Undercooled Liquid Phase Research Articles

Vibrational dynamics in the undercooled liquid of ultra-fragile metallic glasses

The lattice vibrational dynamics in ultra-fragile Pt57Cu23P20 and Pt60Cu20P20 glasses were investigated using in situ inelastic neutron scattering to measure the evolution of the phonon density of states (DOS) through the glass transition, into the undercooled liquid phase, and through crystallization. The phonon DOS curves are remarkably similar for the undercooled liquids and the amorphous solids. The vibrational entropy was obtained with these phonon DOS curves, and changes in vibrational entropy caused by the glass transition are less than 0.05kB/atom. For these highly fragile metal/metalloid glasses, the excess vibrational entropy of the undercooled liquid is about 0.5–1.5 J mol−1 K−1, compared to 0.1 J mol−1 K−1 reported earlier for stronger Cu–Zr glasses. Our work suggests that, consistent with our previous report for less fragile metallic glasses, the fraction of the total excess entropy that is vibrational is small, and the discontinuity in excess entropy at the glass transition is mainly due to configurational entropy.

Microscopic structure evolution and amorphous solidification mechanism of liquid quinary Zr<sub>57</sub>Cu<sub>20</sub>Al<sub>10</sub>Ni<sub>8</sub>Ti<sub>5</sub> alloy

The substantial undercooling and rapid solidification of liquid quinary Zr<sub>57</sub>Cu<sub>20</sub>Al<sub>10</sub>Ni<sub>8</sub>Ti<sub>5</sub> alloy are achieved by electromagnetic levitation (EML) technique. The amorphous solidification mechanism is revealed with molecular dynamics (MD) simulation. It is observed in EML experiment that the containerlessly solidified alloy is characterized by a core-shell structure, with mainly amorphous phase becoming the core and crystalline ZrCu, Zr<sub>2</sub>Cu and Zr<sub>8</sub>Cu<sub>5</sub> phases forming the shell. The volume fraction of amorphous core structure increases with undercooling and attains a value up to 81.3% at the maximum experimental undercooling of 300 K, which indicates that the critical undercooling required for complete amorphous solidification is 334 K. TEM analyses show that the alloy microstructure is mainly composed of Zr<sub>8</sub>Cu<sub>5</sub> phase, whereas the ZrCu phase and Zr<sub>2</sub>Cu phase are suppressed when liquid undercooling approaches this threshold. Once the critical undercooling is reached, amorphous solidification prevails over the crystallization of Zr<sub>8</sub>Cu<sub>5</sub> phase. In addition, a small quantity of amorphous phases are found in the crystalline shell and a little trace of Zr<sub>8</sub>Cu<sub>5</sub> nano-cluster is detected among the amorphous core. It is further verified by MD simulation that the formation of amorphous phase in the shell is caused by the microsegregation-induced solutal undercooling when liquid alloy attains the critical undercooling, while the nano-clusters within the core is mainly ascribed to the micro-thermal fluctuation effect inside highly undercooled liquid phase.

Modeling of Diffusion-Controlled Crystallization Kinetics in Al-Cu-Zr Metallic Glass

Crystallization is a major challenge in metallic glass production, and predictive models may aid the development of controlled microstructures. This work describes a modeling strategy of nucleation, growth and the dissolution of crystals in a multicomponent glass-forming system. The numerical model is based on classical nucleation theory in combination with a multicomponent diffusion-controlled growth model that is valid for high supersaturation. The required thermodynamic properties are obtained by coupling the model to a CALPHAD database using the Al-Cu-Zr system as a demonstrator. The crystallization of intermetallic Al,CumZrn phases from the undercooled liquid phase were simulated under isothermal as well as rapid heating and cooling conditions (10−1–106Ks−1). The obtained time–temperature transformation and continuous-heating/cooling transformation diagrams agree satisfactorily with the experimental data over a wide temperature range, thereby, demonstrating the predictability of the modeling approach. A comparison of the simulation results and experimental data is discussed.

Eutectic growth kinetics and microstructure evolution of highly undercooled Ti86.33Si13.67 alloy at electrostatic levitation state

Eutectic growth kinetics and microstructure evolution of highly undercooled Ti86.33Si13.67 alloy were systematically investigated using electrostatic levitation technique, and the maximum undercooling achieved was up to 343 K (0.21 TE). The experimental undercooling is rather low if the overheating degree is below a critical value, while that is high at the overheating degree beyond this value. The critical overheating degree is in the range from 60 to 86 K. The average eutectic growth velocity increases dramatically with the undercooling. The microstructure was proven to be consisted of α-Ti phase and Ti5Si3 phase. A small amount of metastable β-Ti phase inside the α-Ti phase was found for the alloy solidified at 343 K undercooling, and the orientation relationship between α-Ti phase and β-Ti phase is (110) β-Ti // (002) α-Ti. For the primary Ti5Si3 phase, it can be obtained in the microstructure solidified at low undercoolings, but cannot be found when the undercooling is larger than 200 K. The cluster like eutectic gradually occupied the whole alloy space with the increasing undercooling, and the irregular eutectic microstructure is confirmed to be directly generated from the undercooled liquid phase at high undercoolings.

