Supercooled Metallic Liquids Research Articles

Crystallization of Metallic Glasses and Supercooled Liquids.

This is an overview of recent findings on the structural changes observed upon heating, including crystallization processes in conventional metallic glasses, bulk metallic glasses, and their corresponding supercooled liquids. This paper encapsulates the various crystallization behaviors in metallic glasses by primary, eutectic, and polymorphous mechanisms, highlighting the complexity and diversity of the nucleation and growth mechanisms involved. Mechanically induced room-temperature crystallization is also discussed.

Fragility crossover mediated by covalent-like electronic interactions in metallic liquids

Abstract Fragility is one of the central concepts in glass and liquid sciences, as it characterizes the extent of deviation of viscosity from Arrhenius behavior and is linked to a range of glass properties. However, the intervention of crystallization often prevents the assessment of fragility in poor glass-formers, such as supercooled metallic liquids. Hence experimental data on their compositional dependence are scarce, let alone fundamentally understood. In this work, we use fast scanning calorimetry to overcome this obstacle and systematically study the fragility in a ternary La-Ni-Al system, over previously inaccessible composition space. We observe fragility dropped in a small range with the Al alloying, indicating an alloying-induced fragility crossover. We use x-ray photoelectron spectroscopy, resistance measurements, electronic structure calculations, and DFT-based deep-learning atomic simulations to investigate the cause of this fragility drop. These results show that the fragility crossover can be fundamentally ascribed to the electronic covalency associated with the unique Al-Al interactions. Our findings provide insight into the origin of fragility in metallic liquids from an electronic structure perspective and pave a new way for the design of metallic glasses.

Momentum transfer resolved electron correlation microscopy

Electron correlation microscopy (ECM) characterizes local structural relaxation dynamics in fluctuating systems like supercooled liquids with nanometer spatial resolution. We have developed a new type of ECM technique that provides moderate resolution in momentum transfer or k space using five-dimensional scanning transmission electron microscopy. k-resolved ECM on a Pt57.5Cu14.7Ni5.3P22.5 metallic supercooled liquids measures rich spatial and momentum structure in the relaxation time data τ(r,k). Relaxation time maps τ(r) at each azimuthal k are independent samples of the material's underlying relaxation time distribution, and τ of radial k shows more complex behavior than the de Gennes narrowing observed in analogous X-ray experiments. We have determined the requirements for electron counts per k-pixel, number of k-pixels per speckle, and time sampling to obtain reliable k-resolved ECM data.

Relevance of structural defects to the mechanism of mechanical deformation in metallic glasses

It is known that deformation in disordered materials such as metallic glasses and supercooled liquids occurs via the cooperative rearrangement of atoms or constituent particles at dynamical heterogeneities, commonly regarded as point-like defects. We show via molecular-dynamics simulations that there is no apparent relationship between atomic rearrangements and the local atomic environment as measured by the atomic-level stresses, kinetic and potential energies, and the per-atom Voronoi volume. In addition, there is only a weak correlation between atomic rearrangements and the largest and smallest eigenvalues of the dynamical matrix. Our results confirm the transient nature of dynamical heterogeneities and suggest that the notion of defects may be less relevant than that of a propensity for rearrangement.

The Nanoscale Density Gradient as a Structural Stabilizer for Glass Formation

The rapid cooling of a metallic liquid (ML) results in short-range order (SRO) among the atomic arrangements and a disordered structure in the resulting metallic glass (MG). These phenomena cause various possible features in the microscopic structure of the MG, presenting a puzzle about the nature of the MGs’ microscopic structure beyond SRO. In this study, the nanoscale density gradient (NDG) originating from a sequential arrangement of clusters with different atomic packing densities (APDs), representing the medium-range structural heterogeneity in Zr60Cu30Al10 MG, was characterized using electron tomography (ET) combined with image simulations based on structure modeling. The coarse polyhedrons with distinct facets identified in the three-dimensional images coincide with icosahedron-like clusters and represent the spatial positions of clusters with high APDs. Rearrangements of the different clusters according to descending APD order in the glass-forming process are responsible for the NDG that stabilizes both the supercooled ML and the amorphous states and acts as a hidden rule in the transition from ML to MG.

