Thermophysical and structural investigations of a CuTi- and a Zr-based bulk metallic glass, the influence of minor additions, and the relation to thermoplastic forming

Bulk metallic glasses (BMGs) surpass the strength of steels and at the same time possess the elasticity and formability of thermoplastic polymers. These favorable properties make them interesting candidates for industrial applications. In this work, the thermophysical properties and the structure of the CuTi-based BMG Vit101 (Cu47Ti34Zr11Ni8) and the Zr-based BMG Vit105 (Zr52.5Cu17.9Ni14.6Al10Ti5) are investigated. Special focus lies on the influence of minor additions of sulfur and phosphorus, as they increase the thermal stability of the alloys in the supercooled liquid region. The thermodynamic functions of the alloys are determined, and viscosity and kinetic fragility are measured around the glass transition and in the stable liquid. In-situ synchrotron X-ray scattering experiments are performed, elucidating the crystallization sequence upon heating and cooling. Minor additions retard the formation of the primary crystalline phase upon heating. Based on the diffraction data, the temperature evolution of structural differences between the alloys is discussed. Thermoplastic forming experiments on a variety of BMGs are performed and the deformation is discussed with respect to their thermophysical properties, leading to a description of the thermoplastic formability and the ideal processing region. These findings are transferred to thermoplastic consolidation experiments on amorphous powder, evaluating this technique for additive manufacturing. Finally, thermoplastic deformation experiments are conducted on the CuTi- and Zr-based alloys with minor additions. Minor additions can be used to significantly improve the thermoplastic formability and hence ease the industrial processability.Metallische Massivgläser (MMG) übertreffen Stähle in ihrer Festigkeit und besitzen gleichzeitig die Elastizität und Formbarkeit von thermoplastischen Polymeren. Diese vorteilhaften Eigenschaften machen sie für industrielle Anwendungen interessant. In dieser Arbeit werden die thermophysikalischen Eigenschaften und die Struktur des CuTi-basierten MMG Vit101 (Cu47Ti34Zr11Ni8) und des Zr-basierten MMG Vit105 (Zr52.5Cu17.9Ni14.6 Al10Ti5) untersucht. Der Fokus liegt dabei auf dem Einfluss von geringen Schwefel- und Phosphorzusätzen, da diese die thermische Stabilität der Legierungen in der unterkühlten Schmelze erhöhen. Die thermodynamischen Funktionen der Legierungen werden bestimmt und Viskosität und kinetische Fragilität werden um den Glasübergang und in der stabilen Flüssigkeit gemessen. In-situ Synchrotron Röntgenstreuexperimente werden durchgeführt, um die Kristallisationssequenz beim Erhitzen und Abkühlen aufzuklären. Schwefel- und Phosphorzusätze verzögern die Bildung der primären kristallinen Phase beim Erhitzen. Basierend auf den Beugungsdaten wird auch die Temperaturentwicklung von Strukturunterschieden zwischen den Legierungen diskutiert. Thermoplastische Umformversuche an verschiedenen MMG werden durchgeführt und die Verformung in Bezug auf die thermophysikalischen Kennwerte der Legierungen diskutiert, was zu einer Beschreibung der thermoplastischen Umformbarkeit und des idealen Verarbeitungsbereichs führt. Diese Erkenntnisse fließen in Konsolidierungsexperimente an amorphem Pulver ein und erlauben eine Bewertung dieser additiven Fertigungstechnik. Schließlich werden an den Legierungen auf CuTi- und Zr-Basis mit Schwefel- und Phosphorzusätzen thermoplastische Verformungsexperimente durchgeführt. Diese Zusätze können verwendet werden, um die thermoplastische Formbarkeit signifikant zu verbessern und damit die industrielle Verarbeitbarkeit zu erleichtern

Disentangling structural and kinetic components of the {\alpha}-relaxation in supercooled metallic liquids

The particle motion associated to the {\alpha}-relaxation in supercooled liquids is still challenging scientists due to its difficulty to be probed experimentally. By combining synchrotron techniques, we found the existence of microscopic structure-dynamics relationships in Pt42.5Cu27Ni9.5P21 and Pd42.5Cu27Ni9.5P21 liquids which allows us to disentangle structural and kinetic contributions to the {\alpha}-process. While the two alloys show similar kinetic fragilities, their structural fragilities differ and correlate with the temperature dependence of the stretching parameter describing the decay of the density fluctuations. This implies that the evolution of dynamical heterogeneities in supercooled alloys is determined by the rigidity of the melt structure. We find also that the atomic motion not only reflects the topological order but also the chemical short-range order, which can lead to a surprising slowdown of the {\alpha}-process at the mesoscopic length scale. These results will contribute to the comprehension of the glass transition, which is still missing

Disentangling structural and kinetic components of the α-relaxation in supercooled metallic liquids

The particle motion associated to the α-relaxation in supercooled liquids is still challenging scientists due to its difficulty to be probed experimentally. By combining synchrotron techniques, we report the existence of microscopic structure-dynamics relationships in Pt42.5Cu27Ni9.5P21 and Pd42.5Cu27Ni9.5P21 liquids which allows us to disentangle structural and kinetic contributions to the α-process. While the two alloys show similar kinetic fragilities, their structural fragilities differ and correlate with the temperature dependence of the stretching parameter describing the decay of the density fluctuations. This implies that the evolution of dynamical heterogeneities in supercooled alloys is determined by the rigidity of the melt structure. We find also that the atomic motion not only reflects the topological order but also the chemical short-range order, which can lead to a surprising slowdown of the αprocess at the mesoscopic length scale. These results will contribute to the comprehension of the glass transition, which is still missing

