The Measurement and Interpretation of Transformation Temperatures in Nitinol

A previous paper (Duerig and Bhattacharya in Shap Mem Superelasticity 1:153–161, 2015) introduced several engineering considerations surrounding the R-phase in Nitinol and highlighted a common, if not pervasive, misconception regarding the use of the term Af by the medical device industry. This paper brings additional data to bear on the issue and proposes more accurate terminology. 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, Differential Scanning Calorimetry and Bend Free Recovery, are tested against the observed behavior. Light is also shed upon the origin of the Clausius–Clapeyron ratio (dσ/dT), the triple point, and why such large variations are reported in superelastic alloys.