Crystal Growth Velocity Research Articles

Atomistic insights into sluggish crystal growth in CoNi-containing multi-principal element alloys

Understanding the sluggish kinetics is of great significance for improving the properties of multi-principal element alloys (MPEAs). In this paper, the crystal growth in undercooled CoNi, CoNiFe and CoNiPd alloys was studied by molecular dynamics (MD) to show atomistic insights into sluggish crystal growth kinetics. The added Fe and Pd lead to a decrease in the crystal growth velocity and more significant sluggish crystal growth kinetics was observed in CoNiPd. After minimizing the instantaneous potential energy of atoms adjacent to the Solid/Liquid (S/L) interface, it was found that the decreased crystal growth velocity as the number of principal elements could be accounted by the accompanying change of inherent structure. The exploration of bulk undercooled liquid showed that the diffusion kinetic in liquid does not play a critical role on the sluggish crystal growth kinetics. Besides, the investigation of atomic structure in front of the smooth S/L interface revealed that the sluggish crystal growth kinetics induced by properties of element was associated with the atomic spontaneous ordering degree.

The study of hydroxyapatite growth kinetics on CP – Ti and Ti65Zr treated by Plasma electrolytic oxidation process

The kinetics of hydroxyapatite growth on titanium alloys (CP–Ti and Ti65Zr) was studied. Specimens were treated by Plasma Electrolytic Oxidation (PEO) and immersed into the Hanks' solution. The PEO method was performed at AC mode with the current density of 4 A/dm2 in an electrolyte containing calcium acetate and calcium glycerophosphate, allowing the inclusion of calcium and phosphorus in the obtained oxide layer, which promotes the osseointegration when applied to bone tissue. The treated specimens were immersed in the Hanks’ solution with different exposure times, followed by studying changes in morphology, chemical, and phase composition. In addition, the surface wettability was tested, and the corrosion behavior of the treated samples was investigated. Tiny HA crystals can be seen on the surface of CP-Ti samples treated by PEO for 5 and 30 min. The morphology with a distinct layer of HA was obtained for the Ti65Zr sample after 5 min of PEO treatment. As for the immersion times, the highest crystal growth velocity was observed for the Ti65Zr of 5 min of PEO and 14 days of immersion. The change in morphology and EIS analysis confirms the observed behavior. Finally, based on the XRD analysis for the Ti65Zr of 5 min PEO treatment samples, the Srilankite phase in an amount of 60% is responsible for the crystal growth, resulting in the best crystal growth velocity and good corrosion properties (Rp = 0.732 MΩ cm2).

Multi-fidelity Bayesian optimization to solve the inverse Stefan problem

In this work, we propose an efficient solution of the inverse Stefan problem by multi-fidelity Bayesian optimization. We construct a multi-fidelity Gaussian process surrogate model by combining many low-fidelity estimates of a solidification problem with only a few high-fidelity measurements. To solve the inverse problem, we employ the Gaussian process model in a Bayesian optimization approach based on a multi-fidelity knowledge gradient acquisition function. To account for the specific structure of the target function, we reformulate it as a composite function and thus significantly improve the stability of the optimization procedure. Target values can be switched easily, and previously obtained samples and surrogate models can be reused. The proposed method iteratively improves the recommended solution of the inverse problem. Explicitly adding recommended points of previous iterations to the solution procedure enhances the convergence properties of the algorithm.We demonstrate the applicability of the algorithm by solving the inverse problem for a planar solidification front in a single-fidelity setting. Process parameters are identified for targeted crystal-growth velocities during directional dendritic solidification. The relation between these velocities and process parameters, such as undercooling, thermal diffusivity, capillarity, and capillary anisotropy, defines the thermo-mechanical properties of the solidified part in metal-based additive manufacturing. Material design is based on the corresponding inverse problem. Cost-efficiency of solving the inverse problem is improved by introducing a fidelity hierarchy based on coarse-grid approximations of high-fidelity numerical simulations. The open-source simulation framework ALPACA for multiphase flows allows to generate data at all fidelities. We demonstrate the superior convergence properties of the presented multi-fidelity approach by comparison with an approach solely based on high-fidelity measurements of the tip velocity.