Strong-to-fragile transition in a metallic-glass forming supercooled liquid associated with a liquid–liquid transition

Liquid–liquid transitions are present in a variety of substances. However, investigating the liquid–liquid transitions occurring in a supercooled liquid is difficult because of the interference from rapid crystallization. Here, we report a strong-to-fragile transition in a Pd32Ni52P16 metallic glass-forming supercooled liquid associated with a liquid–liquid transition. Since the liquid–liquid transition takes place at temperatures smaller than the crystallization temperature, the liquid viscosity can be acquired by creep experiments conducted at temperatures close to the glass transition temperature without interference from crystallization. The strong-to-fragile transition results in a 37% increase of the fragility index and a 56% elongation after thermal-plastic processing. An investigation on the loss-modulus peaks by a dynamic mechanical analyzer implies that the enhanced thermal plasticity is contributed by both glass transition and strong-to-fragile transition. This work highlights how liquid–liquid transition affects liquid fragility and how it may aid the thermal-plastic processing of metallic glass.

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 α-process at the mesoscopic length scale. These results will contribute to the comprehension of the glass transition, which is still missing.

Formation energetics/dynamics of icosahedral clusters in supercooled metallic liquids in the dynamic equilibrium regime: Gibbs free energy, entropy, enthalpy, and connection to coordination shells

Icosahedral (ICO) clusters are known to exist in many supercooled metallic liquids and believed to play an important role in stabilizing the liquid before it transitions into a glassy, crystalline or quasicrystalline solid. However, a detailed understanding of their formation energetics/dynamics is currently lacking and a set of key questions regarding these clusters remains to be answered. Here, we report our study on the formation energetics/dynamics of ICO clusters in liquid Cu64Zr36 and Ta by combining MD simulations with statistical and theoretical analysis. We present the formation Gibbs free energy, entropy, enthalpy of ICO clusters in the two liquids in the dynamic equilibrium regime (T > 0.75 Tm), determine the size of the spatial domain (number of coordination shells) surrounding the clusters from which the formation enthalpy is originated, and discuss the results in connection with liquid composition, degree of randomness, potential energy landscape, and glass transition.Graphical abstract

Thermoplastic embossing device to probe rheological changes of supercooled metallic liquids during rapid heating.

The setup and working principle of a device designed for thermoplastic embossing and investigating rheological changes during fast-resistive heating of bulk metallic glasses (BMGs) is presented. The device was developed and custom-built at the Leibniz Institute for Solid State and Materials Research Dresden (Leibniz IFW Dresden) and is integrated into a universal testing machine. By varying the electrical-current pulse signal, the rate at which BMGs are resistively heated is controlled. Next to temperature and electrical resistance, the displacement of the punch, which penetrates the sample during rapid heating, is monitored. Additional pre-heating controlled by thermocouples allows for stable heating conditions to minimize heat extraction from the resistively heated specimen, which could eventually lead to damage of the device. The main focus of this device is to study fundamental phenomena under kinetic constraints evoked by fast heating conditions and to structure the surface of BMGs by thermoplastic embossing. A case study, which is carried out with Zr52.5Cu17.9Ni14.6Al10Ti5 BMG, demonstrates the device's performance covering heating rate ranges of three magnitudes and the access to rheological changes during the devitrification and subsequent crystallization of the BMG during rapid heating. Furthermore, the present device was successfully used to nanostructure the surface of the BMG at elevated heating rates.

ELECTROCHEMICAL POLYMORPHIC PHASE FORMATION IN METALS

The phenomenon of electrochemical phase formation in metals and alloys via a supercooled liquid state stage was discussed. Assuming the electrodeposited metal to be a product of formation and ultrarapid solidification of supercooled metallic liquid, a possibility of metastable phase formation during electrodeposition of polymorphous metals was suggested. It was anticipated that the polymorphic transition of the metal’s metastable form to the stable one occurs by shear, as does the martensitic transformation. To enable revealing an orientation relationship between grains of the two phases, a method for X-ray texture analysis of metals was developed using a combination of direct pole figures. It was established that the phase formation during electrodeposition of polymorphous metals produces metastable modifications typical of entities that crystallized from a liquid state at extremely high rates. In regards polymorphic transitions in metal electrodeposition, certain orientation relationships were observed between grains of the stable and the metastable phase, which is typical of phase transformations proceeding at extremely high rates. The results obtained provided additional arguments in favor of the phenomenon under discussion.

