Search

Metal matrix composites (MMC) have many technical applications, e. g. in automotive industry, and have still a great potential. Usually they are produced by expensive powder metallurgy. Cost effective solidification of the material directly from a particle containing melt requires an understanding of the interaction of the ceramic particles with the advancing solidification front. The knowledge is limited about dendritic solidification with particles. The experimental idea in this work is to deeply undercool the melt below its melting point, resulting in a fast propagating dendritic solid/liquid interface. Due to the rapid solidification, microstructure is frozen and the spatial distribution of particles in the microstructure can be analyzed postmortem. Experimental techniques are processing in a terrestrial electromagnetic levitation facility (EML) and, under low gravity conditions during parabolic flight, in the TEMPUS facility. Convection has a strong influence on the interaction between particles and dendritic solidification front. The experiments under low gravity show the importance of the reduction of melt convection on particle distribution within the rapidly solidified sample.

A simple gain-scheduling approach to the position control of electrostatically levitated materials is presented. An electrostatic levitator is currently being constructed at the German Aerospace Center (DLR). It can be used for containerless processing of metals and nonconducting materials. Time-varying sample charges and the fact that the plant is open-loop unstable make this a difficult and challenging control problem. The nonlinear parameter-dependent system is approximated by two local linear models. Based on these models, two local linear controllers are designed. LMI-based pole region assignment is used to reduce the design procedure to the tuning of three parameters with intuitive meaning. The sample charge is estimated with a discrete-time extended Kalman filter and used as scheduling parameter. Experimental results demonstrate that gain-scheduled control achieves a significantly better performance than fixed-gain, observer-based state feedback control.

43 Dr. Nikolaus Nestle, Munchen W issenschaftliches Hauptthema der Physikertagung war die Festkorperphysik. Sie ist bekannterweise eine sehr aufregende, aber auch eine sehr anwendungsorientierte Wissenschaft. Besonders dieser Anwendungsaspekt wurde auch in Hamburg wieder deutlich. Festkorperphysik ist der zahlenmasig groste Teilbereich der modernen Physik. Das ausert sich auch in der grosen Zahl der Studierenden, die sich auf diesem Gebiet betatigen. Zusammen mit der Anwendungsorientierung ergibt dies eine Mixtur, die das Gebiet und die darin arbeitenden jungen Wissenschaftler zu einem „begehrten Sammlerobjekt“ machen und zwar besonders fur jede Art von Unternehmen, die sich mit zukunftsorientierter Technologie im weitesten Sinne befassen. Das reicht von den Chipherstellern, den Unternehmensund Finanzberatern bis hin zu Banken und nicht zuletzt auch den Biotech-Unternehmen, deren Erfolge unter anderem sehr stark auf der Anwendung physikalischer Methoden und Verfahren beruhen. Leider werden wir in den nachsten Jahren nicht annahernd den qualifizierten naturwissenschaftlichen Nachwuchs ausbilden konnen, der von Wirtschaft und Forschung in unserem Lande benotigt wird. Schon jetzt ist es sehr schwierig, genugend qualifizierten Nachwuchs fur unsere Forschergruppen in den Universitaten zu finden – der ja bekannterweise notwendig ist, um auch den Ausbildungsauftrag wahrnehmen zu konnen (Stichwort: Einheit von Forschung und Lehre) – weil die Industrie den Arbeitsmarkt „abraumt“. Die sehr gute Resonanz der Unternehmen auf die Jobborse mit taglich drei Parallelveranstaltungen ist ein deutlicher Beleg fur diesen Sachverhalt. Uber das Programm der Fachverbande hinaus gab es neun fachverbandsubergreifende Symposien, die zumeist von zwei bis drei Fachverbanden getragen wurden: Dynamik von Ummagnetisierungsvorgangen (B. Hillebrands, G. Schonhense) Lasergestutzte Schichtabscheidung und Oberflachenmodifizierung (G. Reise) Neue Transportphanomene – Vom Maxwell-Damon zum Verkehrsstau (P. Hanggi, P. Reimann) Organische Festkorper (K. Leo, N. Karl, M. Sokolowski, W. Brutting) Oxidische Dunnschichten (J. Schubert, B. Holzapfel) Rekonstruktion und Wachstum von III-V-Halbleiteroberflachen (F. Bechstedt) Simulation von Wachstum und Strukturbildung (K. Samwer) Wissenschaftliche Information und Kommunikation der Physik (D. Gorlitz) 30 Jahre Gutzwiller-Spurformel (H. Grabert) Bernhard Kramer Ludwig Schultz

