Establishing a Modern Time-Temperature-Transformation-Stress Diagram

Over 2 decades later, Pelton’s et al (MITAT 2000) Time-Temperature-Transformation (TTT) diagram showing the shift of A_f corresponding to variations in heat treatment time and temperature still provides the most widely used guidance to nitinol device manufacturers today. Although that diagram has been appended with additional data and presented in a variety of forms since then, updating and expansion of the TTT diagram is warranted. In the present study, we derive a modern and more complete diagram using material that is commonplace in the medical device community – namely superelastic tubing and wire (rather than only wire originally used) heat treated in molten salt to promote rapid heat exchange, thereby minimizing the influence of heating and cooling kinetics. Adopting the terminology proposed by Duerig et al, this modern diagram quantifies the shift of all of the transformation temperatures (R_s, R_p, R_f, M_s, M_p, M_f, M*_s, M*_p, M*_f, A_s, A_p, and A_f) not just the A_f. Additionally, we characterize the concomitant shift in fundamental tensile and compressive mechanical properties (UPS, LPS, UTS, E_a, E_m, El_r, and El_p) and the component-specific attribute of radial resistive forces and chronic outward force of a generic stent. Characterization of this Time-Temperature-Transformation-Stress (TTTS) dependent behavior is quantified over a range intended to bracket the most commonly-used manufacturing procedures: 0.5 – 10 minutes and 450 – 550°C, as well as a broader academic range up to 1 hour and with temperatures between 200 and 600°C.