Multiple structural factors to influence the dynamics of icosahedral clusters in the Al90Sm10 super-cooled metallic liquid

The correlation between local structures and dynamics of the central Al atoms of the icosahedral clusters in the Al90Sm10 super-cooled liquid was investigated by classic molecular dynamics simulations. Three typical order parameters (i.e. the chemical composition, cluster penetration, and quasi-nearest atom) of the Al-centered icosahedral clusters were calculated and we demonstrated that the correlation between each of these parameters and dynamic propensities of the central Al atoms of the icosahedral clusters is weak. Then a simple combined parameter including these three parameters was proposed and it is found that there is a strong correlation between this new parameter and dynamic propensities of the central Al atoms of the icosahedral clusters. The present observations imply that the dynamic property of icosahedral clusters was mainly determined by multiple structural features of local structures. Our results are helpful to understand the relationship between atomic structures and dynamics in super-cooled liquids.

Quantized aging mode in metallic glass-forming liquids

Physical aging exhibits complex off-equilibrium dynamics and its studies are of particular importance for understanding the nature of glassy state. We investigate the aging process of supercooled Cu50Zr50 glass-forming liquid numerically and theoretically. The relaxation time spectra based on molecular dynamics simulations show that the aging dynamics features the superposition of discrete homogeneous dynamic modes. The visiting probabilities of these modes decay toward slower relaxation regime, which is tuned by aging time from the compressed to the stretched exponential manner. Such multi-mode dynamics is found to be accompanied by the quantized hierarchy of mobile clusters with identical size increment. With this finding, we propose an aging model, which describes the aging dynamics as the aging-dependent weighted average over quantized dynamic modes. The two-time correlation functions associated with aging thus are separated into aging-time dependent factor and aging independent relaxation-time term. The model well predicts the heterogeneous dynamics in aging and equilibration of supercooled metallic liquids, and reveals the intrinsic aspects influencing aging process.

Universal Scaling in the Temperature-Dependent Viscous Dynamics of Metallic Glasses.

The essential query about glass formation is how to understand the sheer temperature dependence of viscous dynamics of glass-forming liquids near the liquid-to-glass-transition temperature Tg. In this work, we report a universal scaling in the temperature-dependent viscous dynamics of metallic glasses (MGs) in the form of the Williams-Landel-Ferry equation on the basis of compiled data on the temperature-dependent viscosity and structural relaxation times of 89 MGs ever-reported in the past decades. Implications of this universal scaling are illustrated in the framework of the Adam-Gibbs relation, suggesting a universal vitrification mechanism in MGs mediated by configurational entropy wherein configurational entropy vanishes universally for all supercooled metallic liquids after a further decrease in temperature of ∼170.7 K (whereas with a relatively large error of ±150 K) below Tg. This result corroborates the thermodynamic origin of glass formation and suggests that MGs are an ideal research subject for understanding in depth the nature of glass transition for their relatively simple molecular structures.

Quantized Aging Mode in Metallic Glass-Forming Liquids

Physical aging exhibits complex off-equilibrium dynamics and its studies are of particular importance for understanding the nature of glassy state. We investigate the aging process of supercooled Cu50Zr50 glass-forming liquid numerically and theoretically. The relaxation time spectra based on molecular dynamics simulations show that the aging dynamics features the superposition of discrete homogeneous dynamic modes. The visiting probabilities of these modes decay toward slower relaxation regime, which is tuned by aging time from the compressed to the stretched exponential manner. Such multi-mode dynamics is found to be accompanied by the quantized hierarchy of mobile clusters with identical size increment. With this finding, we propose an aging model, which describes the aging dynamics as the aging-dependent weighted average over quantized dynamic modes. The two-time correlation functions associated with aging thus are separated into aging-time dependent factor and aging independent relaxation-time term. The model predicts the heterogeneous dynamics in aging and equilibration of supercooled metallic liquids and reveals the intrinsic aspects influencing aging process.

