WHAT IS THE BIG DEAL ABOUT THE A_f TEMPERATURE

Ming H. Wu, Mark Polinsky and Neal Webb

Memry Corporation, 3 Berkshire Boulevard, Bethel, CT 06801

Abstract

The austenite transformation finish temperature, A_f, has been specified as a key characteristic in product specifications for an increasing number of Nitinol medical devices. Many literatures also interpret the A_f temperature as a sole parameter for predicting the Nitinol material properties. As a result, the industry is spending a significant amount of resource to test this transformation temperature as a means for product quality assurance.  In the present study, cold drawn wires of Ti-55.8wt.%Ni alloy were heat treated in a temperature range of 350-600°C for durations up to 120 minutes.  Mechanical properties were determined by tensile tests while the transformation temperatures were measured by differential scanning calorimetry (DSC), bend and free recovery (BFR) and constant load dilatometry (CLD).  The fatigue properties of selective specimens were also evaluated using a rotating beam fatigue test method.  The purpose of the study is to test the validity of the correlation between tensile mechanical properties and the A_f temperatures measured using the various techniques.  The results will also access the influence of the Af temperature on the Nitinol fatigue life.

Key Words:            Transformation temperatures, A_f temperature, differential scanning calorimetry, bend and free recovery test, constant load dilatometry, mechanical properties, fatigue