acquisition of kinetic diagrams of SAXS/WAXS enables the direct construction of the overall phase transformation route from room temperature to the onset temperature of solid state phase transformation upon heating. It is revealed that the studied alloy undergoes multiple steps (at least five steps) before reaching the high-temperature thermodynamic equilibrium. This is distinct from the commonly believed simple path of glass → Cu 10 Zr 7 + CuZr 2 → B2-CuZr predicted by the Cu-Zr equilibrium phase diagram. In addition to thermodynamics, we emphasize that kinetic factors play a prominent role in the phase transformation process of Cu-Zr-Al BMGs. Furthermore, isothermal crystallization kinetics analyses conducted by in-situ HEXRD and DSC indicate that the formation of the initial crystalline phase, specifically the Cu 10 Zr 7 , follows an interface-controlled quasi-polymorphic process with an increasing nucleation rate. The findings of this study may provide novel insights into the crystallization mechanism of glass-forming supercooled liquids from a thermophysical perspective. • The phase transformation route and kinetics of Cu 47.5 Zr 45.1 Al 7.4 BMG were systematically studied. • In-situ synchrotron small- and wide-angle X-ray scattering were used to study the crystallization behavior. • The studied Cu-Zr-Al glass experiences five steps until reaching the high-temperature thermodynamic equilibrium. • Kinetic factors play a prominent role in the phase transformation process of Cu-Zr-Al BMGs. • The formation of the primary Cu 10 Zr 7 phase follows an interface-controlled polymorphic phase transition mechanism. [ABSTRACT FROM AUTHOR]

In this work, the crystallization pathways and kinetics of the Cu 47.5 Zr 45.1 Al 7.4 bulk metallic glass (BMG) are studied systematically using a broad collection of laboratory and synchrotron-based techniques, such as differential scanning calorimetry, transmission electron microscopy, in situ high-energy synchrotron X-ray diffraction (HEXRD), simultaneous small- and wide-angle X-ray scattering, and atom probe tomography. The crystallization sequence during heating at conventional slow rates and isothermal annealing near the glass transition temperature was determined to consist in preferential formation of (Cu, Al) 10 Zr 7 followed by the formation of CuZr 2 and AlCu 2 Zr induced by long-range atomic diffusion. In situ HEXRD shows that the formation of the latter two phases is accompanied by the partial dissolution of the primary Cu-rich phase. Microstructural characterizations of annealed samples suggest that a new liquid phase with enhanced local order and obscure compositional changes has formed from the homogeneous supercooled liquid before crystallization. A concept analogous to the disorder-to-order transition in crystalline materials is applied here to describe the observed transition in liquid, resolving the longstanding apparent contradiction between observing a nanoscale phase separation versus a liquid–liquid phase transition during devitrification. Furthermore, we suggest that the formation of an intermediate liquid phase prior to crystallization is the structural origin of the rapid devitrification kinetics during the primary crystallization. On the basis of our combined experimental results, a scheme is proposed to generalize the complex multistep devitrification mechanisms of the Cu 47.5 Zr 45.1 Al 7.4 BMG during isothermal annealing or slow heating. This work may provide new insights into the crystallization behavior and thermal stability of Cu–Zr–Al BMGs in terms of thermodynamics and kinetics and might lead to deeper understanding of the underlying mechanisms of the often observed rapid crystallization processes of deeply undercooled metallic liquids. Graphical Abstract [Display omitted]. [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]