Structure and Crystallization Kinetics of As‐Deposited Films of the GeTe Phase Change Compound from Atomistic Simulations

Models of the amorphous phase of the GeTe compound have been generated by depositing individual atoms on a substrate by molecular dynamics simulations. This material is of interest for applications in phase change memories. The kinetic energy of the atoms impinging on the surface is in the range 1.5–10 eV which is typical of magnetron sputtering growth. The simulations with up to 8000 atoms exploit a machine learning potential devised previously by the group. The structural properties of the films depend very little on the energy of the impinging atoms in the range 1.5–10 eV and they are also very similar to those of amorphous models generated by quenching from the melt, but for a larger fraction of GeGe bonds and Ge atoms in tetrahedral geometries in the films. The crystal nucleation and growth are then simulated at 600 K which is close to the temperature of maximal crystal growth velocity exploited in the memory devices. Despite the structural differences mentioned above, the kinetics of crystallization at 600 K is very similar for the as‐deposited and melt‐quenched amorphous models.

The Effect of Cryoprotectants Concentration on Ice Crystal Propagation Velocity.

Background: Two physicochemical effects occur during vitrification: nucleation and crystallization. Nucleation is a statistical occurrence by its nature. Thus, the more water molecules that are present the higher are the chances for nucleation to occur. Crystallization is a first-order transition where a water molecule is incorporated into ice crystal. Intracellular viscosity, which is the combination of water, salts, and cryoprotectants (CPs), affects both the nucleation and crystal growth rates. Ice velocity is inversely correlated with the viscosity and directly proportional to the function of the system's supercooling. However, little is known about the speed of ice crystal propagation in vitrification solutions containing different concentrations of CPs. Methods: This article describes the ice crystal propagation velocity while referring to vitrification. Ice crystal propagation velocity was measured in solutions containing different CP (dimethyl sulfoxide [DMSO], propylene glycol [PG], ethylene glycol [EG], and glycerol) concentrations at a supercooled temperature. The different CPs solutions were inserted into 0.25 mL straws and placed in different temperatures of an alcohol bath (Tc) at supercooling temperatures of -8°C to -10°C. Results: We found that ice crystal propagation is inversely correlated to CP concentrations. Interestingly, PG showed, with statistically significant results, lower ice crystal growth velocities up to concentrations of 30% (v/v), compared with DMSO, and EG at the same concentrations. The combination of EG with PG showed better results (0.25 mm/s) than EG with DMSO (0.39 mm/s) in terms of decreasing the ice crystal growth velocity. When the concentration was increased to 40% (v/v), EG showed the lowest ice crystal propagation velocity (0.09 mm/s), although not significantly different than PG and glycerol but significantly lower than DMSO (0.13 mm/s). Conclusion: These results suggest that current vitrification solutions are not optimized. Based on our results, we suggest that combining PG with EG has advantages over the combination of DMSO and EG, which might promote successful cell and tissue vitrification.

Anomalous kinetics, patterns formation in recalescence, and final microstructure of rapidly solidified Al-rich Al-Ni alloys

From thermodynamical consideration, rather a monotonically increasing crystal growth velocity with increasing undercooling is expected in the crystallization of liquids, mixtures, and alloys [P.K. Galenko and D. Jou, Physics Reports 818 (2019) 1]. By contrast to this general theoretical statement, Al-rich Al-Ni alloys show an anomalous solidification behavior: the solid-liquid interface velocity slows down as the undercooling increases [R. Lengsdorf, D. Holland-Moritz, D. M. Herlach, Scripta Materialia 62 (2010) 365]. It is also found that besides the anomalous growth behaviour, changes in the shape of the recalescence front as the growth front morphology occur. In the light of recent measurements in microgravity with an Al-25at.% Ni alloy sample onboard the International Space Station (ISS) results confirming this anomalous behavior as an unexpected trend in solidification kinetics are presented. The measurements show multiple nucleation events forming the growth front, a mechanism that has been observed for the first time in Al-Ni alloys [D. Herlach et al., Physical Review Materials 3 (2019) 073402; M. Reinartz et al. JOM 74 (2022) 2420] and summarized with detailed analysis in the present publication over a wider range of concentrations. Particularly, the experimental measurements and obtained data directly demonstrate that the growth front does thus not consist of dendrite tips (as in usual rapid solidifying samples), but of newly forming nuclei propagating along the sample surface in a coordinated manner. Theoretical analysis on intensive nucleation ahead of crystal growth front is made using the previously developed model [D.V. Alexandrov, Journal of Physics A: Mathematical and Theoretical 50 (2017) 345101]. Using equations of this model, quantitative calculations confirm the interpretation of experimentally observed propagation of the recalescence front and obtained data on the microstructure of droplets solidified in electromagnetic levitation facility (EML) on the Ground, under reduced gravity during parabolic flights, and in microgravity conditions onboard the ISS.

