Limitations on leveraging Af to predict the mechanical response of Nitinol

Tailoring the A_f temperature for implantable medical devices is one of the major considerations when selecting process parameters for Nitinol components. Besides enabling a superelastic response under in vivo conditions, A_f is often correlated to mechanical properties such as tensile behavior and fatigue performance. Measuring A_f of Nitinol components is relatively simple in contrast to mechanical testing and therefore it can be convenient to monitor process fluctuations by periodically measuring A_f. However, the practice of tailoring heat treatment process parameters to achieve specific A_f temperatures can result in undesirable mechanical behaviors of the Nitinol component. Experimental data will be presented demonstrating how a single lot of medical grade Nitinol wire can be heat treated to achieve similar A_f temperatures but exhibit large variations in tensile response and fatigue life. The limitations of leveraging A_f temperatures to estimate mechanical properties will be demonstrated and discussed.