The Measurement and Interpretation of Transformation Temperatures in Nitinol

A previous paper introduced several engineering considerations surrounding the R-phase in Nitinol and highlighted a common, if not pervasive, misconception regarding the misuse of the Austenite finish temperature (A_f) by the medical device industry.  This paper brings additional data to bear on the issue and proposes a more accurate set of terminologies that aligns tensile properties to transformation temperature and avoids the ambiguity of A_f.  Moreover, a variety of tools are used to establish the forward and reverse stress-temperature phase diagrams for a superelastic wire typical of that used in medical devices.  Once established, the two most common methods of measuring transformation temperatures, DSC and BFR, are tested against the observed behavior and phase diagram.