In situ studies of non-equilibrium crystallization of AlxCoCrFeNi (x = 0.3, 1) high-entropy alloys

To understand the effect of aluminum addition on the non-equilibrium solidification of the equiatomic CoCrFeNi alloy, the Al0.3CoCrFeNi and AlCoCrFeNi high-entropy alloys (HEAs) have been investigated in situ using electromagnetic levitation and high-energy synchrotron X-ray diffraction (XRD). Because the five-component alloys could not be deeply undercooled, the studies were confined to the low undercooling regime. The in situ XRD revealed that a fcc single-phase solid solution is the primary crystalline phase nucleating in Al0.3CoCrFeNi melt undercooled up to 80 K. In contrast, the B2 ordered phase appears as the primary phase in the AlCoCrFeNi melt, at least at a low undercooling of about 30 K. These results correlate with the equilibrium phase diagrams available. Crystal growth velocities of the Al0.3CoCrFeNi alloy are shown to be smaller than for CrFeNi and CoCrNi equiatomic alloys; nonetheless, they are in the same order of magnitude. Spontaneous grain refinement was observed in the Al0.3CoCrFeNi alloy solidified at a low undercooling of ΔT = 70 K.

P-220 Ice crystals propagation velocity is related to the type and concentration of cryoprotectants and impacts cells and tissue vitrification success

Abstract Study question How different cryoprotectants at various concentrations affect ice crystals formation and propagation during vitrification? Summary answer The velocity of ice crystal propagation is inversely related to concentration and type of cryoprotectants and is responsible for downstream cell damage during vitrification. What is known already Vitrification has replaced slow freezing for oocyte and embryo cryopreservation and is now being considered also for gonadal tissue. However, the results in terms of both vitrification and rewarming survival rates are still not uniform. Understanding the complex physical events controlling ice crystals formation, propagation and expansion are key for improving cryoprotectant solutions and overcoming challenges. Study design, size, duration Basic science research. Ice crystal propagation velocity was measured in solutions of four cryoprotectants: dimethyl sulfoxide (DMSO), propylene glycol (PG), ethylene glycol (EG) and glycerol (Gly), at various concentrations in supercooled temperatures. Each experiment used five 0.25ml and five 2ml straws (volume 0.1 ml and 1 ml) containing different cryoprotectant combinations (DMSO:EG, EG:PG, DSMO:PG, DMSO:Gly, EG:Gly, PG:Gly and EG+PG vs EG+DMSO), immersed in alcohol baths prepared at 8-10 °C below the freezing points of each solution. Participants/materials, setting, methods Alcohol baths were set as follows: (1) between -10 °C and -12 °C for straws with 0% and 10% (v/v) DMSO, EG, PG and Gly; (2) -15 °C to -19 °C for the 20%; (3) -19.5 to -24 °C for the 30%, and for the mixture of 15% EG and 15% PG and 15% EG and 15% DMSO (most used vitrification solutions); and (4) -25 to -36 °C for the 40%. A 50µm thermocouple measured supercooling. Student’s T-test analyzed differences. Main results and the role of chance Seeding was done by touching the straw end with LN- precooled forceps until visualizing a small ice crystal. Following 10 to 60s the distance to which the ice crystals grew inside the straws was evaluated using a fine caliber. Ice crystal propagation was inversely correlated to type and cryoprotectant concentrations. Propylene glycol showed statistically significant lower ice crystals growth velocity up to concentrations of 30% (v/v), compared to DMSO and EG at the same concentrations (p < 0.05). Moreover, the ice crystal velocity of PG was also lower than Gly at these same concentrations. The combination of EG with PG decreased ice crystal propagation better than EG with DMSO. At concentrations of 40%, there were no differences. These results suggest that current cryopreservation vitrification solutions are not optimized and combining PG with other CPs has advantages. Using a vitrification solution of 40% DMSO or EG, the velocity of crystal growth was in the range of 0.1mm/sec (6mm/min). This means that to vitrify large tissues, a cooling rate of over 1000 °C/min from RT to a temperature of -196 °C (ΔT of 220 °C in 13 seconds) is necessary to avoid a single ice nucleus to grow to a size of 1.3mm during cooling or warming. Limitations, reasons for caution Assessing the velocity of ice propagation is a novel concept to perfect vitrification/warming solutions. Wider implications of the findings These results pave the way to develop protocols and platforms able to minimize the deleterious impact of cryoprotectant concentrations on both cells and tissues survival after vitrification. Trial registration number not applicable

