Undercooled Droplets Research Articles

Crystalline Microstructure, Microsegregations, and Mechanical Properties of Inconel 718 Alloy Samples Processed in Electromagnetic Levitation Facility

The solidification of Inconel 718 alloy (IN718) from undercooled liquid is studied. The solidification kinetics is evaluated in melted and undercooled droplets processed using the electromagnetic levitation (EML) technique by the temperature–time profiles and solid/liquid (S/L) interface movement during recalescence. The kinetics is monitored in real time by special pyrometrical measurements and high-speed digital camera. It is shown that the growth velocity of γ-phase (the primary phase in IN718), the final crystalline microstructure (dendritic and grained), and the mechanical properties (microhardness) are strongly dependent on the initial undercooling ΔT at which the samples started to solidify with the originating γ-phase. Particularly, with the increase in undercooling, the secondary dendrite arm spacing decreases from 28 μm to 5 μm. At small and intermediate ranges of undercooling, the solidified droplets have a dendritic crystalline microstructure. At higher undercooling values reached in the experiment, ΔT>160 K (namely, for samples solidified with ΔT=170 K and ΔT=263 K), fine crystalline grains are observed instead of the dendritic structure of solidified drops. Such change in the crystalline morphology is qualitatively consistent with the behavior of crystal growth kinetics which exhibits the change from the power law to linear law at ΔT≈160 K in the velocity–undercooling relationship (measured by the advancement of the recalescence front in solidifying droplets). Study of the local mechanical properties shows that the microhardness increases with the increase in the γ″-phase within interdendritic spacing. The obtained data are the basis for testing the theoretical and computational of multicomponent alloy samples.

Statistical learning for evaluation of crystal growth in low-melting alloy droplets with application to quasicrystal-forming Ti–Zr–Ni alloys

The evaluation of the crystal growth in undercooled alloy melts is essential for investigations of their solidification behavior. Low-melting alloy systems usually exhibit little self-illumination and cinematography is not often feasible. A time-temperature data based statistical learning approach is presented to evaluate the crystal growth in an undercooled droplet, where a spatial geometry-informed, errors-in-variables probabilistic model is developed based on the prediction of the dendrite growth paths. The preferable growth mechanism and growth velocities can be evaluated from statistical model selection and parameter estimation. Based only on the pyrometry data of two Ti–Zr–Ni alloys, a bulk growth mechanism is justified and the growth velocities in both the stable C14 phase and metastable icosahedral phase are able to be measured and statistically interpreted.

Emulsion droplet crystallinity attenuates early in vitro digestive lipolysis and beta-carotene bioaccessibility

The impacts of lipid crystallinity on in vitro digestive lipolysis and bioaccessibility of encapsulated (0.1 wt%) beta-carotene (BC) were investigated for a 15 wt% cocoa butter emulsion prepared as crystalline (i.e. solid emulsions, SE & SE-BC) or undercooled (liquid emulsions, LE & LE-BC) droplets at 25 °C. Particle size distributions (D4,3 ∼0.7 μm), morphology (spherical), polymorphism (beta-V), thermal behavior (peak melting ∼30 °C), zeta potential (∼−44 mV) and BC degradation under accelerated lighting conditions were similarly extensive. Following exposure to simulated gastric conditions, duodenal hydrolysis and BC bioaccessibility were lower for SE-BC up to 2 h (P < 0.05). Ultimately, samples with both solid and liquid droplets were hydrolyzed extensively and BC bioaccessibility did not differ (P > 0.05). Therefore, for compositionally equivalent emulsions, lipid droplet solid state delayed digestive lipolysis and bioactive solubilization. These results help to clarify the role of lipid physical state on dietary lipid digestion and bioactive release.

Modelling of nucleation in Impulse atomized undercooled droplets

Numerous previous researchers have used nucleation models as part of their simulation of droplet solidification. None of these have verified the nucleation portion of these models with experimental results. In this work, a heterogeneous nucleation model based on the classical nucleation theory was coupled with a thermal model of a falling droplet through a stagnant gas. The pre-exponential factor, which is usually unknown, was determined to obtain an undercooling similar to experimental results for a 196 μm droplet solidified in helium. A stochastic element was added by using a cumulative density probability function, leading to a range of possible nucleation undercoolings and nucleation points. The primary and eutectic undercoolings as well as the number of nucleation points were then compared with additional experimental results of Al-Cu alloy droplets. Good agreement was found between the calculated and the experimental undercoolings. Modeling results also suggest the presence of one nucleation point in the majority of cases and are in agreement with experimental observations.