Thermophysical properties of the TiAl-2Cr-2Nb alloy in the liquid phase measured with an electromagnetic levitation device on board the International Space Station, ISS-EML

Abstract Thermophysical properties of the γ-TiAl alloy Ti-48Al-2Cr-2Ni in the liquid phase were investigated with a containerless electromagnetic processing device on board the International Space Station. Containerless processing is warranted by the high liquidus temperature T liq = 1 776 K and the high dissolution reactivity in the liquid phase. Thermophysical properties investigated include the surface tension and viscosity, density, specific heat capacity and the electrical resistivity. The experiments were supported by magnetohydrodynamic fluid flow calculations. The Ti-48Al-2Cr-2Ni alloy could be stably processed over extended times in the stable and undercooled liquid phase and exhibited an exceptional degree of undercooling before solidification. Experimental processes and thermophysical properties so obtained will be described. The experiments demonstrate the broad experimental capabilities of the electromagnetic processing facility on the International Space Station for thermophysical investigations in the liquid phase of metallic alloys not achievable by other methods.

A review on the theory of stable dendritic growth.

This review article summarizes the main outcomes following from recently developed theories of stable dendritic growth in undercooled one-component and binary melts. The nonlinear heat and mass transfer mechanisms that control the crystal growth process are connected with hydrodynamic flows (forced and natural convection), as well as with the non-local diffusion transport of dissolved impurities in the undercooled liquid phase. The main conclusions following from stability analysis, solvability and selection theories are presented. The sharp interface model and stability criteria for various crystallization conditions and crystalline symmetries met in actual practice are formulated and discussed. The review is also focused on the determination of the main process parameters-the tip velocity and diameter of dendritic crystals as functions of the melt undercooling, which define the structural states and transitions in materials science (e.g. monocrystalline-polycrystalline structures). Selection criteria of stable dendritic growth mode for conductive and convective heat and mass fluxes at the crystal surface are stitched together into a single criterion valid for an arbitrary undercooling. This article is part of the theme issue 'Transport phenomena in complex systems (part 1)'.

The first order L-G phase transition in liquid Ag and Ag-Cu alloys is driven by deviatoric strain

An undercooled liquid-phase (L-phase) can undergo a first order configurational phase transition to either a crystal phase (X-phase) or a metastable, configurationally heterogeneous, rigid glassy phase (G-phase). To investigate the underlying mechanism of the L-G transition, we employ molecular dynamics simulations to study G-phase formation in a binary Cu-Ag system. We find that G-phase formation is driven by the reduction of local distortion energy arising from deviatoric strains in the liquid phase and demonstrate its local distribution. Reduction of distortion energy contributes over 80% of the latent heat of the L-G transition, suggesting that condensation of spatially varying random elastic fields in the liquid is primarily responsible for the first order L-G transition. By applying this analysis to crystallization and G-phase formation in elementary Ag, we show that deviatoric strain energy is the dominant driving force for the L-G and L-X transition also in the case of the pure metal.

Glass-forming ability correlated with the liquid-liquid transition in Pd42.5Ni42.5P15 alloy

Alloy melts can solidify into metallic glasses if cooled fast enough to avoid crystallization. Glass-forming ability (GFA), a measure of the ease of vitrification, is vital for the fundamental understanding of glass formation, and is also crucial for the application of metallic glasses. Previous studies of GFA mainly focused on the undercooled liquid phase, while the influence of the evolution of the stable melts on GFA is rarely addressed. Here we show that the Pd42.5Ni42.5P15 glass-forming liquid, in which a first-order liquid-liquid transition (LLT) takes place at TLL = 1063 K high above its liquidus temperature, shows significantly different GFA when quenched from the temperature above or below TLL. Moreover, the pathway and kinetics of crystallization of the melt are strongly related to the kinetics of the LLT. Our work provides new insights into the vitrification process and the kinetics of crystallization, and contributes to designing more stable metallic glasses.

