Undercooled Liquid Research Articles

Crystallization Kinetics of Supercooled Liquid Ge–Sb Based on Ultrafast Calorimetry

The crystallization kinetics of phase-change materials (PCMs) entails a crucial aspect of phase-change memory technology, and their study is also of interest to advance the understanding of crystallization in general. Research on crystallization of PCMs remains challenging because of the short (nanosecond) time and small (nanometer) length scales involved. Ultrafast differential scanning calorimetry (DSC) offers a powerful tool to study crystallization via ultrahigh heating rates. Here, we used this tool to study the crystallization kinetics of growth-dominant Ge7Sb93. Two models describing the viscosity of the undercooled liquid were used to interpret the data and were subsequently crosschecked by independent growth-rate data. With both models the data in Kissinger plots could be fitted well, but one of the models resulted in a large discrepancy with the independent data. These results demonstrate that great care is needed when deriving crystal-growth rates from ultrafast DSC measurements because orders ...

Analysis of thermodynamic properties for Pd-based bulk metallic glasses

The analysis of temperature dependent thermodynamic parameters; Gibb's free energy difference (ΔG), entropy difference (ΔS) and enthalpy difference (ΔH) between the undercooled liquid and the corresponding equilibrium solid phases has been proved to be highly beneficial in the study of the thermodynamic behaviour of bulk metallic glass (BMG) forming melts. In last two decades, Pd based alloys were found to form bulk glass phases. In present study, the three thermodynamic parameters viz., ΔG, ΔS and ΔH are calculated theoretically in the entire temperature range Tm (melting temperature) to Tg (glass transition temperature) for Pd-based three samples of BMGs; Pd40Ni40P20, Pd40Cu30Ni10P20 and Pd43Cu27Ni10P20 on the basis of Taylor's series expansion. A comparative study is also made between the present result and the result obtained experimentally as well as on the basis of expressions proposed by the earlier researchers. An attempt is also been made to relate the reduced glass transition temperature with glass forming ability for all three BMGs.

Ternary eutectic and peritectic solidification of undercooled liquid Fe–Mo–Si alloys

Liquid ternary Fe–6.5%Mo–18.5%Si eutectic and Fe–4%Mo–19%Si peritectic alloys were highly undercooled and rapidly solidified under containerless and microgravity condition inside the drop tube. In this situation, the cooling rate and undercooling of the alloy droplets increased obviously when their diameters decreased, and the alloy droplets exhibited maximum undercoolings up to 184K and 468K respectively. For the Fe–6.5%Mo–18.5%Si eutectic alloy, the ternary eutectic is composed of Fe3Si, Fe2Si and Fe5MoSi4 intermetallic phases. With the decrease of the droplet diameter, the volume fraction of the ternary (Fe3Si+Fe2Si+Fe5MoSi4) eutectic increases and the morphology of the ternary eutectic vary from lamellar structure to granular structure. In the Fe–4%Mo–19%Si peritectic alloy, the primary phase is Fe2Si intermetallic phase, and the peritectic structure consists of Fe3Si and FeSi intermetallic phases. With the increase of the undercooling, the primary phase vanishes, the peritectic transformation nearly accomplishes, and the growth of (Fe3Si+FeSi) peritectic phases transform from faceted blocks into non-faceted granules.

Structural origin underlying the effect of cooling rate on solidification point**Project supported by the National Basic Research Program of China (Grant No. 2011CB012900), the National Natural Science Foundation of China (Grant No. 51171115), the Natural Science Foundation of Shanghai City, China (Grant No. 10ZR1415700), the Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20100073120008), the

Solidification behaviors of liquid aluminum at different cooling rates were examined via classical molecular dynamics simulation with an embedded atom method potential. The results demonstrate that solidification point decreases with increasing cooling rate. To explain this phenomenon, solid-like cluster in liquid was analyzed by the structural analysis method of bond order parameters. The results reveal that the size of the largest solid-like cluster in deeply undercooled liquid decreases with the increase of cooling rate, which can provide a structural interpretation to the above phenomenon.

On the formation of metallic glass coatings by means of Cold Gas Spray technology

Novel Cold Gas Spray technology has been used to build-up coatings of Fe-base metallic glasses on low alloyed carbon steel. Impact morphologies have been studied demonstrating extensive shear band formation but also homogeneous plastic flow in the metallic glass particles despite high strain rates, 108–109 s−1, involved in the technique. Interestingly, thick coatings with good deposition efficiencies have been built-up in conditions of homogeneous flow. Coating formation is discussed in terms of fundamentals of dynamics of undercooled liquids, plastic flow mechanism and crystallization kinetics of metallic glasses. Therefore, the method herein established is valid for any metallic glass.