The origins of spontaneous grain refinement in deeply undercooled metallic melts

Spontaneous grain refinement in undercooled metallic melts has been a topic of enduring interest within the solidification community since its discovery more than 50 years ago. Here we present a comparative study of the solidification microstructures and velocity-undercooling behaviour in two dilute Cu-Ni alloys (3.98 & 8.90 wt.% Ni), which have been undercooled by a melt encasement (fluxing) method. Cu-3.98 wt.% Ni shows grain refinement at both low and high undercooling, with a dendritic growth regime separating the two grain refined regions. Within the grain refined region dendritic fragments are clearly evident in the centres of the refined grains and on the surface of the undercooled droplet, suggesting a dendritic fragmentation mechanism. Cu-8.90 wt.% Ni displays also grain refinement at both high and low undercoolings. In the low undercooling grain refined region the samples display curved grain boundaries with a dendritic substructure that extends across grains, indicative of a recovery and recrystallisation mechanism. Conversely, prior to the onset of the high undercooling grain refinement transition extensive regions of dendritic seaweed are observed, suggesting that it is remelting of a dendritic seaweed that gives rise to this structure. Consequently, in two closely related Cu-based systems we have strong microstructural evidence for the operation of all three mechanisms currently considered to give rise to grain refinement. This may help to resolve the grain refinement controversy, although it remains to be determined what factors determine which mechanism operates in any given system.

Bulk synthesis of amorphous and nano‐crystalline materials by spray forming

Bulk amorphous and nano‐crystalline metallic materials have been observed to possess excellent mechanical and physical properties. The conventional process routes, to synthesize such materials, are restricted by their ability to achieve rapid solidification, which limits the dimensions of the materials produced. In the last 10–12 years, spray forming has been employed to avoid these limitations by using its capability of layer by layer deposition of undercooled droplets. The current literature indicates that the opportunities provided by this process can be effectively utilized to produce bulk materials in a single step. In this paper, an attempt has been made to bring out the developments in the synthesis of bulk amorphous and/or nano‐crystalline materials by spray forming. The effect of process parameters, droplet size distribution in the atomized spray, the thermal conditions of droplets prior to deposition and the deposition surface conditions have been discussed. It has been demonstrate that a layer by layer deposition of undercooled droplets of glass forming alloys on a relatively cold deposition surface is the suitable condition to achieve bulk amorphous/nano‐crystalline materials.

Nucleation Catalysis in Aluminum Alloy A356 Using Nanoscale Inoculants

Different types of nanoparticles in aluminum (Al) alloy A356 nanocomposites were shown to catalyze nucleation of the primary Al phase. Nanoparticles of SiC β, TiC, Al2O3 α, and Al2O3 γ were added to and dispersed in the A356 matrix as nucleation catalysts using an ultrasonic mixing technique. Using the droplet emulsion technique (DET), undercoolings in the nanocomposites were shown to be significantly reduced compared to the reference A356. None of the nanocomposites had a population of highly undercooled droplets that were observed in the reference samples. Also, with the exception of the A356/Al2O3 α nanocomposite, all nanocomposites showed a reduction in undercooling necessary for the onset of primary Al nucleation. The observed nanocomposite undercoolings generally agreed with the undercooling necessary for free growth. The atomic structure of the particles showed an influence on nucleation potency as A356/Al2O3 γ nanocomposites had smaller undercoolings than A356/Al2O3 α nanocomposites. The nucleation catalysis illustrates the feasibility of, and basis for, grain refinement in metal matrix nanocomposites (MMNCs).

Microstructure Characteristics of Spray-Formed and Melt-Spun Al85Nd5Ni10 and Al89La6Ni5 Bulk Hybrid Composites

The microstructure characteristics of the spray-formed and melt-spun Al85Nd5Ni10 and Al89La6Ni5 alloys were studied. The spray forming process was demonstrated to produce a bulk scale hybrid composite consisting of amorphous and nanostructured phases directly without the need of an amorphous precursor. The spray-formed Al89La6Ni5 deposit (~1 mm in thickness) were partially amorphous, and the amorphous phase came from the undercooled liquid droplets upon deposition. The as-spray-formed Al85Nd5Ni10 deposit (~20 mm in thickness) was completely crystallized due to the devitrification of the retained amorphous phase to nano-scale secondary crystals upon deposition. Primary crystals (~1 μm) are dispersed uniformly in the bulk spray-formed amorphous/or partial amorphous composites and many distinctive deformation twins also are observed in the crystals, however, not twins found in the corresponding completely devitrified ribbon. This is mainly because of the stirring and impacting force offered by high velocity droplets during spray forming and the mismatch of thermal expansion coefficient between primary crystals and adjacent amorphous matrix.

