Abstract
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.
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Notes
While this is often written as dσ/dT = − ∆H/TΔ∆ε, care must be taken in that ΔH here is not the latent heat of transformation but includes strain energy considerations.
Even in the absence of Lüders deformation, care must be taken in assuming that the plateau length is a measure of transformational strain. Neutron diffraction experiments have demonstrated that transformation continues well beyond the apparent end of the plateau [13]. Further evidence for this is seen in the temperature dependence of the plateau lengths in Fig. 3. It should be further noted that bands of Martensite do contain some small amount of retained Austenite, and some Martensite can still be found outside of the bands.
Not that, this midpoint stress value could be different from the unloading stress values dictated by ASTM F2516. Here, we are invoking thermodynamic principles, whereas the ASTM standard provides a convenient, yet arbitrary, strain (2.5%) to select an unloading stress..
Extracting the \(M_{\text{s}}^{*}\) temperature is difficult because shape recovery is so gradual. One must project the recovery profile to estimate the horizontal tangent, as shown in this figure. The alternative of drawing a tangent to the tail of recovery is even more problematic since then the temperature at which one begins to record data dictates the slope of the tangent.
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References
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Duerig, T.W., Pelton, A.R. & Bhattacharya, K. The Measurement and Interpretation of Transformation Temperatures in Nitinol. Shap. Mem. Superelasticity 3, 485–498 (2017). https://doi.org/10.1007/s40830-017-0133-0
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DOI: https://doi.org/10.1007/s40830-017-0133-0