Formation of Icosahedral Phase in Bulk Glass Forming Ti-Zr-Be-Cu-Ni Alloy

Formation of an icosahedral phase in the bulk glass forming alloy during crystallization from amorphous phase and solidification from melt is investigated. The icosahedral phase with a size of 10 to 15 nm forms as a thermodynamically stable phase at intermediate temperature during the transformation from amorphous to crystalline phases such as Laves and -(Ti-Zr) phases, indicating that the existence of the icosahedral cluster in the undercooled liquid. On the other hand, the icosahedral phase forms as a primary solidification phase even though the Laves phase is stable at high temperature, which is can be explained based on the high nucleation rate of icosahedral phase relative to that of competing crystalline Laves phase due to lower interfacial energy between icosahedral and liquid phases.

Synergistic stabilization of metastable Fe23B6 and γ-Fe in undercooled Fe83B17

Previous investigations of undercooled liquid Fe83B17 near the eutectic composition have found that metastable crystalline phases, such as Fe23B6, can be formed and persist down to ambient temperature even for rather modest cooling rates. Using time-resolved high-energy x-ray diffraction on electrostatically levitated samples of Fe83B17, we demonstrate that the Fe23B6 metastable phase and fcc γ-Fe grow coherently from the undercooled Fe83B17 liquid and effectively suppress the formation of the equilibrium Fe2B + bcc α-Fe phases. The stabilization of γ-Fe offers another opportunity for experimental investigations of magnetism in metastable fcc iron.

A computational study of diffusion in a glass-forming metallic liquid.

Liquid phase diffusion plays a critical role in phase transformations (e.g. glass transformation and devitrification) observed in marginal glass forming systems such as Al-Sm. Controlling transformation pathways in such cases requires a comprehensive description of diffusivity, including the associated composition and temperature dependencies. In the computational study reported here, we examine atomic diffusion in Al-Sm liquids using ab initio molecular dynamics (AIMD) and determine the diffusivities of Al and Sm for selected alloy compositions. Non-Arrhenius diffusion behavior is observed in the undercooled liquids with an enhanced local structural ordering. Through assessment of our AIMD result, we construct a general formulation for Al-Sm liquid, involving a diffusion mobility database that includes composition and temperature dependence. A Volmer-Fulcher-Tammann (VFT) equation is adopted for describing the non-Arrhenius behavior observed in the undercooled liquid. The composition dependence of diffusivity is found quite strong, even for the Al-rich region contrary to the sole previous report on this binary system. The model is used in combination with the available thermodynamic database to predict specific diffusivities and compares well with reported experimental data for 0.6 at.% and 5.6 at.% Sm in Al-Sm alloys.

Microgravity metal processing: from undercooled liquids to bulk metallic glasses.

Bulk metallic glasses (BMGs) are a novel class of metal alloys that are poised for widespread commercialization. Over 30 years of NASA and ESA (as well as other space agency) funding for both ground-based and microgravity experiments has resulted in fundamental science data that have enabled commercial production. This review focuses on the history of microgravity BMG research, which includes experiments on the space shuttle, the ISS, ground-based experiments, commercial fabrication and currently funded efforts.

Solute–solute correlations responsible for the prepeak in structure factors of undercooled Al-rich liquids: a molecular dynamics study

We have performed molecular dynamics simulations on a typical Al-based alloy Al90Sm10. The short-range and medium-range correlations of the system are reliably produced by ab initio calculations, whereas the long-range correlations are obtained with the assistance of a semi-empirical potential well-fitted to ab initio data. Our calculations show that a prepeak in the structure factor of this system emerges well above the melting temperature, and the intensity of the prepeak increases with increasing undercooling of the liquid. These results are in agreement with x-ray diffraction experiments. The interplay between the short-range order of the system originating from the large affinity between Al and Sm atoms, and the intrinsic repulsion between Sm atoms gives rise to a stronger correlation in the second peak than the first peak in the Sm–Sm partial pair correlation function (PPCF), which in turn produces the prepeak in the structure factor.

Linking structure to fragility in bulk metallic glass-forming liquids

Using in-situ synchrotron X-ray scattering, we show that the structural evolution of various bulk metallic glass-forming liquids can be quantitatively connected to their viscosity behavior in the supercooled liquid near Tg. The structural signature of fragility is identified as the temperature dependence of local dilatation on distinct key atomic length scales. A more fragile behavior results from a more pronounced thermally induced dilatation of the structure on a length scale of about 3 to 4 atomic diameters, coupled with shallower temperature dependence of structural changes in the nearest neighbor environment. These findings shed light on the structural origin of viscous slowdown during undercooling of bulk metallic glass-forming liquids and demonstrate the promise of predicting the properties of bulk metallic glasses from the atomic scale structure.

