Effect of test variables on DSC and BFR techniques for phase transformation temperature measurements and implications to finished nitinol components

Understanding the relationship between Nitinol phase transformations and performance is critical for the successful development and manufacturing of a Nitinol medical device. The two industry standard test methods for measuring these phase transformations are ASTM F2004 and ASTM F2082. These standards were originally intended for ingot or semi-finished material. More specifically, ASTM F2004 describes the standard test method for determining phase transformations via the thermal analysis technique, differential scanning calorimetry (DSC), of an annealed sample to verify the Ni:Ti ratio of the Nitinol ingot. ASTM F2082 describes the standard test method for determining Austenite and R-phase phase transformations via bend and free recovery (BFR) of wire, strip, tube, and sheet semi-finished materials. The ASTM standards allow application of these methods to finished devices, but do not clarify how to adapt them correctly and how the results should be used. Despite that, the method is often utilized for specification verification at finished component level with potential for erroneous conclusions.

In this work, we will examine the influence of common test variables on measured phase transformation temperatures for both DSC and BFR to understand the limits of both methods. For DSC, we will evaluate the impact of sample mass, heating/cooling temperatures and rates. For BFR, we will examine the influence of strain, volume under strain and cooling temperature. We will discuss the results and observed limits of these test methods in the context of medical device manufacturing.