Transverse-field measurements of the muon-spin precession frequency and relaxation rate in liquid supercooled and solid Co 1−x Pd x (0.1⩽x⩽0.3) show the characteristic behaviour of a ferromagnet. On decreasing the sample temperature from above its Curie-temperature TC, the precession frequency decreases, while the relaxation rate increases and diverges on approaching TC. Close to TC the temperature dependence of the inverse frequency shift is in good agreement with susceptibility measurements. In the solid phase, the magnetic field at the muon strongly decreases with the addition of palladium; for x=0.2 it is only 10% of the field in pure cobalt.

The undercoolability of bulk samples of Co100 − xPdx alloys is investigated at four compositions (x = 18, 25, 30, 50) by application of the electromagnetic levitation technique. Each alloy is undercooled approximately 100 times under same experiment conditions. A model developed by Skripov is utilized to statistically analyze the distribution of maximum undercoolings in order to gain insight into the origin of nucleation. The results indicate that crystal nucleation is influenced by the onset of magnetic ordering when the nucleation temperature is approaching the Curie temperature.

Zr57Cu20Al10Ni8Ti5 has very high glass forming ability (GFA) due to its low crystal growth rate which limits development of crystallites arising from container walls or nuclei. Effect of ZrO2 and SiO2 on GFA was studied by investigating the solidification microstructures of alloys containing different amount of the oxides. Amorphous ingots were formed by melting and freezing alloys without addition of the oxides using an arc furnace. Partially amorphous ingots were formed for addition of the oxides up to 0.2 wt %. Amorphous forming conditions of alloys containing greater concentration of the oxides were studied using a drop tube. GFA decreases with increasing concentration of the oxides. However, superheating of melts eliminates some effects of the oxides. Different types of oxides leads to different microstructures due to their different heterogeneous nucleation rates and nucleation temperature.

Liquid samples of Co80Pd20 were cooled below their liquidus temperature Tl and supercooled towards the paramagnetic-ferromagnetic transition temperature TC(l) by means of the Electromagnetic Levitation (EML) technique. Muon-Spin Rotation (μSR) measurements were used to determine the spin precession frequency f and the transverse muon-spin relaxation rate Γ in the liquid supercooled sample at temperatures T > TC(l). Γ increases by a factor of 5 when T approaches TC(l), indicating large spin fluctuations caused by the onset of spontaneous magnetization. The temperature dependence of the inverse frequency shift Δf−1 reflects a Curie-Weiss behaviour of the paramagnetic susceptibility. The experiments strongly suggest the existence of ferromagnetic ordering in a liquid metal.

Containerless solidification of germanium has been realized by electromagnetic levitation and drop-tube processing, respectively. The effect of undercooling in the range of 40–426 K on the as-solidified structures of levitation melted Ge drops (∼8.4 mm diameter) was investigated. For undercoolings less than 300 K, the lamellar twins were grown, whereas a microstructural transition to equiaxed grains was observed at undercoolings ≥300 K. Further increasing the undercooling to 400 K, a significant reduction in grain size was achieved. In addition to a similar microstructural development among the particles solidified during free fall in a 8.5 m drop-tube, high-undercooling-induced single crystals were found for some droplets less than 200 μm in diameter. The results on the transition from twins to fine equiaxed grains are accounted for by theories of solidification kinetics and a dendrite break-up model.

