transformation kinetics
Thermodynamics tells you a soggy log will eventually rot; kinetics tells you how fast. Transformation kinetics is the study of how much of a phase transformation has happened as a function of time, at fixed temperature. It is the rate story that the phase diagram completely leaves out.
If you hold a steel at a fixed temperature and measure the fraction transformed, you get an S-shaped (sigmoidal) curve: slow at first (nuclei are still few), fast in the middle (many particles all growing at once), then slow again as the remaining untransformed regions run out of room and impinge on each other. Plotting fraction versus log(time) makes this S especially clear.
This sigmoidal behaviour is captured mathematically by the Avrami equation, and it underlies the TTT and CCT diagrams that engineers actually use to choose a heat treatment. Kinetics is temperature-sensitive: raise the temperature and the whole S-curve usually shifts to shorter times, until diffusion-versus-driving-force effects reverse it near the transformation temperature.
For a steel held isothermally you might see 1 percent transformed after 1 second, 50 percent after 10 seconds, and 99 percent after 100 seconds. The same reaction, tracked purely in time.
Transformation is a rate process, not an instant switch.
Equilibrium thermodynamics predicts the final phases; kinetics predicts whether you will actually reach them in the time available, and fast cooling can bypass the equilibrium product entirely.