Relationship between mechanical properties and phase transformation temperatures of Nitinol

The A_f temperature is still widely used as the primary material specification for devices with the intent to ensure superelasticity at body temperature and to ensure consistent device performance. Cold worked and aged Nitinol used in medical devices undergoes two-staged phase transformation from austenite to R to martensite. For a given heat treatment condition, the importance of R-phase in determining the temperature threshold for superelasticity, the effective modulus as well as the sensitivity of plateau stress to test temperature have been discussed by Duerig et al (Shap Mem Superelasticity 1:153–161, 2015 and Shap Mem Superelasticity 3:485–498, 2017). In our present study, we explore the role of R-phase in determining device relevant mechanical properties across a range of heat treatment conditions in commercial Nitinol.

For specimens fabricated from commercially available Nitinol tubing, we measured phase transformation temperatures and mechanical properties for a range of heat treatment temperatures and times. Transformation temperatures were determined using differential scanning calorimetry (DSC), and mechanical properties were characterized by tensile testing dogbone specimens under constant temperature conditions. While both the M to R’ temperatures (R’_s and R’_f) and R’ to A temperature (A_s) correlate with the plateau stresses, the former correlation is much stronger than the latter. Both plateau stresses are linearly related to M-R’ transformation temperatures. Based on the results, we will further discuss the potential implications for process development and specifications.