The short-range order in undercooled melts of the intermetallic Zr2Pd glass-forming alloy is investigated by combining electrostatic levitation (ESL) with high-energy X-ray diffraction and neutron diffraction. Experimentally determined structure factors are measured and analyzed with respect to various structures of short-range order. The comparative X-ray and neutron scattering experiments allow for investigations of topological and chemical short-range order. Based on these studies, no preference of a specific short-range order is found for the liquid Zr2Pd glass-forming alloy, even in the metastable state of the deeply undercooled melt. This is in agreement with an earlier report from X-ray diffraction and molecular-dynamics studies of a Zr75.5Pd24.5 liquid, which showed a broad distribution of cluster types. The results for the Zr2Pd liquid are discussed with respect to the glass-forming ability of this melt.

The electromagnetic levitation technique was combined with the diagnostic means at the European Synchrotron Radiation Facility in Grenoble to study in situ the phase selection during solidification of undercooled Nd–Fe–B melts. By energy dispersive diffraction experiments with synchrotron radiation on levitated Nd–Fe–B alloys complete diffraction spectra were recorded within a time interval of a few seconds. Owing to the short measuring time the primary crystallization of a metastable phase was observed that initiates the solidification of hard magnetic Nd2Fe14B1 phase. The metastable phase dissolves subsequently and cannot be detected in the as-solidified sample at the ambient temperature. By analyzing the diffraction spectra the metastable phase is identified as a ternary extension of the rhombohedral Nd2Fe17 phase being stable in binary Nd–Fe alloys.

The interrelationship undercooling-growth velocity-solidified structure was studied for Ge-1at%Si solid solution. Three structural morphologies, faceted twins, coarse dendrites and refined equiaxed grains, were observed in a wide range of undercoolings from 80 to 394 K achieved by a levitation technique. Corresponding to low, intermediate and high undercoolings, these types of microstructures originated from lateral, continuous and rapid crystal growth, respectively.

The solidification behavior of undercooled Fe-Cr-Ni melts is analyzed with respect to the competitive formation of body-centered cubic (bcc) phase (ferrite) and face-centered cubic (fcc) phase (austenite). The activation energies of homogeneous nucleation and growth velocities for both phases as functions of undercooling of the melt are calculated on the basis of current theories of nucleation and dendrite growth using data of thermodynamic properties available in the literature. As model systems for numerical calculations, the alloys Fe-18.5Cr-11Ni forming primary ferrite and Fe-18.5Cr-12.5Ni forming primary austenite under near-equilibrium solid-ification conditions are considered. Nucleation of the bcc phase is always promoted in the under-cooled primary ferrite alloy, whereas the barrier for bcc nucleation falls below that for fcc nucleation for large undercooling in primary austenite alloys. With rising undercooling, tran-sitions of the fastest growth mode were found from bcc to fcc and subsequently from fcc to bcc for the primary ferrite forming alloy and from fcc to bcc for the primary austenite forming alloy. The results of the calculations provide a basis for understanding contradictory experi-mental findings reported in the literature concerning phase selection in rapidly solidified stainless steel melts for different process conditions.

Dendrite growth velocities have been measured as a function of undercooling in droplets of dilute alloys of B in Ni undercooled by up to 306 K using an electromagnetic levitation technique. At a critical undercooling, depending strongly on the B concentration, a steep rise of the dendrite growth velocity is observed. The analysis of the results within current theories of dendrite growth gives evidence for a transition from diffusion-limited to purely thermally controlled growth with partitionless solidification as a consequence.

Drop-tube processing in inert gas is used to study grain refinement in highly undercooled melts. The high cooling rate in fine droplets can give rise to grain-refined but directional microstructures. The general recrystallization associated with “spherical” solidification morphologies may be prevented by a sufficiently high post-solidification cooling rate. The diameter of the spherical elements can be related to calculated local solidification times.

