Nitinol SMA Sheets

The shape memory effect is closely related to the thermoelastic martensitic transformation. In TiNi memory alloys, the high-temperature phase and low-temperature phase martensite (M), as well as an intermediate phase, undergo mutual transformation during phase transition, achieving a self-coordinated state among the various variants. When external forces are applied, deformation occurs; upon removal of the external forces and heating above the Af temperature, the material returns to its shape at high temperatures. Subsequent cooling maintains the shape, known as "one-way" shape memory. If the material undergoes special "training," enabling it to repeatedly remember the two shapes of the high-temperature phase and low-temperature phase during subsequent heating and cooling cycles, it is referred to as "two-way" shape memory.

The elastic deformation of titanium-nickel shape memory alloys can reach 5% to 20%, often referred to as superelasticity. According to thermodynamic equations, both the memory effect and superelasticity are related to stress and temperature. Below the As temperature, it exhibits a shape memory region; between As and Af, it's a mixture of shape memory and superelasticity; from Af to Md (the stress-induced martensitic transformation limit temperature), it's the superelastic region. Above the Af temperature, slip deformation occurs, and the shape cannot fully recover. When external stress is applied in the temperature range above Ar and below Md, stress-induced martensitic phase transformation occurs, resulting in significant strain under low stress. Upon removal of the external force, the stress-induced martensite immediately undergoes reverse transformation, the strain disappears, and it returns to the original shape of the high-temperature phase, known as superelasticity.