Time-temperature-transformation diagram for the martensitic transformation in a titanium-nickel shape memory alloy

• A TTT diagram for the B2-B19′ transformation is constructed by holding experiments.
• Free energy difference between the B2 and B19′ is evaluated by specific heat measurements.
• The size of nuclei of the B19′ martensite is estimated using a statistical thermodynamic model.

We have constructed a time-temperature-transformation (TTT) diagram for the martensitic transformation in a Ti-51.3Ni (at%) alloy. The transformation proceeds when the specimen is held at fixed temperatures between 110 K and 140 K, but not at other temperatures. The time required for the formation of 0.1% of the martensite phase is plotted as a TTT diagram. It exhibits a C-curve with a nose temperature near 130 K. Application of a thermal activation model to the TTT diagram suggests that the size of a nucleus of the martensite phase is about (2.2 nm)3.