Time-Temperature-Transformation (TTT ) Diagram

T (Time) T(Temperature) T(Transformation) diagram is a plot of temperature versus the logarithm of time for an alloy of definite composition. In case of steels, it is used to determine when transformations begin and end for an isothermal (constant temperature) heat treatment of a previously austenitized alloy. When austenite is cooled slowly to a temperature below LCT (Lower Critical Temperature), the structure that is formed is Pearlite. As the cooling rate increases, the pearlite transformation temperature gets lower. The microstructure of the material is significantly altered as the cooling rate increases. By heating and cooling a series of samples, the history of the austenite transformation may be recorded. TTT diagram indicates when a specific transformation starts and ends and it also shows what percentage of transformation of austenite at a particular temperature is achieved.

Upper half of TTT Diagram (Austenite-Pearlite Transformation Area)

Lower half of TTT Diagram (Austenite-Martensite and Bainite Transformation Areas)

Cooling rate that permits both pearlite and martensite formation

TTT Diagram and microstructures obtained by different types of cooling rates

Type of phase transitions1st order phase transitions2nd order phase transitions1st order phase transitionsInvolve latent heatExamples: Melting of ice, melting of an alloy, solidification of an alloy

b) 2nd order phase transitionNo latest heat involvedExamples: Ferromagnetic ordering is an example of 2 transitions

Diffusionless and Diffusive transformationMartensitic transformations are very different from those involving diffusion of atoms, i.e. reconstructive transformations. In martensitic transformations, the atoms move in an organised manner relative to their neighbours. This homogeneous shearing of the parent phase creates a new crystal structure, without any compositional change (no diffusion).Martensitic transformations are also known as diffusionless, displacive or military.

The Martensitic Reaction and TemperingMartensite is a phase that forms as the result of a diffusionless solid-state transformation. In this transformation there is no diffusion and, hence, it does not follow the Avrami transformation kinetics. The growth rate in martensitic transformations (also known as displacive or athermal transformations) is so high that nucleation becomesthe controlling step. The phase that forms upon the quenching of steels was named martensite by Floris Osmond in 1895 in honor of German metallurgist Adolf Martens. Similar martensitic phase transformations occur in other systems as well. Cobalt, for example, transforms from a FCC to a HCP crystal structure by a slight shift in the atom locations that alters the stacking sequence of close-packed planes. Because the reaction does not depend on diffusion, the martensite reaction is an athermaltransformationthat is, the reaction depends only on the temperature, not on the time. The martensite reaction often proceeds rapidly, at speeds approaching the velocity of sound in the material.

These transformations can also be driven by the application ofmechanical stress. Other than martensite that forms in certain type of steels, the Ni-Ti alloy, known as nitinol (which stands for Nickel Titanium Naval Ordinance Laboratory, developed by the U.S. Naval Ordinance Laboratory in the 1940s) is perhaps the best-known example of alloys that make use of martensitic phase transformations. These materials can remember their shape (i.e. shape memory effect) and are known as shape-memory alloys (SMAs).

Tempering of Steel Martensite: Martensite is not an equilibrium phase. This is why it does not appear on the Fe-Fe3C phase diagram. When martensite in a steel is heated below the eutectoid temperature, the thermodynamically stable and Fe3C phases precipitate. This process is called tempering. The decomposition ofmartensite in steels causes the strength and hardness of the steel to decrease while the ductility and impact properties are improved

Keep in mind!

Martensite in steel is not the only martensitic transformationAny diffusionless transformation (occurring because cooling/heating rate is rapid enough to avoid any diffusion-controlled process)Can occur in other metals