Formation of nano-porous GeOx by de-alloying of an Al–Ge–Mn amorphous alloy

The present study shows that nanometer-scale amorphous phase separation occurs by spinodal decomposition of the undercooled liquid in a melt-spun Al 60 Ge 30 Mn 10 alloy, although there is no atomic pair with positive enthalpy of mixing. By adopting a proper de-alloying process, an interconnected nano-porous germanium oxide with an amorphous structure is successfully synthesized. The present study shows that nano-porous amorphous germanium oxide can be easily obtained by de-alloying of Al-based amorphous alloys with nm-scale composition fluctuation.

Structural and dynamical properties of liquid Ag74Ge26 alloy studied by experiments and ab initio molecular dynamics simulation

The structures and dynamics of liquid Ag74Ge26 alloy at different temperatures were investigated by high-energy X-ray diffraction and ab initio molecular dynamics simulation. The calculated structure factors agree well with the experimental data. Local atomic structure orders have been characterized by angular distribution functions, Honeycutt-Andersen index, Voronoi tessellation and the atomic cluster alignment. We reveal that (i) 〈0,3,6,4,0〉, 〈0,2,8,4,0〉, 〈0,3,6,5,0〉, and 〈1,3,4,5,1〉 polyhedral dominate in the undercooled Ag74Ge26 liquid. (ii) The icosahedral-like short-range order is enhanced as the temperature decreases while 〈0,3,6,4,0〉, 〈0,2,8,4,0〉, 〈0,3,6,5,0〉, and 〈0,1,10,2,0〉 polyhedral become the major short-range orders at low temperature. (iii) The medium-range order which was formed for amorphous phase at ambient temperature persists in the undercooled liquid. In addition, it is found that the diffusion constant of Ag is lower than that of Ge over the studied temperature range. Our findings elucidate that highly dominated short-range order associated with nearest-neighbors shell could empower the development of medium-range order and the formation of metallic glasses.

Characteristics of bulk liquid undercooling and crystallization behaviors of jet electrodeposition Ni–W–P alloy

The undercooling of Ni–W– P ternary alloy coating melt was investigated by in situ differential scanning calorimeter (DSC) with the flux processing technique. The results showed that the highest undercooling of Ni–W–P ternary alloy with 359 K was obtained as the thermal treatment temperature of the melt being 1679 K and the cooling rate being 50 K min−1. When cooling rate is fixed, the change of undercooling depends on the melt processing temperature, and the undercooling will increase rapidly at the first stage. The effects of thermal treatment temperature and cooling rates on the undercooling were discussed.

Crystallization of undercooled liquid fenofibrate.

Formulation of hydrophobic drugs as amorphous materials is highly advantageous as this increases their solubility in water and therefore their bioavailability. However, many drugs have a high propensity to crystallize during production and storage, limiting the usefulness of amorphous drugs. We study the crystallization of undercooled liquid fenofibrate, a model hydrophobic drug. Nucleation is the rate-limiting step; once seeded with a fenofibrate crystal, the crystal rapidly grows by consuming the undercooled liquid fenofibrate. Crystal growth is limited by the incorporation of molecules into its surface. As nucleation and growth both entail incorporation of molecules into the surface, this process likely also limits the formation of nuclei and thus the crystallization of undercooled liquid fenofibrate, contributing to the good stability of undercooled liquid fenofibrate against crystallization.

Thermal behavior of Se-rich Ge2Sb2Se(5−y)Tey chalcogenide system

The thermal behavior of the Ge2Sb2Se(5−y)Tey chalcogenide system (a compositional analog of the famous GST-225 PCM material) was thoroughly studied. Glass transition, cold crystallization, and melting phenomena were investigated in dependence on composition and various experimental conditions (heating rate, particle size). The kinetics of the structural relaxation and crystallization processes are described in terms of the Tool–Narayanaswamy–Moynihan and Autocatalytic models. The complexity of these processes is thoroughly discussed with regard to the compositionally given changes of molecular structures. Heat capacity measurements were performed for the compositions with y=0, 0.5, and 1 in the 50–550°C temperature range. All glassy and crystalline Cp dependences were found to be identical within the experimental error. The compositional dependence of the P-type undercooled liquid Cp evolution was found to contribute significantly to the changes in glass-forming ability (GFA). The GFA trend was discussed in terms of the mutual interplay between the thermodynamics and kinetics in this system.

Thermal behavior in Se-Te chalcogenide system: interplay of thermodynamics and kinetics.

