Recalescence Temperature Research Articles

Formation of Y xNd 1− xBa 2Cu 3O 7− δ (0≤ x≤0.9) superconductors from an undercooled melt via aero–acoustic levitation

This paper presents the results of rapid solidification experiments performed on the copper oxide superconductors Y x Nd 1− x Ba 2Cu 3O 7− δ (0≤ x≤0.9). Spherical rare earth (RE) 123 specimens were levitated in O 2 using aero–acoustic levitation (AAL), melted with a laser, undercooled, and solidified. The peritectic transformation temperature for the reaction RE 2BaCuO 5+liquid→REBa 2Cu 3O 7− δ corresponding to the maximum recalescence temperature during solidification was determined. RE123 was formed directly from the melt for Y–Nd binary alloy compositions with Nd concentration greater than 20% (Y concentration less than 80%). A minimum in the peritectic transformation temperature for the Nd/Y123 system corresponding to a composition Y 0.3Nd 0.7123 was determined at 66 °C below the peritectic of pure Nd123.

Modelling of the thermal history of a non-spherical molten metal droplet during solidification

A mathematical model to describe the thermal history of an atomised non-spherical droplet during solidification is formulated. Four distinct solidification stages, the initial transition stage, the rapid solidification stage, the slow solidification stage, and the final transition stage, have been identified. As the initial undercooling increases, the extent of the initial transition stage is smaller, and the recalescence temperature range becomes larger. The extent of the rapid solidification stage is larger, and the extents of the slow solidification and the final transition stages are smaller for larger initial undercooling. The final recalescence temperature is lower for larger initial undercooling. The release rate of the latent heat is smaller and the cooling rate is more dominant for the elliptical metal droplets than for the spherical droplets. The extent of the initial transition stage is smaller and the final recalescence temperature is lower for elliptical droplets.

Peritectic solidification under high undercooling conditions

The solidification characteristics of highly undercooled Cu-7.77% Co peritectic alloy has been examined by glass fluxing technique. The obtained undercoolings vary from 93 to 203 K(0.14 TL). It is found that the α(Co) phase always nucleates and grows preferentially, which is followed by peritectic transformation. This means that the peritectic phase cannot form directly, even though the alloy melt is undercooled to a temperature far below its peritectic point. The maximum recalescence temperature measured experimentally decreases as undercooling increases, which is lower than the thermodynamic calculation result owing to the actual non-adia-batic nature of recalescence process. The dendritic fragmentation of primary α (Co) phase induced by high undercooling is found to enhance the completion of peritectic transformation. In addition, the LKT/BCT dendrite growth model is modified in order to make it appllcable to those binary alloy systems with seriously curved liquidus and solidus lines. The dendrite growth velocities of primary α (Co) phase are subsequently calculated as a function of undercooling on the basis of this model.

Modeling of solidification of molten metal droplet during atomization

A mathematical model to describe the solidification behavior of an atomized droplet during flight, in terms of nucleation temperature, recalescence temperature, nucleation position, solid fraction at nucleation temperature, and droplet temperature and velocity, is formulated. The concept of transient nucleation is applied to model the short nucleation event. A maximum droplet velocity exists, beyond which the droplet velocity shows an inflection phenomenon during the flight. The velocity of smaller droplets is higher at a shorter flight distance and lower at a longer flight distance. Variations of the gas flow patterns have more effects on smaller droplets, and the effects are more significant at a longer flight distance. A minimum surface heat-transfer coefficient exists as the droplet flies. Prior to nucleation or recalescence, smaller droplets have lower temperature at a given flight distance. Smaller droplets have lower nucleation temperature. Medium-size (around 80-µm) droplets fly over the shortest flight distance before the nucleation starts. Smaller droplets have a larger solid fraction at the end of recalescence. Atomization gas has more effects on the droplet momentum than on the heat content of the droplet.

Undercooling and solidification of metallic melts suspended in a non-nucleating matrix: Experiment NUCAL of the German Spacelab Mission D-2

During the German Spacelab Mission D-2 bulk alloy samples, surrounded by a non-nucleating liquid B 2O 3 matrix, were molten and solidified in the isothermal heating facility. In reduced gravity, sedimentation and thus heterogeneous nucleation at container walls were suppressed. By measuring the recalescence temperature with thermocouples, it was possible to determine the degree of undercooling for eutectic Ag-Ge and for iron-rich Fe-Ni samples. Deep undercoolings of around 100 K for Ag-Ge and up to 392 K for Fe-Ni have been achieved. The metallographic microstructure analysis of the Fe-Ni samples indicates the formation of a metastable δ-phase in the Fe-Ni system.

Micro-Heat Transfer of Solidification of Mixtures with Supercooling. Experiment with Two-Dimensional Solidification.

Two-dmensional solidification of mixtures with supercooling was studied experimentally using aqueous solution tilled in filling a circular freezing cell. It was clarified that the solidification process consisted of the following characteristic stages: free crystal growth in a supercooling state, recovery of the recalescence temperature field with development of a mushy zone, and solidification under the quasi-steady state condition. Microbehaviors such as the crystal growth velocity and their number density were also clarified in connection with the supercooled fields and the geometrical configuration of the freezing cell.

Solidification Characteristics of Undercooled Ni and Ni-Co Binary Alloys.

An investigation on the static undercooling behavior of Ni and Ni-Co alloys using an alumina crucible under both air and Ar atmosphere was carried out. The degree of undercooling in both Ni and Ni-Co alloys decreased with increasing oxygen content. Impurities of spinels NiAl2O4 and CoAl2O4 were detected in the low undercooled specimens by XMA and X-ray diffraction methods. Both spinels have low value of planar disregistry which means they have a good potency to be heterogeneous uncleation sites. These inclusions are created as results of reactions between an alumina crucible, soluble oxygen, and molten metal. Furthermore, an equation which gave the fraction solid was derived considering the effect of the crucible. The equation is a function of undercooling, cooling rate, thermal properties of crucible and melt, and time. This equation gives a relationship between undercooling and solidification time by substituting '1' in place of the fraction solid. The calculated solidification time is in good agreement with the experimental data. In low undercooling, the maximum recalescence temperature was almost constant and a refining of casting structure by oxygen was achieved.

Dendritic solidification of undercooled melts: mushy zone recalescence dynamics

The nucleation and growth of crystals in deeply undercooled melts leads to rapid dendritic growth accompanied by recalescence. The recalescence temperature provides an internal degree of freedom if the thermodynamic system is closed to all energy and mass transfers over relevant observational time scales. Precision adiabatic recalescence experiments on both pure and binary undercooled melts were conducted. The adiabatic constraint limits the amount of net phase transformation from the metastable undercooled state, and also sets bounds on the microstructural parameters of the mushy zone, i.e. the volume fraction of the phases, their length scales, and the interfacial compositions. As many as three distinct kinetic time scales are observed in these experiments: (1) a short time scale ( ca. less than 1 s) of rapidly increasing temperature from the initial nucleation temperature, associated with propagation of the dendrites and release of latent heat; (2) an intermediate time scale ( ca. 10 3–10 4 s) of slowly rising temperature, associated with coarsening of the mushy zone; (3) a long time scale ( ca. days) of steadily falling temperature, associated with solid-state diffusional adjustments near the solid-liquid interface. Interpretation of these experiments is provided in an attempt to understand the overall process of non-equilibrium freezing of undercooled alloy melts.