THE EFFECT OF PRESSURE ON NUCLEATION IN UNDERCOOLED LIQUIDS

• Main Author: YOON, WOOYOUNG
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-1987
• Subjects: Materials science; Metallurgy
• ISBN: 9798207001401

Recent progress of the study of nucleation during solidification has been related largely to undercooling work. By taking advantage of other thermodynamic and kinetic factors such as pressure which is relatively easy to control and to measure, an enhanced understanding of the nucleation behaviors may be possible. The pressure dependence of the nucleation temperature for the pure metals, (e.g. Bi and Sn) was considered in terms of classical nucleation theory and a structural model for the liquid-solid interfacial energy. With a suitable selection of kinetic parameter values, the calculated trend of nucleation temperature on pressure can be shown to reproduce the experimental observations. However, for a complete understanding of the nucleation kinetics, the structural model may require modification to interpret the liquid-solid interfacial energy. A technique to obtain extra-undercooling induced by a rapid pressure change was examined. Undercooled Bi and Sn droplet samples experienced about $9\sp\circ{\rm C}$ of extra-undercooling for a few seconds following a rapid pressure drop from $\sim3.5$ kbar to $\sim0.2$ Kbar in $\sim1$ second. The pressure related thermal behaviors that occurred during the rapid pressure drop can be monitored at high speed and with accuracy by an oscilloscope. The combined effect of undercooling in pressurized liquids has been studied in several pure metals and alloy systems. Polymorphism is often observed in metallic systems at elevated pressure. Rapid solidification is also an effective way to form a metastable phase. The metastable $\alpha\sb1$ (bct)-phae has been reported by RSP and confirmed by droplet studies in the Pb-Sn system. The metastable $\alpha\sb1$ phase was suggested to be identical with the high-pressure X phase. Moreover, the X (or $\alpha\sb1)$ phase can be considered as a solid solution of the high-pressure Sn(II) phase based on comparisons of crystal structures. Hydrostatic pressure can have a considerable influence on the phase diagrams of binary metallic systems. Also, undercooling has been considered a critical factor of the phase selection and the nucleation kinetics. Therefore, the combined application will enhance not only the general properties of the metallic systems but also the understanding of the nucleation kinetics. (Abstract shortened with permission of author.)