Technical Papers and Notes - Institute of Metals Division - Nucleation of Solid-State Transformations

THERE seems to be a natural urge for the human mind to wonder about the beginning of things. When an explosion occurs, we immediately inquire "what set it off?" If a person contracts a disease, we are likely to ask "how did it happen?" On a much loftier plane, when we reflect about the universe, there is the inevitable question "what, or rather Who, started it all?" In principle, these varied queries have at least one attribute in common: they are attempts, whether earnest or casual, to search out the origin of events in nature. Accordingly, they involve not only scientific, but philosophical and even re1igious matters of deep concern. It is evident that, in a broad sense, the phenomenon of nucleation is enmeshed in this enticing concept of "the beginning", but we must extract the scientific component of the problem for our present purposes. Nucleation, as in the case of phase transformations, is best regarded from the standpoint of kinetics. There are then two main steps, which may come in either order: (a) Experimentally, one attempts to measure the frequency with which transformation centers spring into existence; and (b) theoretically, one seeks a model that will account for this observed birth-rate. Temperature, composition, strain, and prior history are some of the important variables that must be considered both experimentally and theoretically when dealing with nucleation in the solid state. Such interaction of theory and observation is an integral part of science in general, but it is particularly significant in the nucleation problem because the very beginning of a phenomenon invariably occurs on too localized a scale in both space and time to be detected with available techniques. Thus, there is little hope of making direct measurements on the nucleus during its birth or to ascertain the exact nature of the process; one can only infer the details of nucleation from the model whose predicted behavior seems to harmonize best with the measured rate of nucleation and its dependence on known parameters. In the field of solid-state nucleation, both theory and experiment call for much ingenuity, but somehow the conceptual schemes have spawned faster than they can be tested by observation. This is not to say that there are many basically new ideas in modern nucleation theory; rather, as we shall see, it is because the required fine-scale measurements are of immense technical difficulty. Quite likely, some of the nucleation models existent today have survived only because of the latitude permitted by meager data. There is great need to bring theory and experiment into better balance in this important area of physical metallurgy. Although one can