Effect of time on the isothermal viscosity of metallic glass supercooled liquids

The viscosity of metallic glasses is considered to remain constant during thermoplastic forming operations. Here, we quantify the time dependent change in isothermal viscosity of Pt57.5Cu14.7Ni5.3P22.5 and Zr35Ti30Cu8.25Be26.75 glass formers in the supercooled liquid state. The samples were isothermally held prior to thermoplastic molding to probe the viscosity at different times. The viscosity values were extracted from the filling length of mold cavities. The viscosity increased with annealing time and the rate of increase was found to be largely independent of temperature and type of glass former. However, the viscosity increase was higher in the outer parts of samples which experienced higher shear rate during molding. Crystal nucleation and growth contributions are considered to explain the observed changes in effective viscosity of remaining amorphous phase. Our results suggest that thermoplastic forming is an effective tool in analyzing subtle changes in structure and viscosity of metallic glass supercooled liquids.

Ultrastability of metallic supercooled liquid induced by vibration

The range of supercooled liquid region (ΔT) is the key parameter for evaluating the thermal stability and glass-forming ability of metallic glasses. The enhancement of ΔT is of significance not only for the glass formation and stability, but also for their thermoplastic forming ability. Traditional approaches to increase ΔT are composition regulation or dynamical modulation, which are either time-consuming or difficult to be controlled. Here, through a high-frequency ultrasonic vibration treatment, we show that the ΔT of a La-based metallic glass can be significantly widened from 64 K to 83 K, around 30% increment. This behavior is physically originated from the denser atomic packing and a lower stable basin in the energy landscape induced by the ultrasonic vibration energy. This work might provide a ready route to stabilize the supercooled liquid and glass, and give an insight into the understanding of the glass-forming of amorphous materials.

Wave-Vector Dependence of the Dynamics in Supercooled Metallic Liquids.

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. Aballisticlike 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.

Effect of loading rate on crystallization of metallic glass supercooled liquids

Metallic glasses exhibit unique thermoplastic processing capability, which is enabled by their metastable supercooled liquid state below the crystallization temperature. The thermoplastic processing critically depends on the crystallization time (processing time window), temperature (viscosity), applied load, and strain-rate. Among these parameters, the effects of crystallization time and processing temperature have been extensively studied. However, the effects of load and loading rate have not been thoroughly investigated. In this work, we performed a systematic study of load on the supercooled liquid state of three metallic glass formers: Pt-based, Zr-based, and Pd-based. The results show that the load-response of a metallic glass supercooled liquids is strongly composition dependent. The onset temperature of crystallization decreases with increasing load in Pt-based metallic glass whereas for Zr-based and Pd-based metallic glasses the onset temperature remains unchanged. The crystallization peak time is reduced for all three metallic glasses after thermoplastic forming. The results are discussed in terms of nucleation and growth of crystallites in metallic glasses.

Effect of loading rate on crystallization of metallic glass supercooled liquids

Effect of loading rate on crystallization of metallic glass supercooled liquids

Electrochemical Phase Formation of Metals and Alloys at Chemically Identical Solid or Liquid Cathode: Part 1. Metals

The existence of the phenomenon of electrochemical phase formation in metals and alloys via a supercooled liquid state stage is discussed. In dealing with that subject in terms of emergence and ultrarapid solidification of a deeply supercooled metallic liquid represented by a multitude of liquid atomic clusters, an idea was put forth that an identity of the state of aggregation between the cathode and the electrodepositing metal/alloy at the cathode???deposit interface must facilitate phase transformations at the interface. It was assumed that replacing a solid cathode with a chemically identical liquid one would lead to a wider spectrum of intermetallics formed due to interactions of the electrodepositing metal with elements of the liquid metallic cathode. Precision X-ray phase characterization was performed for the interface between the electrodepositing metal and a cathode of Wood???s alloy that persisted in either a solid or a liquid condition through the electrodeposition process. Relying on the data obtained, it was concluded that the phenomenon in point is proved by the formation enhancement of intermetallic compounds involving metallic cathode elements and the electrodepositing metal plus evolvement of additional intermetallics enriched in cathode elements, on replacing a solid cathode with a chemically identical liquid one.