Isothermal transformation diagrams (also known as time-temperature-transformation or TTT diagrams) are plots of temperature versus time (usually on a logarithmic scale). They are useful for understanding the transformations of an alloy steel that is cooled isothermally. Isothermal transformation diagrams are generated from percentage transformation-vs logarithm of time measurements.
An isothermal transformation diagram is only valid for one specific composition of material, and only if the temperature is held constant during the transformation, and strictly with rapid cooling to that temperature. Though usually used to represent transformation kinetics for steels, they also can be used to describe the kinetics of crystallization in ceramic or other materials. Time-temperature-precipitation diagrams and time-temperature-embrittlement diagrams have also been used to represent kinetic changes in steels.