Correlation between Differential Scanning Calorimetry and Bend Free Recovery test methods when evaluating Nitinol Wire Austenite Finish transformation temperature

Problem: There are two primary methods used in the industry to evaluate nitinol phase transformations, bend free recovery (BFR) per ASTM F2082 and differential scanning calorimetry (DSC) per ASTM F2004. Both methods are widely used, but the correlation between the method outputs is not fully characterized. This can be challenging when different test methods are specified or preferred by suppliers, customers, and manufacturers. DSC is a test method that measures the change in energy as a function of temperature; it is a strictly thermal method. On the other hand, BFR monitors the displacement of a sample as temperature is gradually increased. This output is an Active Af as the wire actively changes shape/displacement during the test. There is a gap in published data discussing how different wire inputs (i.e., amount of cold work, prior heat treat history, etc.) impact the correlation between these two outputs.

Methods: This paper will analyze the correlation between BFR and DSC test data for nitinol wires that have undergone various heat treatments in temperature range of 300°C-500°C and durations of 0.5 min – 240 min. The paper will also include discussion about how different factors contribute to the correlation of the methods. These factors may include amount of cold work present in the wire, tension/compression experienced during BFR testing, and presence of R-Phase.