Convective effects on columnar dendritic solidification – A multiscale dendritic needle network study

Gravity-induced buoyancy, inevitable in most solidification processes, substantially alters the dynamics of crystal growth, such that incorporating fluid flow in solidification models is crucial to understand and predict key aspects of microstructure selection. Here, we present a multi-scale Dendritic Needle Network (DNN) model for directional solidification that includes buoyant flow in the liquid, and apply it to a range of alloys and growth conditions. After a brief presentation of the model, we study the selection of stable primary dendrite arm spacings in Al-4at.%Cu and in Ti-45at.%Al alloys under different gravity levels, comparing both applications to published phase-field results and experimental measurements. Then, we simulate the oscillatory growth behavior recently reported via X-ray in situ imaging of directional solidification of nickel-based superalloy CMSX-4. In this last application, the DNN simulations manage to reproduce the oscillatory growth behavior, and hence permit identifying the fundamental mechanisms behind the oscillatory growth regime. In particular, we show that sustained oscillations occur when the average liquid flow velocity is close to the crystal growth velocity, and that primary dendritic spacings also play a crucial role in the oscillatory behavior.

Accelerated crystal growth in a lithia aluminosilicate glass

No viscosity change in the vicinity of the crystal/melt interface is to be expected for crystal growth in silicate systems at constant temperature in a deeply undercooled isochemical melt. In most experimental studies, however, absolute chemical conformity is not realized and the formation of an interfacial zone with presumably strongly altered viscosity will occur. In the present investigation isolated Li2O⋅Al2O3⋅4SiO2 (LAS4) single crystals grew in a slightly alumina deficient base glass at temperatures above glass transition. Due to the depletion of alumina and the enrichment of excess components and impurities of the continuously growing interfacial zone (“shell”), its viscosity will change. Dedicated experiments showed that the viscosity of the interfacial zone is up to 7 orders of magnitude lower than the viscosity of the base glass. The decreased viscosity at the crystal growth front is assumed to cause the experimentally observed increase of the crystallisation velocity with the crystal radius. This behaviour can be quantitatively modelled introducing a linear decrease of the activation enthalpy with the crystal radius. Obviously, the proposed model can be likewise used in the opposite case, i.e. for an increasing viscosity of the interfacial zone during crystal growth. According to the standard concept of a fractional viscosity relation, a coupling factor can formally be extracted from the comparison between the experimentally determined activation enthalpies of the crystal growth velocity and of the base glass viscosity. The resulting unrealistically low numerical value indicates that it is not the high viscosity of the base glass, but the lower viscosity of the interfacial zone that must be considered for the determination of a physically meaningful coupling factor.

Stochastic multi-fidelity surrogate modeling of dendritic crystal growth

In this work, we propose a novel framework coupling state-of-the-art multi-fidelity Gaussian Process modeling techniques with input-space warping for a cost-efficient construction of a stochastic surrogate model. During model generation, we achieve high computational efficiency by combining a large number of cheap estimates (low-fidelity model) with only a few, computationally expensive, high-fidelity measurements. We base the fidelity hierarchy on coarse-grid approximations of high-fidelity numerical simulations and show its successful application within the proposed framework. Utilizing coarse-grid approximations for multi-fidelity modeling is attractive for many practical applications, since it often allows for multi-fidelity data generation with a single simulator. As benchmark, we apply this framework to generate a surrogate model for crystal growth velocities in directional dendritic solidification. The derivation of a relation between this tip velocity and process parameters, such as undercooling, thermal diffusivity, capillarity, and capillary anisotropy, has been in the focus of research for decades due to its important role on microstructure evolution during solidification. It defines the thermo-mechanical properties of the solidified part and influences its behavior in subsequent manufacturing steps.As data generator, we use the open-source simulation framework ALPACA, applying a conservative sharp-interface level-set model. We assess the accuracy of the multi-fidelity tip velocity model by using cross-validation techniques. Compared to single-fidelity models purely based on high-fidelity data, our approach improves prediction accuracy significantly but only requires a little cost overhead for data generation. The stochastic nature of the resulting surrogate model allows for quantifying the uncertainty associated with predictions. This motivates the application of the model in Bayesian-optimization algorithms for inverse problems. Also, it may serve as input for microstructure simulations which rely on accurate relations between local solidification velocities and process parameters such as undercooling to predict grain-scale crystalline structures and which need material-dependent model calibration.

