Study of Nucleation and Growth in Al-Zn Alloys Using Tem
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G.SUNDAR-, E.A.KENIK-,J.J.HOYT- AND S.SPOONER"Dept.of Mechanical and Materials Engg., Washington State Univ., Pullman,WA 99164 Metals and Ceramics Division and -Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 ABSTRACT Nucleation and growth studies were conducted on Al-Zn alloys at several temperatures using transmission electron microscopy (TEM) with an in-situ furnace. The value of the critical undercooling was established by noting the lowest temperature at which precipitates were no longer observed, following a quench into the two-phase metastable region. These results were compared with the Langer-Schwartz model of nucleation and growth in which it is predicted that the half-completion time (i.e, the time required for the supersaturation to reach half its initial value) diverges for initial supersaturations which are higher than those predicted by the classical nucleation theory. INTRODUCTION For most systems, the nucleation rate i.e the rate of formation of minority phase droplets, predicted by the classical theory of nucleation [1,2] varies from extremely low values to very large values through a narrow range of supersaturation, thus effectively defining the onset of nucleation, or cloud point. Although the classical theory is now over 60 years old, there have been very few studies done so far to compare its prediction with experimental data on solid-solid nucleation. Perhaps the first serious attempt made to compare theory and experiment in solid metals is that of Servi and Turnbull [3], reported in 1966. They investigated homogeneous
nucleation kinetics in f.c.c Cu-rich Cu-Co alloys using resistivity measurements. Although their experiment was well designed, several factors such as the complicated steps involved in obtaining the particle number density and high Co concentrations leading to excessively rapid transformation kinetics, rendered their data imprecise. Another study of the comparison between
homogeneous nucleation theory and experiment was conducted by Kirkwood and coworkers [4,5,6] not long afterwards, on ordered f.c.c Ni3AI in disordered f.c.c a, Ni-Al solid solutions, using transmission electron microscopy (TEM). Again, as in the previous case, over-rapid transformation kinetics were such a serious problem that nearly all data reported were obtained in the coarsening regime. Considering the meager amount of success achieved in comparisons between theory and experiment in solid-solid transformations, one would therefore ask what has been achieved with parallel efforts on liquid-liquid and liquid-vapor transformations. From the outset, it would seem that these would follow nucleation theory better since they support no internal strains and the much higher diffusivities involved permit nucleation at very low supersaturations, where one would imagine the theory to be most accurate. Somewhat surprisingly, past experiments on liquid systems have also demonstrated serious discrepancies between theory and experiment [7-10]. It was Binder and Stauffer (BS) [11] who first recog
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