Electromagnetic levitation has been used to melt and undercool Ni100−xBx alloys (x = 0.7, 1.0). Dendrite growth velocities in the levitated droplets have been measured as a function of undercooling. Owing to the extremely small solubility of boron in solid nickel, the growth velocities in the alloys are much lower than those in pure nickel. At a critical undercooling ΔT ∗ a sudden rise in the growth velocities with increasing undercooling is observed, indicating a transition from growth controlled both by solutal and thermal diffusion to a purely thermally controlled and segregation free growth of the supersaturated NiB dendrites.

The controlled solidification of a metal from an undercooled melt into a predetermined metastable product phase has been demonstrated for the iron-nickel alloy system. This was achieved by introducing an external nucleation seed of suitable catalytic potency. The metastable b.c.c. phase was identified through its melting point well below the equilibrium f.c.c. value, in agreement with a calculated metastable phase diagram.

Small liquid droplets of Al 1−x Mn x (x = 8, 11.8, 17, 21.5 at.%), with diameters in the range 250 μm – 1000 μm have been undercooled and solidified by containerless processing during free fall in a drop tube. Undercoolings larger than 150 K have been estimated. The solidified droplets show crystalline phases and metastable quasicrystalline T-phase. However, no quasicrystalline I-phase has been found. The analysis of the results indicates primary nucleation of I-phase which transforms into T-phase during recalescence.

The purpose of the present work was to measure the velocity of dendrite growth in undercooled Ni–Al alloy melts as a function of undercooling. The experiments were performed both by containerless electromagnetic levitation on Earth and under reduced gravity conditions during parabolic flight campaigns. While under terrestrial conditions, strong magnetic fields are required to compensate the gravitational force, the forces to compensate disturbing accelerations are decreased by orders of magnitude in reduced gravity. In turn, the alternating electromagnetic fields induce convection, which is strong under terrestrial conditions while much weaker in reduced gravity. The heat and mass transport in front of the solid-liquid interface during solidification controls the dynamics of dendrite growth. By comparing results obtained on Earth and in reduced gravity, it was demonstrated that the change of transport conditions by convection significantly alters the kinetics of solidification and the evolution of grain refined microstructures at undercoolings less than 100 K.

To improve magnetic properties of the Nd-Fe-B magnets, it is crucial to eliminate soft magnetic inclusions such as soft magnetic α-Fe phase. Recently, several efforts have been devoted to producing the Nd-Fe-B alloys free from the α-Fe phase by undercooling solidification. In these works, it was expected that the direct crystallization of the Nd2Fe14B phase would suppress the formation of the α-Fe phase. Gao and co-workers, however, suggested that the metastable Nd2Fe17Bx phase is formed as the primary phase [1]. In addition, they showed that, even in the Nd14Fe79B7 alloy, in which the a-Fe phase is unstable, the Nd2Fe17Bx phase decomposes into the Nd2Fe14B and α-Fe phases at the post-recalescence cooling stage. Our previous works showed that the post-processing heat-treatment transforms the metastable phase into the Nd2Fe14B phase without forming the α-Fe phase [2]. These results imply that the kinetics of the transformation in the metastable phase at the post-recalescence cooling stage is different from that at the post-solidification heat-treatment. In the present study, the undercooling solidification of NdxFe(100-1.5x)B0.5x alloys was carried out in a drop tube. The objective of this investigation is to clarify the mechanism of the phase transformation in the rapidly solidified Nd-Fe-B alloys from the undercooled melt. Segments of ingot were induction melted in a helium atmosphere, and the molten alloy was then ejected into the 26m drop tube to obtain the metastable phase. The changes of the microstructure and the phase constituent during the heat-treatment of the as-dropped sample were carefully investigated as a function of sample diameters. A large amount of the Nd2Fe17Bx metastable phase was contained in the as-dropped samples. The heat-treatment of the as-dropped samples induces a dual-stage phase transformation. The 1st stage of the phase transformation is the diffusive phase transformation from Nd2Fe17Bx and Nd-rich phases into the Nd2Fe14B phase at 950K, and the 2nd stage of the transformation is the decomposition of the metastable phase into the Nd2Fe14B and α-Fe phases at 1100K. Although both stages of the phase transformation is controlled by the diffusion of Nd atoms, transformation temperature for the 1st stage is decreased due to the large existence of the Nd-rich phase adjacent to the metastable phase. [1] J. Gao, T. Volkmann and D. M. Herlach, Acta Mater. 50, 3003 (2002) [2] S. Ozawa, M. Li, S. Sugiyama, I. Jimbo, S. Hirosawa and K. Kuribayashi, Mater. Sci. Eng. A. 382, 295 (2004)

