Thomas Volkmann

Undercooling and Solidification of Si by Electromagnetic Levitation

Abstract Pure Si droplets were containerlessly undercooled using an electromagnetic levitation method. An undercooling up to 330 K prior to solidification has been reproducibly achieved for bulk samples in size of 10 mm. A transition from faceted growth at lower undercoolings to continuous growth at higher undercoolings was observed through analyses of changes in phase morphologies on the surface of the samples. The transition was caused by existence of a large kinetic undercooling. The nucleation frequency and the crystal/melt interfacial energy are discussed within the frame of Spaepen’s model in terms of the structure of the interface.

Undercooling-dependent solidification behavior of levitated Nd14Fe79B7 alloy droplets

Abstract Bulk Nd 14 Fe 79 B 7 alloy droplets were processed using electromagnetic levitation technique for the purpose of studying their metastable solidification behavior at significant melt undercoolings. The results show that γ-Fe solid solution, Nd 2 Fe 14 B compound and the metastable Nd 2 Fe 17 B x compound were solidified as primary phase in sequence of increasing bulk undercooling level. The critical undercoolings were determined to be 45 K and 60 K, respectively. Following primary γ-Fe formation, the Nd 2 Fe 17 B x compound was solidified peritectically prior to the Nd 2 Fe 14 B compound. However, primary Nd 2 Fe 14 B formation was quite predominant over the whole sample volume. In case of primary Nd 2 Fe 17 B x formation, the Nd 2 Fe 14 B compound was solidified also in a peritectic manner. The metastable Nd 2 Fe 17 B x compound was found to decompose into α-Fe plus Nd 2 Fe 14 B during the post-solidification process. The phase selection mechanisms were discussed in terms of in-situ observations on the solidification process.

Nonequilibrium solidification of hypercooled Co–Pd melts

Co–Pd alloy melts are characterized by a low heat of fusion and, as a consequence, by a comparably small critical undercooling for the hypercooling limit ΔThyp of the order of 300 K. It is shown that containerless processing of bulk melts by electromagnetic levitation offers undercooling levels of ΔT≈350 K thus exceeding the hypercooling limit considerably. Solidification of undercooled melts from the hypercooling regime leads to rapid crystallization of the entire sample under nonequilibrium conditions. The electromagnetic levitation technique in combination with time-resolved recalescence detection was used to measure the growth velocity of Co–Pd alloy melts as a function of undercooling prior to solidification. The growth velocities of undercooled metallic melts were measured in an undercooling range exceeding the hypercooling limit. The experimental results are discussed within current theory of dendritic growth.

Nucleation and metastable phase formation in undercooled FeCrNi melts

The non-equilibrium solidification behaviour of undercooled FeCrNi melts is analysed theoretically with respect to the competitive formation of b.c.c. and f.c.c. phases. Nucleation as well as dendrite growth processes in undercooled melts are considered. Thermodynamic modelling of homogeneous nucleation as a function of the melt undercooling revealed that the b.c.c. phase may be promoted even in primary f.c.c. type solidification alloys. The homogeneous nucleation temperatures for different cooling conditions are calculated as functions of the alloy composition. Containerless solidification experiments, electromagnetic levitation melting as well as drop-tube solidification experiments of gas atomized droplets, confirmed the predicted tendency of metastable b.c.c. phase formation. In order to prevent the decay of metastable phases by solid state transformations, high cooling rates of samples were achieved by applying the new technique of liquid metal quenching of as-solidified levitated drops. Calculated dendrite growth velocities agree reasonably with experimentally determined values up to a critical undercooling of approximately 150 K. For selected alloy compositions, transitions between the fastest dendrite growth modes as a function of undercooling from b.c.c. to f.c.c. and vice versa were derived from dendrite growth theory. The implications of the present results for rapid solidification processing of stainless steels are discussed briefly.

Observation of a metastable phase during solidification of undercooled Nd–Fe–B alloy melts by in situ diffraction experiments using synchrotron radiation

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.

Solidification of levitated Nd–Fe–B alloy droplets at significant bulk undercoolings

Abstract Nd–Fe–B alloys with composition of Nd16Fe76B8, Nd18Fe73B9 and Nd22Fe67B11 were melted and solidified using an electromagnetic levitation technique. Two types of solidification behavior were observed depending on the bulk undercooling achieved prior to solidification. The samples with small undercoolings were solidified by the predominant primary formation of the Nd2Fe14B compound, whereas those with large undercoolings were solidified by the primary formation of the metastable Nd2Fe17Bx compound (x∼1) plus the subsequent formation of Nd2Fe14B. The critical undercooling for the primary Nd2Fe17Bx formation was determined to be 40, 70 and 130 K in the three alloy compositions, respectively. The liquidus temperature of the metastable Nd2Fe17Bx compound was estimated to be 1423, 1393 and 1283 K, respectively. The Nd2Fe17Bx compound was found to decompose into a mixture of Fe plus Nd2Fe14B due to the slow cooling rates of the samples. It was suggested that the metastable Nd2Fe17Bx compound may have a lower interfacial energy than that of the stable Nd2Fe14B compound, hence being favored in nucleation-controlled phase selection at large undercoolings.