Ni-Nb-P-based bulk glass-forming alloys: Superior material properties combined in one alloy family

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

Changes in the crystallization sequence upon sulfur addition in the Zr52.5Cu17.9Ni14.6Al10Ti5 bulk metallic glass-forming liquid revealed by in situ high-energy x-ray diffraction

Bulk metallic glasses (BMGs) surpass the strength of steels and possess the elasticity and formability of polymers. The key to obtain these properties is to conserve the amorphous structure of a metallic melt and avoid crystallization during processing. In this work, a change in the crystallization sequence in the widely used BMG Zr52.5Cu17.9Ni14.6Al10Ti5 (Vit105) upon the addition of sulfur (Zr51.45Cu17.54Ni14.31Al9.8Ti4.9S2; Vit105 S2) is revealed by in situ high-energy x-ray diffraction, both upon heating from the glassy state and upon cooling from the liquid state during electrostatic levitation. This methodology proves to be a powerful tool to elucidate the complete crystallization behavior of complex BMG-forming liquids. The experiments show that the addition of sulfur changes the crystallization sequence and phases in a different manner upon cooling from the liquid state than upon heating from the glassy state. The thermal stability at low temperatures upon heating is increased, as the supercooled liquid region is extended from 60 to 77 K and the glass transition temperature increases from 671 to 692 K. However, the thermal stability is decreased upon cooling, causing a reduced glass-forming ability

Sulfur-bearing metallic glasses: A new family of bulk glass-forming alloys

Metallic glasses constitute a class of engineering materials having an enormous potential for many fields of application due to their superior properties. Here, we report on a new family of sulfur-bearing bulk metallic glasses. So far, sulfur was not considered as alloying element for the synthesis of bulk metallic glasses. We observe bulk glass formation in a variety of sulfur-containing systems, including titanium-based bulk glass-forming systems with an extremely high titanium content of 70 at.%. These findings allow the development of a whole new class of amorphous metals, having good processability and consisting of alloying elements suitable for industrial applications

The kinetic fragility of Pt-P- and Ni-P-based bulk glass-forming liquids and its thermodynamic and structural signature

The thermodynamic and kinetic properties of Pt-P- and Ni-P-based bulk glass-forming liquids are investigated using differential scanning calorimetry and three-point beam bending. The kinetic fragility of the alloys is determined by measuring relaxation times and equilibrium viscosities. In the Pt-P-based alloy family a Pt-rich alloy is one of the most fragile bulk glass-forming liquids in the vicinity of the glass transition reported so far. The fragility parameter D$^∗$ increases significantly with decreasing Pt-content, which is due to a more pronounced loss of excess entropy upon undercooling for the fragile liquids. Relying on previous observations made for Pd-P-based alloys, it is argued that a bifurcation of the P environment into Pt-Ni-P and Pt-Cu-P structural units stabilizes the deeply undercooled liquid due to formation of medium range order (MRO). This is supported by synchrotron X-ray scattering experiments, which reveal the formation of a pre-peak in the total structure factor with increasing fragility. Adapting a previously reported empirical correlation, we show that the more fragile liquid alloys are characterized by a more pronounced dilatation of atomic pair correlations on the length scale of 1 nm, corroborating the idea that the observed fragility behavior of Pt-P-based bulk glass-forming liquids is inherently related to structural changes on the length scale of MRO

On the bulk glass formation in the ternary Pd-Ni-S system

We report on the formation of bulk metallic glasses in the ternary Pd-Ni-S system. In a large compositional range, glass formation is observed by copper mold casting with a glass forming ability of up to 2 mm in diameter for the composition $Pd_{37}Ni_{37}S_{26}$. The best compromise of thermal stability upon heating from the as-cast state and glass forming ability was found for $Pd_{31}Ni_{42}S_{27}$, having a critical diameter of 1.5 mm and an extension of the supercooled liquid region of 27.2 K $(ΔTx = Tx – Tg)$. Differential scanning calorimetry and X-ray diffraction experiments were conducted in order to study the influence of the composition on the glass forming ability and thermal stability. The primary precipitating crystalline phases $Ni_{3}S_{2}$ and $Pd_{4}S$ are identified by in-situ high energy synchrotron X-ray scattering experiments upon heating from the glassy state as well as upon cooling from the equilibrium liquid. Finally, the origin of the bulk glass formation in this novel system is discussed regarding thermodynamics and kinetics and compared to current models for the prediction of the glass forming ability. Furthermore, the mechanical properties are investigated and discussed with respect to the rather fragile kinetic behavior. All in all, we gain new insights into the process of glass formation in this novel alloying system and give conclusions about the determining contributions for the glass forming ability and glass forming range

Signatures of structural differences in Pt–P- and Pd–P-based bulk glass-forming liquids

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

On the thermodynamics and its connection to structure in the Pt-Pd-Cu-Ni-P bulk metallic glass forming system

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