Nitinol and Shape-Memory Alloys: Metals With a Memory

Nitinol (about 50.8 at percent Ni - Ti) exhibits two remarkable effects: the shape-memory effect (recovering a trained shape on heating through the austenite finish temperature) and superelasticity (large recoverable strain from stress-induced martensite). Buehler, Gilfrich and Wiley (1963) discovered the anomalous modulus drop at the transformation while working at the Naval Ordnance Laboratory - the name Ni-Ti-NOL. Otsuka and Wayman (1998) is the definitive treatise on the crystallography and applications.

The reversible B2 to B19' martensitic transformation, with a small thermal hysteresis, underlies stents, guidewires, orthodontic wires, couplings and actuators. Morgan (2004) reviewed the medical market and its products. Composition must be controlled to about 0.1 at percent because transformation temperature is extremely sensitive to Ni content; processing (cold work + age) sets the actuation temperature and fatigue life.

For buyers, define the required Af (austenite finish) temperature and bias force. JLHSteel supplies Nitinol wire, strip and tube for medical and actuator use, with transformation-temperature certificates.

References
1. Buehler, W.J., Gilfrich, J.V., Wiley, R.C. (1963). Effect of low-temperature phase changes on the Young's modulus of NiTi. Journal of Applied Physics 34(5): 1475-1477. https://doi.org/10.1063/1.1729603
2. Otsuka, K., Wayman, C.M. (1998). Shape Memory Materials. Cambridge University Press.
3. Morgan, N.B. (2004). Medical shape memory alloy applications - the market and its products. Materials Science and Engineering A378(1-2): 16-23. https://doi.org/10.1016/j.msea.2003.11.087