Changes in short- and medium-range order in metallic liquids during undercooling

Abstract

Precise Measurements of Thermophysical Properties of Liquid Ti–6Al–4V (Ti64) Alloy On Board the International Space Station

The titanium alloy Ti–6Al–4V (Ti64) is a very widely applied alloy with extreme importance in the fields of aerospace, automotive, and biomedicine. For a large range of manufacturing methods that involve the liquid phase, such as casting, metal injection molding, thermal spraying, and additive manufacturing, process simulations are an important tool for the optimization and development of the fabrication process. Reliable thermophysical property data of Ti64 in the solid and the liquid phase are necessary inputs to such models. Due to the high melt reactivity of titanium alloys with container materials, measurements performed using conventional thermophysical equipment are usually impossible. Precise benchmark measurement of thermophysical properties (surface tension, viscosity, specific heat, total hemispherical emissivity, thermal conductivity, electrical resistivity, mass density) of Ti64 is presented in the undercooled liquid phase using containerless electromagnetic levitation on board the International Space Station (ISS). The obtained values are compared with predictive models that can help to analyze the data for other compositions of the alloy system. The property values are applied to simulations of industrial fabrication processes, to optimize these processes and thus the product quality.

Enhanced crystallization resistance and thermal stability via suppressing the metastable superlattice phase in Ni-(Pd)-P metallic glasses

Virtually, glass formation is to avoid crystallization during solidification, i.e., a consequence of the competition between the undercooled liquid and primary crystalline phases. It is found that the crystallization resistance of the binary Ni-P system was drastically enhanced with alloying of Pd and correspondingly, the critical size for glass formation increased significantly from the micrometer to millimeter scale. Thermodynamically, the introduction of Pd could effectively increase the atomic size mismatch and heat of mixing, which are beneficial to stabilize the supercooled liquid. Kinetically, the introduction of Pd not only successfully suppresses the formation of metastable superlattice phase, which is prone to nucleation and growth in the supercooled liquid state, but also changes the crystallization mechanism from the primary to eutectic mode. The current finding sheds light on understanding glass formation of the most studied Pd-Ni-P system and the glass-forming ability in general.

A microscopic look at the Johari-Goldstein relaxation in a hydrogen-bonded glass-former

Understanding the glass transition requires getting the picture of the dynamical processes that intervene in it. Glass-forming liquids show a characteristic decoupling of relaxation processes when they are cooled down towards the glassy state. The faster (βJG) process is still under scrutiny, and its full explanation necessitates information at the microscopic scale. To this aim, nuclear γ-resonance time-domain interferometry (TDI) has been utilized to investigate 5-methyl-2-hexanol, a hydrogen-bonded liquid with a pronounced βJG process as measured by dielectric spectroscopy. TDI probes in fact the center-of-mass, molecular dynamics at scattering-vectors corresponding to both inter- and intra-molecular distances. Our measurements demonstrate that, in the undercooled liquid phase, the βJG relaxation can be visualized as a spatially-restricted rearrangement of molecules within the cage of their closest neighbours accompanied by larger excursions which reach out at least the inter-molecular scale and are related to cage-breaking events. In-cage rattling and cage-breaking processes therefore coexist in the βJG relaxation.

Switching between Crystallization from the Glassy and the Undercooled Liquid Phase in Phase Change Material Ge2Sb2Te5

Controlling crystallization kinetics is key to overcome the temperature-time dilemma in phase change materials employed for data storage. While the amorphous phase must be preserved for more than 10 years at slightly above room temperature to ensure data integrity, it has to crystallize on a timescale of several nanoseconds following a moderate temperature increase to near 2/3 Tm to compete with other memory devices such as dynamic random access memory (DRAM). Here, a calorimetric demonstration that this striking variation in kinetics involves crystallization occurring either from the glassy or from the undercooled liquid state is provided. Measurements of crystallization kinetics of Ge2 Sb2 Te5 with heating rates spanning over six orders of magnitude reveal a fourfold decrease in Kissinger activation energy for crystallization upon the glass transition. This enables rapid crystallization above the glass transition temperature Tg . Moreover, highly unusual for glass-forming systems, crystallization at conventional heating rates is observed more than 50 °C below Tg , where the atomic mobility should be vanishingly small.

Relaxation dynamics induced in glasses by absorption of hard x-ray photons

X-ray photon correlation is used to probe the slow dynamics of the glass-former B2O3 across the glass transition. In the undercooled liquid phase the decay times of the measured correlation functions are consistent with visible light scattering results and independent of the incoming flux; in the glass they are instead temperature independent and show a definite dependence on the X-ray flux. This dependence can be exploited to obtain information on the volume occupied by the atoms that move in the glass following an absorption event. The length scale derived in this way, of the order of the nanometer, is consistent with that reported for the dynamical heterogeneities, suggesting the existence of a new scheme to get access to this fundamental quantity.

