There are four critical temperatures for SMA which characterize its behavior. These are A_s, A_f, M_s and M_f with A and M indicating Austenite and Martensite phases. The subscripts, s and f denote start and finish of transformation, respectively. These temperatures are stress dependent and their values change depending on the loading conditions.

The Shape Memory Effect (SME) can be summarized as shown in Fig.34.2. At a temperature below M_fo , SMA has twinned Martensitic microstructure (B). On loading SMA, the de-twinning of Martensite starts and it ends at some critical value of the stress (C). Further loading of SMA results in permanent plastic strain in SMA. On complete unloading, SMA still retains strain in it (D), which is recovered if it is heated beyond A_fo (E). At A_so , Martensite phase starts transforming to Austenite. A_fo is the temperature for completing this transformation at zero stress level. Percentage of transformation strain recovered by SMA is proportional to Austenite volume fraction. On cooling SMA, Martensite transformation starts from M_so and for temperatures below M_fo Austenite gets completely transformed to twinned Martensite (B). C_m and C_A are material dependent parameters of SMA providing the relationship between temperature and stress for Martensite and Austenite phases, respectively. Their units are Pa / ⁰K. C_M is typically of higher magnitude than C_A .

Figure 34.2: Shape memory effect

- Cubic crystal structure. Austenite phase

- Twinned martensite crystals. Twinned martensite phase

- De-twinned martensite crystals. De-twinned martensite phase