Using a local nonequilibrium model of solidification, experiments on rapid eutectic growth are analyzed. An analytical solution of a problem of rapid lamellar eutectic growth under local nonequilibrium conditions in the solute diffusion field is found. It is shown that solute diffusion-limited growth of a eutectic pattern is completely finished, and diffusionless growth of the chemically homogeneous crystalline phase begins to proceed at a critical point V = V(D), where V is the solid-liquid interface velocity and V(D) is the solute diffusion speed in bulk liquid. A suppression of eutectic decomposition occurs in the range V > or = V(D) that results in a growth of homogeneous crystal phase with the initial (nominal) chemical composition of the binary system.

Accepted: 2006-08-27, 資料番号: SA1000503000

Electromagnetic levitation was used to melt and solidify containerlessly Nd–Fe–B alloys. A method was developed to reduce and evaporate stable solid Nd oxides at the surface of the liquid samples. The oxides otherwise limit the undercoolability of the highly reactive Nd-based alloys. In such a way, undercoolings up to 150 K of the levitation processed samples were achieved. Owing to the large undercooling range, the intermetallic Nd2Fe14B1 phase with the hard magnetic properties was primarily crystallized at undercoolings exceeding 50 K. Solidification of the pro-peritectic γ-Fe with soft magnetic properties was successfully suppressed. At the largest undercoolings, a metastable phase, likely to be of the composition Nd2Fe17Bx (x≈1) is primarily formed while the crystallization of Nd2Fe14B1 phase is followed up in a subsequent step.

The Cu-Co system shows a metastable miscibility gap in the range of the undercooled melt. In this work the method of electromagnetic levitation (EML) and drop tube experiments have been used to examine the metastable state of Cu-Co alloys. The experiments show that both methods allow deep undercooling of the melt into the range of the miscibility gap. Due to the deep undercooling the velocity of the solidification front is very high and the actual microstructure is frozen in. The process of demixing can be observed and the binodal has been determined with high precision. The microstructure of samples processed in the electromagnetic levitation shows an influence of the electromagnetic stirring due to the induction of electric currents into the melt. Drop tube experiments, which lead to a rapid solidification under reduced gravity conditions, in contrary result in a homogeneous distribution of spherical particles of the minority phase. For this reason space experiments under microgravity conditions in the TEMPUS facility are under consideration. In these experiments the stirring effect would be greatly reduced compared to the EML.

Containerless undercooling by electromagnetic levitation has been used to measure both growth velocity and grain size as a function of undercooling in Cu70Ni30 and Cu69Ni30B1 alloys. At a critical undercooling ΔT* the temperature dependence of the growth velocity changes discontinuously. This is accompanied by a sudden drop of grain size in the solidified product by more than two orders of magnitude. The comparative investigations show that it is not a critical undercooling that initiates the grain refinement process, but rather a critical crystal growth velocity, which approximately equals the diffusive speed.