Calculation of solid–liquid interfacial free energy of silicon based on classical nucleation theory

Abstract

Formation of metal being electrodeposited solely in spherulitic form

The aim of the work was the experimental verification of validity of the phenomenon of phase formation through a stage of liquid state in metals being electrodeposited. The idea of the work is based on a known fact that at super-quick solidification of highly undercooled liquid metallic phase the spherulites appear. For the proof of existence of intermediate liquid phase of metal being electrodeposited it was planned to obtain the deposits in spherulitic form. The conditions for the formation of metal being electrodeposited in spherulitic form are discussed and realized. Practical realization of the idea mentioned above was accomplished by combined nickel and chromium alloying of iron being electrodeposited at high current density. As a result of the model experiment the samples of electrodeposited alloyed iron, consisting solely of spherulites, were obtained. The formation of metal being electrodeposited solely in shperulitic form, typical for the metal solidified from liquid state with very high rate in conditions of significant undercooling, proves validity of the phenomenon of phase formation of metals being electrodeposited through a stage of liquid state.

Formation of spherulites and pentagonal quasicrystals in metals being electrodeposited

The aim of the work was experimental verification of validity of the phenomenon of phase formation through a stage of liquid state in metals being electrodeposited. The idea of the work was based on the known fact, that at minor undercooling of melt solidification of metal usually occurs in dendritic form, and at significant undercooling of melt dendritic form transfers to spherulite one. At that spherulites, as a rule, are being formed on the interface between metal and a melting-pot. Besides, superfast solidification of greatly undercooled melt causes formation of quasicrystals with pentagonal symmetry. Therefore, if metals being electrodeposited really pass through a stage of undercooled liquid state and rapidly solidify at the deposition temperature, than spherulite forms of crystallization and pentagonal quasicrystals should be detected in their layers adjacent to the cathode. As the result of experimental investigations accomplished by the method of scanning electron microscopy formation of spherulites and pentagonal quasicrystals in the adjacent to the cathode layers of metals (copper, lead and cobalt) being electrodeposited was discovered. It is shown, that presence of spherulites and pentagonal quasicrystals in electrodeposited metals is the result of superfast solidification of undercooled liquid metallic phase being formed during electrochemical deposition of metals. Formation of spherulites and pentagonal quasicrystals in the adjacent to the cathode layers of metals being electrodeposited proves the validity of the phenomenon of phase formation through a stage of liquid state in metals being electrodeposited.

A grand potential approach to phase-field modeling of rapid solidification

Abstract. Rapid solidification occurs under large driving force of transformation from the metastable undercooled liquid phase to the stable crystalline state. Using a formalism of extended irreversible thermodynamics, a phase-field model of rapid solidification in binary systems is derived. An entropy approach together with a grand potential density of a binary system is used to obtain the main governing equations of the model. Special attention is paid to equations of a rapidly solidifying binary system which are accompanied by essential deviations from local equilibrium in the transport of the conservative variables (such as inner energy and mass) and in the dynamics of non-conservative variables (such as phase field). The obtained equations are analyzed and compared with recent models and outcomes based on the grand potential approach to solidification.

Acoustic dynamics of supercooled indomethacin probed by Brillouin light scattering.

Acoustics dynamics of the molecular glass-former indomethacin (IMC) have been investigated by Brillouin light scattering (BLS) at GHz frequencies. Elastic response of the system has been tracked from the melting temperature down to the glass transition through the supercooled liquid. Both the structural arrest and the vibrational dynamics are described by modeling the experimentally determined dynamic structure factor within the framework of the Langevin equation, through a simplified choice of memory function which allows one to determine sound velocity and the acoustic attenuation coefficient as a function of temperature. The density fluctuation spectra in the glassy phase, as probed by BLS, are compared with time-domain results from photoacoustics experiments. The arising scenario is discussed in the context of current literature reporting inelastic X-ray scattering and BLS in platelet geometry. The link between the probed elastic properties and the non-ergodicity factor of the glass phase is finally scrutinized.

The Correlation between Reduced Glass Transition Temperature and Glass Forming Ability of Undercooled Polymeric Glasses

The thermodynamic behaviour of glass forming undercooled melt have been studied by analysing the temperature dependence of the Gibbs free energy difference (Δ<i>G</i>) for six polymeric glass forming materials[O-terphenyl, Tri-α-naphthyl benzene, Glycerol, Ethanol, 2-methylpentane and Phenyl salicylate (salol)] between the undercooled melt and the corresponding equilibrium solid phases. The expression obtained by expanding free energies of the undercooled liquid and solid phases of the materials in the form of Taylor’s series expansion. The procedure for the estimation of the ideal glass transition temperature has been reviewed together its importance in the glass forming ability of materials. The glass forming ability of undercooled materials is also explained in terms of ratio of reduced glass transition temperature and ideal glass transition temperature. It has also reported that Glycerol has the largest melting interval with the poorest glass forming ability among these six polymers. It has also been found that the best polymeric glass forming materials are at near eutectic composition. There is a strong correlation between glass forming ability of these polymeric materials.