Limitations on leveraging Af to predict the mechanical response of Nitinol

Tailoring the austenitic finish temperature (Af) for implantable medical devices is a common practice used in selecting heat treat parameters for Nitinol components. In addition to predicting a superelastic response under in-vivo conditions, Af is often correlated to mechanical properties such as tensile behavior and fatigue performance. Measuring the Af of Nitinol is seemingly simple, and therefore, a convenient tool to monitor process fluctuations. However, the reliance on Af can be highly misleading, resulting in undesirable mechanical properties of the Nitinol component. Experimental data presented here demonstrates how a single lot of superelastic Nitinol wire can be heat treated to achieve similar Af temperatures, but exhibiting large variations in tensile responses. Other measures of transformation temperatures, such as Ms and \({\text{M}}_{{\text{s}}}^{*}\), are better indicators, but still not completely successful. It is thus concluded that one should, whenever possible, rely on the direct measurement of tensile properties at body temperature in order to design and validate heat treatment processes, and control those processes once designed.