Isothermal crystallization behavior of undercooled liquid Pd40Cu30Ni10P20 in terms of crystal growth, overall volume crystallization kinetics and their relation to the viscosity temperature dependence

Abstract The crystallization behavior of undercooled liquid Pd40Cu30Ni10P20 metallic alloy has been studied with the aid of differential scanning calorimetry, optical metallography, and electron microscopy methods. The crystallization occurs as a single-stage process starting preferably on the surface of casting voids in the sample volume and on the outer sample cutting planes. The linear crystal growth velocity of crystallizing cells was measured at several constant temperatures in the range 590 –620 K. It is constant at constant temperatures and its temperature dependence is in good agreement with the theoretical concepts of Uhlmann using the temperature dependence of the viscosity of the alloy studied. A differential scanning calorimetry study of isothermal overall volume transformation kinetics of samples pre-annealed at 600 K for 600 s was carried out at several constant temperatures. The experimental data are interpreted on the basis of the equation of Kolmogorov-Avrami-Johnson-Mehl for the case of three-dimensional growth of a fixed number of pre-existing nuclei. The possibility to revitrify the crystallized samples by their melting in differential scanning calorimeter, followed by quick cooling with cooling rate of 5 K τ– 1 is shown. The volume transformation kinetics of revitrified samples has been also studied.

The best diffusivity proxy for crystal nucleation in stoichiometric oxide glasses

Due to the difficulty in measuring the mobility of the “structural units” of supercooled liquids and glasses during crystal nucleation, the effective diffusivity (D) has been frequently estimated via viscosity (Dη) or nucleation time-lags (Dτ). However, it has been recently reported that, below the glass transition temperature (Tg), the experimental steady-state nucleation rates (Ist) can reach much higher values than those predicted by the Classical Nucleation Theory (CNT) when assuming D ≈ Dη or D ≈ Dτ. Hence, another possibility would be inferring D from experimental crystal growth velocity data (D ≈ DU), which is a natural approach since nucleation and growth are likely controlled by the same mass transfer process through the crystal/liquid interface. Therefore, the approximations D ≈ Dη and D ≈ DU were tested in this work using lithium disilicate glass as a model system. To avoid the influence of compositional changes, we measured the nucleation rates, growth velocity, and viscosity using samples of the same glass batch. With this strategy, we found that the very long experimental nucleation times implemented below Tg shifted the experimental temperature of maximum nucleation rate to temperatures lower than those previously reported for this glass former. Most important is that the CNT gives a much better description of the experimental Ist(T) data in the whole analyzed temperature range if D is approximated by DU instead of Dη, indicating that crystal growth is indeed an adequate proxy for nucleation diffusivity.

Rapid eutectic growth: from rod growth to diffusionless solidification.

Numerous experimental data on the rapid solidification of eutectic systems exhibit the formation of metastable solid phases with the initial (nominal) chemical composition. This fact is explained by the suppression of eutectic decomposition due to diffusionless (chemically partitionless) solidification beginning at a high but finite growth velocity of crystals. In the present work, a model is suggested for the diffusionless growth to analyse the atomic diffusion in the rod eutectic couples growing into supercooled liquid. A simplified calculating method for the equation related to the Bessel function in the solution of the growth of rod eutectics is obtained. This method can also be used in the calculation of other rod eutectic growth models. This article is part of the theme issue 'Transport phenomena in complex systems (part 2)'.

Phase constitution and microstructure of the NbTiVZr refractory high-entropy alloy solidified upon different processing

Time- and temperature-resolved phase formation in the equiatomic NbTiVZr refractory high-entropy alloy has been studied in situ using high-energy synchrotron X-ray diffraction and high-speed video imaging during non-equilibrium solidification. Phase formation is shown to be dependent on the solidification conditions. When the melt is undercooled over 80 K it crystallizes as a bcc single-phase solid solution despite solute partitioning between dendrites and interdendritic regions. When the sample is for some time kept in the semisolid state, two additional bcc phases form in the interdendritic regions. The crystal growth velocity for the NbTiVZr alloy, as estimated from the high-speed videos, shows pronounced sluggish kinetics.

