Determination of martensite-to-austenite transformation properties of thin medical grade Ni-Ti wires by high resolution bend and free recovery tests

The application of thin Ni-Ti wires for medical devices and implants requires reliable and specific information about the actual material`s properties as e.g. the transformation temperatures. To warrant the functionality of the implant, the deformation characteristics including the temperature range for the martensite-to-austenite transformation and the corresponding reverse deformation degree are of strong interest, too.

As loading and straining conditions affect the transformation behaviour, the testing method should reproduce the loading and straining conditions of the material during its application. Similar conditions as in the application are used during the bend and free recovery test, which has been chosen for the measurements, accordingly. Literature results of bend and free recovery tests occasionally suffer from low accuracy due to unprecise temperature control and correlation of temperature and deformation.

As thin wires are sensitive to mechanical loading, a non-contact optical method was chosen for the present newly constructed experimental setup. A cooling / heating system that allows for controlling the temperature with an accuracy of ±0.1K in a range of -55°C to 45°C has been built.

With this system the transformation temperatures of thin Ni-Ti wires were determined with ±1K. Changes in the transformation characteristics, e.g. related to the occurrence of the R-Phase and other phenomena are clearly visible. The variations in the transformation characteristics for differently annealed wires are presented and discussed.