When containerless electromagnetic levitation processing was applied to elemental semiconductor Si using a two-step heating method proposed by the present authors, a substantial degree of undercooling up to 420 K (0.25Tm, Tm is the melting temperature, 1685 K for Si) has been reproducibly achieved in slowly cooled silicon droplets ( ~ 7.6 mm in diameter). The microstructural change from faceted twins grown at low or intermediate undercoolings to twin-free grains formed at high undercoolings indicates a transition of solidification mechanisms. The experimental results, of which both the undercooling level and the sample volume surpass the previous work, lead to a reassessment of the nucleation frequency and the crystal/melt interfacial energy of undercooled Si liquid.

Alloy melts of the intermetallic compounds FeSi and CoSi were deeply undercooled using the melt fluxing technique. Crystal-growth velocities were measured as a function of undercooling by a high-speed photosensing infrared device. The experimental results were analyzed in the framework of dendrite growth theories with special emphasis placed on the kinetic interface undercooling. This analysis gives insight into nonequilibrium effects occurring during rapid solidification of undercooled melts of intermetallic compounds due to short-range diffusion-limited dendritic growth.

Liquid ${\mathrm{Co}}_{100\ensuremath{-}x}{\mathrm{Pd}}_{x}$ alloys were undercooled below the Curie temperatures ${T}_{C}(s)$ of the ferromagnetic solid phase by using electromagnetic levitation. A modified Faraday balance was applied to measure the magnetization of the undercooled liquid sample as a function of temperature. A Curie-Weiss behavior is detected. The Curie temperature ${T}_{C}(l)$ is determined for the transition from the paramagnetic to the ferromagnetic state of this metallic liquid with long-range magnetic order. ${T}_{C}(l)$ is slightly smaller than ${T}_{C}(s)$.

Dendrite growth velocities as a function of undercooling on the Al-rich side of the Al-Ni system have recently been measured by electromagnetic levitation. They have shown an anomalous behaviour, which so far cannot be theoretically described. The present work investigates whether properties of the phase diagram could explain this effect. For this purpose two simplified qualitative models of sharp interface theory, one treating the forming AlNi (B2) phase as a solid-solution, one treating it as an intermetallic phase, have been developed. To treat the concentration range between 50 and 70 at% Al, sets of thermodynamical data for six different compositions have been created. Results are compared to the experimental data. In both models low undercoolings at low Ni concentrations lead to very limited velocities, because the constitutional undercooling ΔTC overrides the kinetic undercooling ΔTk. The experimental results for low undercoolings seem to contain an effect which cannot be described by a model of this type.

The dendrite growth velocity during non-equilibrium solidification of tetragonal Ni2B crystals is measured as a function of undercooling on electrostatically levitated droplets. High-speed video imaging is applied to measure in situ the propagation of the solid-liquid interface apparent on the sample surface. To analyze the raw data, a three-dimensional model is developed to determine the growth velocity within the volume of the melt. Microstructure analysis is performed to resolve the pattern formation mechanism of tetragonal Ni2B crystals which differs significantly from the cubic case. The dendrite tips are found to grow perpendicular to the crystallographic {111} planes. As a consequence, the propagation of the solidification front on a macroscopic scale is determined by the lattice parameters of the tetragonal crystal structure. The dendrite growth velocities along the {111} normal directions are analyzed on a mesoscopic scale within a sharp-interface model.

Electromagnetic levitation has been used to undercool an alloy of Co80Pd20 below the Curie temperature of the solid α phase. If the temperature of the undercooled liquid decreases below the Curie temperature an attractive force between the sample and an external permanent magnet was observed. This may indicate the existence of a metallic liquid showing magnetic ordering.

The electromagnetic levitation technique is applied to undercool melts of quasicrystal-forming alloys of Al-Cu-Co and Al-Cu-Fe. Undercooling with respect to the solidification of the decagonal phase (quasiperiodic in two dimensions) is measured in situ by contactless temperature measurements. The undercoolings are analyzed within nucleation theory. The results indicate that the activation energy for the formation of the nuclei of quasicrystalline phases is lower than that of crystalline phases. The decrease of activation energy is more pronounced for the icosahedral phase than for the decagonal phase.

