Glass Flux Research Articles

Liquid–liquid phase separation in undercooled Co–Cu alloys

The liquid–liquid phase separation in undercooled Co–Cu alloy melts has been investigated by differential thermal analysis in combination with glass fluxing technique over a composition range of 16.0–87.2 at.% Cu. The DTA signals, obtained during isochronous cooling, indicate that this separation process is exothermic and proceeds till the rapid solidification of Co-rich liquid phase occurs. The metastable miscibility gap that was determined directly and reproducibly from the onset temperatures of this process is slightly shifted to the Cu-rich side and roughly symmetrical about a Cu concentration of 53 at.%. The measured critical temperature of phase separation is 1547±1.5 K and is about 108 K below the corresponding liquidus temperature. In the present measurements the separated Co-rich liquid always solidified prior to the Cu-rich phase, which crystallized near the peritectic temperature. Lower surface tension and better wetting properties of the Cu-rich liquid phase with glass flux are responsible for the Co-rich phase to be always encased by the Cu-rich phase. In addition, thermodynamic calculations have been accomplished leading to a binodal line, which is in sufficient agreement with the experimental results.

Measuring Magnetic Susceptibility of Undercooled Co-Based Alloys with a Faraday Balance

Co-based melts embedded by glass flux in a crucible can be undercooled nearly as strong as samples processed containerlessly by electromagnetic levitation. This allows to measure magnetic susceptibility on undercooled melts while combining the Faraday method with undercooling technique. The magnetic susceptibility of Co and Co-based alloys will be analysed as a function of temperature from superheated to undercooled state. This will be performed by measuring the change of magnetic force F Z on a sample in a constant magnetic field H 0 and an additional gradient field by means of a Faraday balance. When liquid Co-based alloys are undercooled the magnetisation steeply rises if temperature approaches the Curie temperature. The magnetisation is measured for some Co-based alloys in liquid state as function of alloy system and its concentration. The results show that the magnetic susceptibility of the liquid undercooled samples follows a Curie-Weiss behaviour, from which Curie temperature is inferred.

Highly undercooled germanium: Growth velocity measurements and micro structural analysis

Growth velocities have been measured in samples of Ge (3–5 mm in diameter) which were undercooled using a fluxing technique. Samples of Ge were heated and undercooled in a viscous soda-lime glass flux contained within thin-walled, clean silica crucibles. Using this particular method, undercoolings of up to 250 K below equilibrium melting temperature were obtained. This maximum undercooling may have been restricted by sample size, experimental conditions or impurities present in the sample. It was possible to determine growth velocities up to undercoolings of 250 K The size of the samples used allowed growth velocities measurements to be made, by measuring recalescence times using a linear photo-diode array. These were measured at a variety of undercoolings, where nucleation was initiated using a thin alumina needle, and a maximum velocity of 7.6 ms −1 was recorded at an undercooling of 250 K. Microstructural analysis of samples revealed a range of transitions occurring as the level of undercooling increased. These include a gradual transition from faceted to non-faceted type growth at ΔT > 160 K, characterised by the observance of growth twins a small undercoolings which were not present in samples undercooled greater than 160 K. By transmission electron microscopy (TEM) examination, it was also confirmed that there was no microtwinning present in the highly undercooled samples. A general grain refinement at ΔT> 210 K from a coarse microstructure incorporating an extensive dendritic substructure to a fine structure with an impurity-rich, cross-shape dendrite fragment at the centre of the grains. These transitions could also be associated with trends seen between measured growth velocities and undercoolings.

Cellular growth of a dilute binary alloy at high solidification velocities

Ag–Cu eutectic alloy was undercooled by the glass flux method and the solidification structure was investigated. It is revealed that when undercooling is not more than 70 K, the large difference in composition between two eutectic phases and very large thermal diffusion coefficient of the liquid result in cellular growth of the lamellar eutectics from the nucleation site. The variation in interface temperature during rapid solidification gives rise to a systematic change in microstructure within the sample. With the distance along the growth direction increasing, the finest lamellar spacing across the cellular eutectic rises, which indicates a gradually decreasing growth velocity of the primary eutectics. The primary lamellar eutectics near the nucleation site solidify under conditions far from equilibrium, and therefore are supersaturated with solute, and then partially remelted and ripened into anomalous eutectics. As undercooling increases, the area of the anomalous eutectics enlarges.

Grain refinement in undercooled nickel

In this paper, the microstructures of undercooled Ni that solidified at various initial bulk undercoolings are examined in detail in order to understand the mechanism of grain refinement in metallic systems. Molten Ni contracts on solidification. In the experiment, since it was covered by molten glass flux, upon crystallization cavities had to form to accommodate the rapid volume change if the undercooled specimen remained in contact with the glass flux, which could not flow so readily. The adhesiveness between Ni and glass flux was confirmed by removing them from a furnace after the whole system had been cooled down to room temperature. Furthermore, it was clear from the micrographs that after a cavity had formed, it did not collapse. It can therefore be concluded that smaller grains are found to concentrate near the void along the minor axis. At still higher undercoolings, the effect was so violent that the voids took irregular shapes. Again, the grains near the cavity are somewhat smaller than those further away. Accordingly, the authors conclude that grain refinement in undercooled Ni was brought about by dynamic nucleation as the cavities formed.

Critical solidification behavior of undercooled Ag-Cu alloys

Melts of Ag, Cu, and four different Ag-Cu alloys, including the eutectic composition, with masses between 1 and 2.5 g were undercooled by the glass flux method, and the temperature of the specimen was measured with a pyrometer during cooling and recalescence. For each composition the maximum recalescence rate was determined as a function of the undercooling. Comparison of the results for Ag and Cu with published measurements of the dendrite growth velocities in these metals showed that the maximum recalescence rate provides a good semiquantitative measure of the growth rate. For the Ag-Cu alloys there is a critical solidification temperature—close to T0—at which the maximum recalescence rate rises abruptly by about two orders of magnitude, accompanied by significant changes in the microstructure. It is shown that this behavior can be explained theoretically on the basis of a transition from diffusion-controlled growth of the eutectic or the primary dendrites to dendritic growth of a supersaturated phase, provided that interface kinetics and growth-rate-dependent partition coefficients are taken into account.

Nonequilibrium solidification in undercooled melts of the alloy Ag-39.9 at. % Cu

Melts of the eutectic alloy Ag-39.9 at. %Cu were cooled below the liquidus temperature by the glass flux method. The solidification behavior was followed by measuring the temperature change (recalescence), and the resulting microstructures were investigated by optical and electron microscopy. Up to a certain critical undercooling ΔT*≊76 K, the recalescence rate is less than 100 K s−1, and a feathery, lamellar microstructure is formed, with typically one or two growth centers. When the undercooling exceeds ΔT*, a sudden increase in the recalescence rate (up to 104 K s−1) is observed, and the microstructure consists of a dendritic, irregular two-phase component surrounded by a lamellar eutectic. The critical undercooling temperature coincides with the temperature (T0) at which the Gibbs free energies of the liquid and the supersaturated Ag-Cu solid solution are equal. The critical behavior is therefore thought to be due to the formation of this metastable phase. A model is proposed to explain the observed behavior and microstructures.