Introduction

Whenever a single phase system is put into a two-phase metastable state, for example by quenching from a high temperature or changing pressure, a second phase nucleates, grows, and coarsens. Nucleation of the second phase occurs since the energy of the single phase system can be reduced by forming regions of the second phase. Nanometer-sized precipitates of the new phase thus appear. Growth of nuclei then proceeds due to heat diffusion away from the nucleus into the matrix or mass diffusion from the matrix to the nuclei. This results in a dispersion of second phase particles in a matrix. Coarsening, also called Ostwald ripening or competitive growth, then occurs where large particles grow at the expense of small particles. The nucleation, growth, and coarsening is very common, occurring in systems ranging from solid alloys to the precipitation of drops from clouds [246].