The study of metallic melts, their physical properties, and their solidification behaviour is of relevance to understanding of fundamental principles and to metallurgical applications. Many, if not...

The dendrite growth velocity during solidification is measured on liquid drops of the intermetallic compound Ni50Al50 undercooled by levitation up to 265 K. A sharp increase of the growth velocity is found at a critical undercooling ΔT*≈250 K. In situ diffraction of synchrotron radiation on levitation-processed samples unambiguously shows a transition from ordered to disordered growth at ΔT*. The sharp interface model is extended to describe the transition from ordered to disordered dendrite growth by taking into account the velocity dependence of the order parameter and the kinetic growth coefficient.

Controlled phase selection has been demonstrated in the crystallization of a melt from deep undercooling. In the FeNi alloy system the metastable b.c.c. phase has been solidified by a seeding technique for nickel content up to 30%, using a trigger made of FeMo alloy. The stable f.c.c. phase is shown to be formed spontaneously in primary solidification, while for small nickel concentrations a two-step process prevails, consisting of primary b.c.c. and secondary f.c.c. crystallization.

We report measurements of the thermal conductivity on two amorphous Ni 72 Si 8 B 20 samples of different thicknesses 90 μm and 25 μm in the temperature range between 0.3 K and 25 K. The thickness of the ribbons (Vitrovac 0080) was correlated to the quenching rate q resulting in q = 1.5 × 10 5 K s −1 and q = 8 × 10 5 K s −1 , respectively. The total thermal conductivity κ tot consists of two contributions, the phonon thermal conductivity κ ph and the electronic thermal conductivity κ el . At low temperatures T κ ph is limited by scattering of phonons from electrons and low-energy excitations (LEE). The phonon-LEE scattering coefficient increases with quenching rate by 80% while the phonon-electron scattering remains almost unchanged. These results can be understood by an enhanced density of LEE in the sample quenched with a higher cooling rate, giving further support for the validity of the free volume model to explain the origin of the LEE in amorphous metals.

Rapid solidification is a well established method for the preparation of metals and alloys in metastable states, giving access to a range of novel materials properties [1]. An important crystal-growth mechanism in these techniques is dendritic growth, which provides the most effective means of heat and solute rejection during solidification. Dendritic growth has attracted recent attention both from the theoretical [2 – 5] and the experimental side [6 – 9].

Drop tubes and electromagnetic levitation are described as techniques for containerless processing. The constraints of maximum undercoolability of metals are discussed with respect to thermodynamic considerations and the onset of homogeneous nucleation. Dendrite growth occurring in undercooled melts is described on the basis of experimental investigations and modern dendrite growth theories. The importance of the knowledge of thermophysical properties in the understanding of both nucleation and crystal growth in undercooled melts is pointed out. Experimental techniques are presented which allow the measurement of the dependence of such properties on temperature in particular by using electromagnetic positioning in future space missions.

We present direct experimental observations of growth velocity and morphology in undercooled melts of pure Si and dilute Si100−xCox alloys (x=0.1, 0.5, and 2.5at.%). An anomalous behavior in the growth velocity-undercooling relation of the Si–Co alloys is found, exhibiting an abrupt break at a critical undercooling ΔT* which depends on Co concentration. In contrast to pure Si, the anomalous growth behavior in Si–Co alloys cannot be described by current dendrite growth theory even if nonequilibrium effects at the interface as solute trapping are taken into account. The growth morphology changes essentially with increasing undercooling and Co concentration.

It is proposed that the widely observed transitions in solidification of undercooled melts from a coarse grained dendritic to a grain refined equiaxed microstructure result from the fragmentation of dendrites by remelting during the period following recalescence where the interdendritic melt solidifies. This mechanism is supported by the experimental demonstration in Cu-Ni alloys that the transition undercoolings vary with cooling rate in a way which is relatively well described quantitatively by a simple fragmentation model. The latter also predicts the occurrence to two transitions, both of which are observed.