Precipitate nucleation kinetics at grain boundaries

The essential equations of solid–solid nucleation theory are summarised and the origins of the accelerated nucleation kinetics observed at grain boundaries are examined on this basis. Methods of measuring nucleation kinetics at grain boundaries are critically reviewed and the precautions which must be taken in order to secure accurate measurements are indicated. Available data on the nucleation rates of precipitation and massive transformations at grain faces and grain edges are reviewed and where possible are compared with the predictions of nucleation theory. On the basis of recent work, in particular, it is concluded that critical nuclei in diffusional transformations involving a significant change in crystal structure must be bounded largely or entirely by low energy interfaces. Such nuclei can evidently form at only a very limited proportion of the available grain face area or grain edge length.