Hysteresis is the temperature difference between a material’s phase transformation upon heating or cooling. This spread is typically around 20-30°C (70-85°F) for Nitinol superelastic alloys used in medical applications (Fig. 1). Various heat treatments can shift the hysteresis higher, lower or widen it.
Austenite is nitinol’s stronger, higher-temperature phase. Crystalline structure is body-centered cubic. Superelastic behavior is in the phase, over a 50-60°C (120-140°F) temperature spread. Martensite is nitinol’s weaker, lower temperature phase. Crystalline structure is twinned. Material easily deformed in this phase. Once deformed in martensite it will remain deformed until heated to austenite where it will return to its pre-deformed shape, i.e. “shape memory” effect.
Figure 1 Notes:
1. Af: temperature where material has finished transforming to austenite upon heating
2. As: temperature where material starts to transform to austenite upon heating
3. Ms: temperature where material starts to transform to martensite upon cooling
4. Mf: temperature where material has finished transforming to martensite upon cooling
Nitinol in the cold-work condition (as-drawn or as-rolled) is material that has not been subject to a final heat treatment (called a shape-set anneal). Nitinol wire and tube typically will have 30-40% cold-work reduction during the last drawing steps. In sheet and strip products, that value is usually closer to 20% reduction.
The amount of cold work a material had prior to its shape-set anneal dictates the ultimate strength of the material. Once the Nitinol material has been shape-set annealed, it will exhibit the superelastic and shape-memory properties.
Generally, the shape-set anneal is a straightening process performed under controlled time, temperature and pressure conditions. For example, an SE508 alloy (55.8% Ni, 0.05% O max, 0.02% C max, balance Ti) the aging temperature range is 200-500°C (400-930°F). This process defines the final mechanical properties of the material until it is subject to further processing. Varying the parameters of the shape-set anneal will affect these properties. This annealing process is continuous for wire and strip straightened on spools or in discrete lengths for tubes and sheet and bar-stock material.