Phase separation and rapid solidification of liquid Cu60Fe30Co10 ternary peritectic alloy

The metastable liquid phase separation and rapid solidification of Cu60Fe30Co10 ternary peritectic alloy were investigated by using the drop tube technique and the differential scanning calorimetry method. It was found that the critical temperature of metastable liquid phase separation in this alloy is 1623.5 K, and the two separated liquid phases solidify as Cu(Fe,Co) and Fe(Cu,Co) solid solutions, respectively. The undercooling and cooling rate of droplets processed in the drop tube increase with the decrease of their diameters. During the drop tube processing, the structural morphologies of undercooled droplets are strongly dependent on the cooling rate. With the increase of the cooling rate, Fe(Cu,Co) spheres are refined greatly and become uniformly dispersed in the Cu-rich matrix. The calculations of Marangoni migration velocity (V M) and Stokes motion velocity (V S) of Fe(Cu,Co) droplets indicated that Marangoni migration contributes more to the coarsening and congregation of the minor phase during free fall. At the same undercooling, the V M/V S ratio increases drastically as Fe(Cu,Co) droplet size decreases. On the other hand, a larger undercooling tends to increase the V M/V S value for Fe(Cu,Co) droplets with the same size.

Phase-Field Modeling of Dendritic Solidification in Undercooled Droplets Processed by Electromagnetic Levitation

The results on modeling dendritic solidification from undercooled melts processed by the electromagnetic levitation technique are discussed. In order to model the details of formation of dendritic patterns we use a phase-field model of dendritic growth in a pure undercooled system with convection of the liquid phase. The predictions of the phase-field model are discussed referring to our latest high accuracy measurements of dendrite growth velocities in nickel samples. Special emphasis is given to the growth of dendrites at small and moderate undercoolings. At small undercoolings, the theoretical predictions deviate systematically from experimental data for solidification of nickel dendrites. It is shown that small amounts of impurities and forced convective flow can lead to an enhancement of the velocity of dendritic solidification at small undercoolings.

Metal-like growth of silicon during rapid solidification by quenching undercooled droplets on a chill plate

Abstract Droplets of pure silicon were dropped onto a chill plate of copper within an electromagnetic levitation facility. Through observation of morphologies grown on the quenched surfaces, direct evidence of growth mode transition from a continuous form to a lateral form was provided.

Kinetics of crystal nucleation in undercooled droplets of Sb- and Te-based alloys used for phase change recording

Droplets of molten alloys of composition Ge12Sb88, Ag0.055In0.065Sb0.59Te0.29, Ge4Sb1Te5, and Ge2Sb2Te5, used for optical data storage, surrounded by a molten dehydrated B2O3 flux were undercooled to 40–80K below their liquidus temperature in a differential thermal analyzer. The crystal-melt interfacial energy was calculated from the nucleation temperature using the classical nucleation theory, which gave values of around 0.20 times the heat of fusion per atom in the interface for all alloys. This value should be a lower limit since we did not establish that nucleation was homogeneous in the experiments. The steady-state nucleation rate was calculated between the liquidus and glass transition temperatures and was higher for the GeSbTe alloys than for the Sb-rich alloys. Nevertheless, the nucleation rates appear too high to allow amorphization under operating conditions for the highest achievable cooling rates. Therefore, we conclude that it is the presence of an incubation time that makes amorphization and therefore phase change recording possible in both optical and electronic media.

Crystallization of Spherical Single Crystal of Silicon from Undercooled Droplet under Containerless Processing

The images of the growing crystal in the growth process of the undercooled droplets of silicon were lively recorded by using a high-speed camera. The number of crystal that nucleated spontaneously from the undercooled liquid was found to decrease to 1 when the undercooling was higher than 5K. The morphology of the growing single crystal of silicon was a thin plate. A model for predicting the critical undercooling of growing single crystal of silicon from undercooled liquid has been developed. The theoretically predicted value of the undercooling from present model for silicon is in agreement with the experimentally measured result.

Kinetics of dendritic growth under the influence of convective flow in solidification of undercooled droplets

In the present work, the results on dendritic growth with forced convective flow are discussed. The results are obtained for solidification of a pure nickel melt in an electromagnetically levitated droplet. It is shown that the upstream flow imposed on the growing dendrite enhances the growth velocity of the dendritic tips growing in the opposite direction to the flow. Using the model of dendritic growth with convection, we are computing the dendrite growth velocity versus undercooling and explain in part the disagreement of experimental data with previous model predictions for small undercoolings.

Modèle de transferts thermiques lors de la cristallisation d'une solution binaire dispersée

In this paper, we propose a model concerning the thermal transfers inside an emulsion of aqueous saline binary solution during the crystallization of the dispersed undercooled droplets. This model takes into account the erratic character owed to the probability of water crystallization, and to the probability of salt apparition. It also takes into account the phenomenon of the progressive crystallization owed to the thermodynamic balance in every drop. The model is based on the resolution of the energy equation with a heat source depending on the probability of crystallization of every drop. It has been applied to the scanning calorimetry in the case of an aqueous solution of sodium chloride dispersed within an industrial oil. The studied samples have a cylindrical shape of a few mm 3 in volume. The good agreement between the numerical results and the experimental results permitted to validate the model, and therefore to better describe the thermal transfers in these dispersions. We determined the space–time distribution of the temperature and transformed part of every drop. An evaluation of the different exchange coefficients of involved in the model has also been brought.