Mathematical modeling of bulk and directional crystallization with the moving phase transition layer

This paper is devoted to the mathematical modeling of a combined effect of directional and bulk crystallization in a phase transition layer with allowance for nucleation and evolution of newly born particles. We consider two models with and without fluctuations in crystal growth velocities, which are analytically solved using the saddle‐point technique. The particle‐size distribution function, solid‐phase fraction in a supercooled two‐phase layer, its thickness and permeability, solidification velocity, and desupercooling kinetics are defined. This solution enables us to characterize the mushy layer composition. We show that the region adjacent to the liquid phase is almost free of crystals and has a constant temperature gradient. Crystals undergo intense growth leading to fast mushy layer desupercooling in the middle of a two‐phase region. The mushy region adjacent to the solid material is filled with the growing solid‐phase structures and is almost desupercooled.

A novel preparation of high purity TiO2 from industrial low concentration TiOSO4 solution via short sulfate process

High purity TiO2 was prepared by authigenic seed thermal hydrolysis route from industrial low concentration titanyl sulfate solution via the short sulfate process. The hydrolysis conditions had important influences on the hydrolysis process. The appropriate induced water volume ratio and hydrolysis time were conducive to form appropriate quantity and quality of hydrolysis seeds, adjust and control the precipitation, crystal growth, particle growth and aggregation velocity for the hydrated TiO2, which could obtain the hydrated TiO2 with smaller average particle size and specific surface area. These effects were beneficial to reduce the adsorption of impurities, and improve the purity of TiO2. The hydrolysis conditions had internal influencing relationship between them. The optimized hydrolysis conditions were the induced water volume ratio was of 60%, the hydrolysis time was of 120 min, and the high purity, monodisperse titanium dioxide over 99.98% content could be obtained.

Arrhenius behavior of crystallization at up to 184 000 K/s in Ge2Sb2Te5 thin films

We investigate the glass transition and crystallization of Ge2Sb2Te5 by nanocalorimetry on 32–57 nm layers at temperature scanning rates ranging from 23 to 184 kK/s. Crystallization follows the Arrhenius behavior observed at low heating rates, with an activation energy of 2.7 ± 0.1 eV. The crystal-growth velocity (>500 µm/s at 500 K) confirms fast values found in earlier work and by ab initio molecular dynamics simulations. These experiments also outline the importance of carrying out ultra-fast scanning calorimetry studies with samples in excellent thermal contact with the heating device.

Tendency of crystal orientation rotation toward stable {001} during lateral crystal growth of Si thin film sandwiched by SiO2

Stable crystal orientation (CO) for lateral growth of Si thin film sandwiched by SiO2 was evidenced to be only {001} in normal direction (ND {001}) and 〈100〉 ±5° in scanning direction (SD 〈100〉). Crystal with ND{001} is quasi-stable when angle θ between inplane 〈110〉 and SD is among 15° ≤ θ < 40° and is unstable when θ is θ < 15°. CO other than the stable CO will rotate spontaneously toward the stable CO, i.e. ND{001} with SD〈100〉 ±5°. Most ND{001} crystal was ended by twinning before the CO come to the stable CO. The twinning was triggered by gas ejection or particles, so suppressing of these phenomena would be the key for increasing ND{001}SD〈100〉 crystal occupations. These results have been verified for crystal growth velocity among 0.04–45 mm s−1.

In situ study of non-equilibrium solidification of CoCrFeNi high-entropy alloy and CrFeNi and CoCrNi ternary suballoys

The solidification behavior of the CoCrFeNi high-entropy alloy and the ternary CrFeNi and CoCrNi medium-entropy suballoys has been studied in situ using high-speed video and synchrotron X-ray diffraction on electromagnetically levitated samples. In all alloys, the formation of a primary metastable bcc phase was observed if the melt was sufficiently undercooled. The delay time for the onset of the nucleation of the stable fcc phase, occurring within bcc crystals, is inversely proportional to the melt undercooling. The experimental findings are corroborated by thermodynamic calculations of stable and metastable phase diagrams for the (CoCrNi)-Fe section. Crystal-growth velocities for the CrFeNi, CoCrNi, and CoCrFeNi medium- and high-entropy alloys extracted from the high-speed video sequences in the present study are comparable to the literature data for Fe-rich Fe-Ni and Fe-Cr-Ni alloys, evidencing the same crystallization kinetics. The effect of melt undercooling on the microstructure of solidified samples is analyzed and discussed.