Determination of Transformation Properties of Thin Medical Grade Ni-Ti Wire by High-Resolution Bend and Free Recovery Testing

Thin medical grade Ni-Ti wires, pseudo-elastic at ambient temperature, were annealed to set the final shape and the properties of the material. In order to investigate the influence of the annealing, high-resolution bend and free recovery tests were carried out. With the newly constructed device, the reverse deformation (free recovery) of the bent wires was recorded in detail. Wire segments with a weight of <1 mg are sufficient to attain reliable results. The high resolution of the testing device is based on a precise temperature control and a video monitoring system equipped with a microscope lens. In addition to the determination of the transformation temperatures, which is possible with common bend and free recovery tests, the high resolution allows for the identification and quantification of the reverse deformation stages attributed to the martensite-to-austenite transformation and the occurrence of the R-phase. The variations of the transformation properties attained by evaluating the different types of reverse-deformation curves are presented and discussed.