Directional Solidification and Phase Stabilization

Source: Sina Shahbazmohamadi and Peiman Shahbeigi-Roodposhti-Roodposhti, School of Engineering, University of Connecticut, Storrs, CT

Directional solidification zone melting is a metallurgical process in which a narrow region of a crystal (usually in the form of bar) is melted. The furnace moves along the rod shape sample, meaning that the molten zone is moved along the crystal and the molten zone is moved from one end of the bar to the other. This mechanism is widely used in alloys, however solute atoms tend to segregate to the melt. In this type of alloy, the impurities also concentrate in the melt, and move to one end of the sample along with the moving molten zone. Therefore, zone melting is used most extensively for commercial material refining. Fig. 1. shows how the high-impurity molten-zone moves from one side of the bar to the other. The vertical axis is the impurity concentration and the horizontal axis is the sample length. Due to the tendency for impurities to segregate to the molten region, its concentration in the melt is higher than in the solid. Therefore, as the molten materials travel to the end of bar, the impurity will be transported to the end of bar and leave the high purity solid material behind it.

Figure 1: Schematic of the composition change during zone melting directional solidification.

In this study, a zone melting directional solidification apparatus will be employed to synthesize stable structures of Pb-Cd alloys.

Insert a 100 µm chromel-alumel thermocouple (in a 0.1 cm double bore mullite protection tube) in an 8 mm outside diameter Pyrex tube. The tube length should be around 30 cm. The thermocouple tip should be coated with a boron nitride slurry.
Form rods of the desired composition by first melting the alloy in a crucible, and drawing the molten alloy into the Pyrex tube by means of a partial vacuum. For this, use a bulb attached to the end of the Pyrex tube to suck the melt into the tube.
Place the sample in the v

Figs. 5 and 6 show the microstructures developed from directional zone melting solidification of Pb-55Cd alloy revealed by optical microscope, at two different G/V ratios (G: thermal gradient, V: velocity of the furnace movement along the Pyrex tube).

At low ratio (G/V=1.03×10⁶ (^oC.Sec/Cm²)) the microstructure consisted of branched dendrites of α phase in the matrix of ß phase. At

This experiment demonstrates to employ a specific type of zone melting freezing directional solidification furnace to develop stable microstructures. Unlike the two phase unstable microstructure that is not in equilibrium at room temperature and the structure degrade over a period of months by diffusion at room temperature, the single phase structure obtained in sample grown does not undergo any change.

Sample with stable phases, developed by mentioned furnace may have wide applications in v

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