B.N Roy, Santoshkumar Choudhary, Satyendra Kumarmodi
The martensitic transformation (austenite-to-martensite) occurs when the free energy of martensite becomes less than the free energy of austenite at a temperature below a critical temperature T0 at which the free energies of the two phases are equal. However, the transformation does not begin exactly at T0 but, in the absence of stress, at a temperature M0s (martensite start), which is less than T0. The transformation continues to evolve as the temperature is lowered until a temperature denoted M0f (martensite finish) is reached. This temperature difference M0s - M0f is an important factor in characterizing shape memory behavior.When the SMA is heated from the martensitic phase in the absence of stress, the reverse transformation (martensite-to-austenite) begins at the temperature A0s (austenite start), and at the temperature A0f (austenite finish) the material is fully austenite. The equilibrium temperature T0 is in the neighborhood of (M0s + A0f)/2. The spreading of the cycle (A0f - A0s) is due to stored elastic energy, whereas the hysteresis (A0s - M0f) is associated with the energy dissipated during the transformation. The key characteristic of all SMAs is the occurrence of a martensitic phase transformation. The martensitic transformation is a shear-dominant diffusion less solid-state phase transformation occurring by nucleation and growth of the martensitic phase from a parent austenitic phase.
Critical Temperature, Martensitic Phase, Reverse Transformation, Hysteresis, Nucleation and Growth.