Nucleation transitions in undercooled Cu70Co30 immiscible alloy

High temperature differential scanning calorimetry (DSC) is applied to undercool and crystallize melts of a Cu70Co30 alloy into the metastable miscibility gap. The kinetic prefactor Γ and the activation energy ΔG* of the nucleation rate are determined based on the statistical analysis within classical nucleation theory. The value of Γ reaches 2.64 (0.21) × 1037 m−3 s−1, which is close to that of the value for homogenous nucleation and much larger than that of undercooled pure Co melts. The value of ΔG* is estimated to be 67 (2.5) kBT which is also higher than that of undercooled pure Co melts. The nucleation of the crystallization of the Co-rich phase is governed by homogeneous nucleation or conditions that are indistinguishable from homogeneous nucleation and the Cu-rich liquid phase effectively prevents the occurrence of heterogeneous nucleation for the nucleation of the Co-rich phase in the liquid-phase separated Cu70Co30 alloy. The results indicate that nucleation of the crystalline phase is sensitive...

Containerless Undercooled Melts: Ordering, Nucleation, and Dendrite Growth

Electromagnetic and electrostatic levitation are applied to containerless undercool and solidify metallic melts. A large undercooling range becomes accessible with the extra benefit that the freely suspended drop is accessible directly for in situ observation. The short-range order in undercooled melts is investigated by combining levitation with elastic neutron scattering and X-ray scattering using synchrotron radiation. Muon Spin Rotation (µSR) experiments show magnetic ordering in deeply undercooled Co80Pd20 alloys. The onset of magnetic ordering stimulates nucleation. Results on nucleation undercooling of zirconium are presented showing the limit of maximum undercoolability set by the onset of homogeneous nucleation. Metastable phase diagrams are determined by applying energy-dispersive X-ray diffraction of Ni-V alloys with varying concentration. Nucleation is followed by crystal growth. Rapid dendrite growth velocity is measured on levitation-processed samples as a function of undercooling ∆T by using high-speed video camera technique. Solute trapping in dilute solid solutions and disorder trapping in intermetallic compounds are experimentally verified. Measurements of glass-forming Cu-Zr alloy show a maximum in the V(∆T) relation that is indicative for diffusion-controlled growth. The influence of convection on dendrite growth of Al50Ni50 is shown by comparative measurements of dendrite growth velocity on Earth and in reduced gravity. Eventually, faceting of a rough interface by convection is presented as observed on Ni2B alloys.

Phase selection in the mushy-zone: LODESTARS and ELFSTONE projects

In a collaboration sponsored by ESA and NASA, international partners have developed a work plan to successfully address key issues relating to understanding the role of convection on alloy phase selection for commercially important structural alloys using the MSL-EML facility aboard the International Space Station. The approach is two-pronged. First, ground and space-based experiments will develop a baseline database to anchor subsequent modelling predictions. Tasks include sample preparation and verification, ground-based transformation evaluation, space-based experiments, and thermophysical property evaluation to support modelling activities. Second, modelling and theoretical analysis tasks will lead to a new understanding of the role of convection in phase selection for this class of materials. These models will allow prediction and control of microstructural evolution during solidification processing. Tasks include modelling of macroconvection induced by the EM levitation field, modelling of microconvection within the dendrite array, nucleation modelling, and modelling of the transformation kinetics specific to each alloy system. This paper outlines how two NASA-sponsored projects relate to the goals of the international collaboration.

Metastable phase formation in undercooled Nd-Fe-B alloys investigated by in-situ diffraction using synchrotron radiation

Diffraction experiments on electromagnetically levitated Nd-Fe-B alloys during solidification of the undercooled melt have been performed at the European Synchrotron Radiation Facility (ESRF). By using high intensity synchrotron radiation complete diffraction spectra could be detected within a short period of some seconds thus enabling the observation of metastable solidification products that exhibits a limited lifetime. A metastable phase that crystallizes in wide composition range and that initiates the solidification of the stable Nd2Fe14B1-phase (φ-phase) have been observed.