Most solid-state phase transitions need to go through two stages: nucleation and growth. In the non-diffusion phase transition, it is non-thermal activation nucleation (variable temperature nucleation); The nucleation of diffusion phase transformation is similar to solidification, which accords with the classical nucleation model. Very few non-nuclear transformations, such as amplitude modulation decomposition. There are also two ways to nucleate new phases: uniform nucleation and heterogeneous nucleation.

Homogeneous nucleation also has various ups and downs in homogeneous parent phase. If the configuration, composition and density fluctuation of the parent phase are similar to those of the new phase, these regions may form embryos of the new phase, and when these embryos are large enough, they can grow into stable nuclei.

In the process of solid-state transformation, strain energy (ε) will be generated due to the different specific volumes of the new phase and the parent phase, which plays an important role in solid-solid phase transformation. Assuming that the embryo is spherical with radius r, the change of system free energy caused by the formation of embryo. △ G = (-4/3) π (r 3) (△ GV+ε)+4π (r 2) σ Draw the functional relationship between△ G and R in the above formula.

As can be seen from the figure, when △ GV+ε