Phase Selection In Undercooled Melts Research Articles

Phase selection and microstructure formation in undercooled Co–61.8 at.% Si melts under various containerless processing conditions

The hypoeutectic composition Co–61.8at.% Si was undercooled and solidified using electromagnetic levitation, electromagnetic levitation under a static magnetic field, electrostatic levitation and glass-fluxing. The samples generally showed two thermal events, either separated or continuous depending on undercooling. In situ monitoring of the two thermal events with a high-speed camera revealed a sudden decrease of dendritic growth velocities of primary phases at a critical undercooling of 88K. Scanning electron microscopy studies of the solidified samples showed that the CoSi compound and the CoSi2 compound nucleate as the primary phase for low and high undercoolings, respectively. The microstructure of the samples depends not only on undercooling, but also on the onset temperature or delay time of the second thermal event. Melt convection has no effect on the primary phase selection in undercooled melts, but it has a significant effect on the delay time and therefore on microstructure formation of the samples for high undercoolings.

Phase Selection in Undercooled Melts of Peritectic Cu-Ge Alloys

Phase selection in undercooled melts of Cu-14.8Ge and Cu-18Ge compositions was investigated using the electromagnetic levitation technique in combination with substrate quenching and a drop tube. The results showed that the levitated and gas-cooled samples were all solidified into a microstructure consisting of primary -Cu plus peritectic -Cu5Ge. However, the samples quenched onto a copper substrate showed a segregation-free microstructure in the chilled zone, suggesting direct crystallization of the peritectic -Cu5Ge phase from a highly undercooled liquid. The Cu-18Ge samples quenched onto a glass substrate as well as those solidified in the drop tube also showed a segregation-free microstructure. An analysis of the nucleation kinetics revealed that the -Cu5Ge phase had a larger nucleation barrier than that of -Cu for all accessible undercoolings. It was suggested that phase selection in the undercooled Cu-Ge liquid might be controlled by transient nucleation kinetics.

Nucleation and Phase Selection in Undercooled Melts: Magnetic Alloys of Industrial Relevance (MAGNEPHAS)

Nucleation and Phase Selection in Undercooled Melts: Magnetic Alloys of Industrial Relevance (MAGNEPHAS)

Nucleation and phase-selection in undercooled melts

Abstract Electromagnetic levitation technique is applied to undercool bulk melts of metals and alloys. A large undercooling range becomes accessible by avoiding heterogeneous nucleation on container walls and processing the melts under high purity environment. A freely suspended drop is produced with the extra benefit that it becomes accessible for direct observation and diagnostics. Temperature–time profiles (ttp) are measured during the melting and solidification process. The ttp profiles give insight into the thermodynamics of phase-selection processes. The levitation technique is combined with the diagnostic means at European Synchrotron Radiation Facility (ESRF Grenoble) to record time-resolved diffraction spectra during rapid crystallization of deeply undercooled melts. In such a way, the sequence of solidification of different crystallographic phases and (partly) remelting of primarily formed phases during recalescence is unambiguously determined. All these investigations allow for the construction of nucleation phase-selection diagrams examples of which are presented for Fe-based, Ni–V, and Nd–Fe–B alloys with a competition of phase formation of bcc, fcc, and some intermetallic phases, respectively. They give the conditions for the formation of metastable solids of different crystallographic structures as a function of concentration and undercooling achieved prior to solidification.

Electromagnetic levitation apparatus for investigations of the phase selection in undercooled melts by energy-dispersive x-ray diffraction

The metastable state of an undercooled melt can provide access to the formation of metastable phases whose crystallographic structures differ from their stable counterparts. The electromagnetic levitation technique is used here to undercool bulk melts without containers. This technique, combined with energy dispersive x-ray diffraction of synchrotron radiation, allows in situ study of the rapid crystallization of undercooled Ni–V alloys. The binary Ni–V alloy was chosen as a model system, because it shows phase competition of three different crystallographic phases, bcc, fcc, and an intermetallic phase of tetragonal structure at intermediate concentrations. The diffraction experiments provide for direct identification of the various crystalline phases formed from the nonequilibrium state of the undercooled melt. The phase selection is investigated as a function of undercooling temperature for Ni–V alloys of different concentrations.

A metastable phase crystallized from undercooled NdFeCoZrGaB alloy droplets

Phase formation in rapidly solidified particles of a Nd 13Fe 68.4Co 11Zr 0.1Ga 0.5B 7 alloy was investigated using the drop tube technique in order to gain further understanding of the metastable solidification behavior of undercooled Nd 2Fe 14B-based alloy melts. The results show that primary iron formation prevails in larger particles (360–1200 μm), as expected for a small undercooling achieved prior to solidification. However, a new metastable phase with the TbCu 7-type structure was found to have crystallized directly from the undercooled liquid phase in smaller particles (80–360 μm). Based on the result of compositional analysis, it was assumed that the phase may have the chemical formula Nd 2(Fe,Co) 17B x , where x=0–1. The reasons for the metastable formation of the phase are discussed by taking into account both thermodynamic and kinetic arguments which can explain phase selection in undercooled melts.

Phase selection in undercooled melts of AlCuCo and AlCuFe quasi-crystal-forming alloys

The solidification paths of melts of AlCuCo and AlCuFe alloys of compositions yielding quasi-crystalline phases have been investigated. On the basis of these results, four alloy compositions were selected for a study of the undercoolability of the melts, to be carried out by containerless processing in an electromagnetic levitation apparatus. From the measurements of the undercooling temperatures with respect to solidification of the decagonal quasi-crystalline phase in Al 67Cu 21Co 12 and Al 65Cu 25Co 10, and of the icosahedral quasi-crystalline phase in Al 58Cu 34Fe 8 and Al 60Cu 34Fe 6, very low values of the interfacial energy can be deduced. This indicates a pronounced structural similarity between the quasicrystalline phases, with the melt increasing from the decagonal to the icosahedral structure.