Comparative Analysis of Phase Transformation Measurements in Nitinol: DSC vs. BFR from wire to complex medical device components

The phase transformation temperatures of Nitinol are critical to ensuring appropriate functioning of medical devices that rely on its unique superelastic behavior. Both differential scanning calorimetry (DSC) and/or bend-free recovery (BFR) testing are used to measure transformation temperatures, though the two methods do not yield the same results. While literature sheds some light on the relationship between the two methods in the case of wires, no consistent method to correlate the two or for using these methods interchangeably has been established for complex shaped components used in medical devices. These do not only have two-stage (M-R'-A) transformations but also complex deformation modes when being bent. BFR is widely used in the medical device industry as a fast non-destructive method to determine transformation temperature for components; establishing a quantitative relationship between the two methods would prove invaluable in device-design and manufacturing. Here, we compare DSC and BFR for wires and beams as well as for stents, whose deformation during BFR for different heat treatment conditions that result in overlapping and consecutive M-R’ and R’-A transformations.