Direct observation of the crystal-growth transition in undercooled silicon

Using an electromagnetic levitation facility with a laser heating unit, silicon droplets were highly undercooled in the containerless state. The crystal morphologies on the surface of the undercooled droplets during the solidification process and after solidification were recorded live by using a high-speed camera and were observed by scanning electron microscopy. The growth behavior of silicon was found to vary not only with the nucleation undercooling, but also with the time after nucleation. In the earlier stage of solidification, the silicon grew in lateral, intermediary, and continuous modes at low, medium, and high undercoolings, respectively. In the later stage of solidification, the growth of highly undercooled silicon can transform to the lateral mode from the nonlateral one. The transition time of the sample with 320 K of undercooling was about 535 ms after recalescence, which was much later than the time where recalescence was completed.

Initial crystallization kinetics in undercooled droplets

In large liquid volumes a variety of residual catalysts can limit the attainable undercooling. With a liquid droplet dispersion an effective nucleant isolation has yielded the largest undercooling for solidification and conditions where the nucleation process is the rate limiting step. Calorimetric measurements provide an effective means to determine the nucleation frequency in droplet samples. Since droplet samples are polydisperse in size, a full kinetics evaluation requires also a consideration of the size distribution which often obeys a log–normal relationship. With this information, the isothermal solidification rate constants may be determined to distinguish between a volume dependent process and a heterogeneous surface area dependent mechanism. An additional study of heterogeneous nucleation kinetics is possible in droplets containing well-defined solid catalyst surfaces in contact with the liquid which undercool to a clear nucleation onset. Suitable two-phase liquid+solid droplet samples may be generated in eutectic and peritectic alloys following thermal cycling and allow for a further analysis of the rate controlling kinetics.

Ripening Phenomena in Emulsions — A Calorimetry Investigation

ABSTRACT Ripening phenomena occurring within different kinds of emulsions have been studied. The emulsions concerned are simple water-in-oil (W/O) or oil-in-water (O/W) emulsions, mixed emulsions obtained by the mixture of two simple emulsions, and multiple emulsions water-in-oil-in-water (W/O/W) or oil-in-water-in-oil (O/W/O) emulsions. Composition ripening due to a mass transfer and solid ripening due to the formation of solid particles from the undercooled droplets or due to the formation of solid hydrate around the droplets have been pointed out on using a suitable calorimetric technique. For that purpose a non-diluted emulsion sample is submitted to a cooling and heating cycle during which solidification and melting temperatures and energies of the different phases are analyzed. It has been shown that correlations between these quantities and the properties of the dispersed phase permit one to get information about the ripening phenomena under study. The solution-diffusion model used for mass transfer is in good agreement with the experimental results. From the shell model used for the hydrate formation, it has been possible to deduce the formation energy and the influence of salt upon the temperature of formation.

Non-equilibrium solidification of undercooled droplets during atomization process

Thermal history of droplets associated with gas atomization of melt has been investigated. A mathematical model, based on classical theory of heterogeneous nucleation and volume separation of nucleants among droplets size distribution, is described to predict undercooling of droplets. Newtonian heat flow condition coupled with velocity dependent heat transfer coefficient is used to obtain cooling rate before and after nucleation of droplets. The results indicate that temperature profile of droplets in the spray during recalescence, segregated and eutectic solidification regimes is dependent on their size and related undercooling. The interface temperature during solidification of undercooled droplets rapidly approaches the liquidus temperature of the alloy with a subsequent decrease in solid-liquid interface velocity. A comparison in cooling rates of atomized powder particles estimated from secondary dendrite arm spacing measurements are observed to be closer to those predicted from the model during segregated solidification regime of large size droplets.

Metastable phase formation during solidification of undercooled melt

Undercooling behaviour of micron-sized droplets entrained in the matrix of the primary phase of Ag–Ge, Al–Ge and Cu–Ag alloys was investigated. Thermal cycle consisted of equilibration of hypoeutectic alloys above the eutectic temperature in two phase liquid–solid region followed by their slow cooling or quenching from the mushy state. Repeated thermal cycling generated a wide size range of droplets. During slow cooling droplets exhibited a large undercooling below the eutectic temperature of alloys. Solidification structure of the undercooled droplets revealed presence of metastable phases in Ag–Ge and Al–Ge alloys and a non-equilibrium solid solution phase in Cu–Ag alloy. Solidification behaviour of undercooled melt and formation of metastable microstructure are discussed.