Heat capacity measurements were performed for Se, Se90Te10, Se80Te20, and Se70Te30 materials in the 230-630 K temperature range. Both glassy and crystalline Cp dependences were found to be identical within the experimental error. The compositional dependence of the N-type undercooled liquid Cp evolution was explained on the basis of free-volume theory; vibrational and chemical contributions to heat capacity were found to be roughly similar for all Se-Te compositions. The thermal behavior in the Se-Te chalcogenide system was thoroughly studied: glass transition, cold crystallization, and melting were investigated in dependence on composition and various experimental conditions (heating rate, particle size, and pre-nucleation period). The kinetics of the structural relaxation and crystallization processes are described in terms of the Tool-Narayanaswamy-Moynihan and Johnson-Mehl-Avrami models. The complexity of these processes is thoroughly discussed with regard to the compositionally determined changes of molecular structures. The discussion is conducted in terms of the mutual interplay between the thermodynamics and kinetics in this system.

Modelling the creation and destruction of columnar and equiaxed zones during solidification and melting in multi-pass welding of steel

The authors present a novel meso-scale (mm–cm) numerical model of multi-pass welding of stainless steel based on the front tracking formulation. During a typical multi-pass notched butt weld, multiple cycles of melting and solidification occur throughout the weld pool, where previously solidified equiaxed zones are destroyed by melting and subsequently resolidify in columnar form. These thermal cycles result from the thermal effects of the current deposition of liquid filler metal being superimposed onto those of the previous pass. The advancement of the liquidus isotherm is employed to characterize melting, while both columnar and equiaxed dendritic growth in the undercooled melt are considered in the model of solidification. Columnar solidification is simulated using a front tracking method where computational markers are employed to explicitly define the advancing columnar front. Competing equiaxed solidification is modelled using a volume averaging approach. During the first and second welding passes of a case study, the weld pool solidifies as purely columnar. This is due to the high thermal gradient present in the weld pool, which results in a very small region of undercooled liquid ahead of the columnar front and subsequently low levels of equiaxed grain nucleation/growth. Towards the end of the third and final pass the equiaxed grains have sufficient dwell time in the undercooled liquid to form a coherent network and present a physical barrier to the advancing columnar front. A Columnar to Equiaxed Transition (CET) occurs and the remainder of the weld pool solidifies in a fully equiaxed microstructure. The results are compared with Hunt’s analytical model of CET and the agreement is reasonably good. The occurrence of CET throughout the weld pool is directly predicted via this front tracking model without the need to rely on estimated thermal gradients and solidification rates. Further simulations indicate that CET can be promoted, even in early passes, by increasing the grain refiner level and/or by pre-heating the parent metal.

Experimental determination of the nucleation rates of undercooled micron-sized liquid droplets based on fast chip calorimetry

Accurate thermal analyzes and calorimetry measurements depend on careful calibration measurements. For conventional differential scanning calorimeters (DSC) the calibration procedure is well known. The melting point of different pure metals is measured and compared with literature data to adjust the temperature reading of the calorimeter. Likewise, the measured melting enthalpies of standard reference substances serve for enthalpy calibration. Yet for fast chip calorimetry, new procedures need to be established. For the medium-area and large-area calorimeter chips, this procedure needs to be modified, because the calibration behavior depends on the position of the sample on the measurement area. Additionally, a way to calibrate the calorimeter for measurements performed during cooling will also be shown. For this second aspect, the athermal and diffusionless martensitic phase transformation of Ni49.9–Ti50.1 at% was used. The well-calibrated sensor chips are ideally suited to perform nucleation rate density analyzes based on a statistical approach. Here, the nucleation rate densities of micron-sized pure Sn droplets that had been coated with a non-catalytic coating have been determined by experimental analysis of the statistical variance of the undercooling response.

Composition-dependent stability of the medium-range order responsible for metallic glass formation

The competition between the characteristic medium-range order corresponding to amorphous alloys and that in ordered crystalline phases is central to phase selection and morphology evolution under various processing conditions. We examine the stability of a model glass system, Cu–Zr, by comparing the energetics of various medium-range structural motifs over a wide range of compositions using first-principles calculations. We focus specifically on motifs that represent possible building blocks for competing glassy and crystalline phases, and we employ a genetic algorithm to efficiently identify the energetically favored decorations of each motif for specific compositions. Our results show that a Bergman-type motif with crystallization-resisting icosahedral symmetry is energetically most favorable in the composition range 0.63<xCu<0.68, and is the underlying motif for one of the three optimal glass-forming ranges observed experimentally for this binary system (Li et al., 2008). This work establishes an energy-based methodology to evaluate specific medium-range structural motifs which compete with stable crystalline nuclei in deeply undercooled liquids.