M. Frotscher, J. Burow, M. Wagner, K. Neuking, Ruhr-University Bochum, Bochum, Germany; G. Eggeler, Ruhr University Bochum, Bochum, Germany
The pseudoelastic properties of ternary NiTiCr shape memory alloys (SMAs) with 1 wt.-% Cr have been studied. The addition of Cr to NiTi alloys results in an improvement of tensile strength. There is a need to work on a better understanding of microstructural and mechanical aspects of fatigue especially in the area of medical applications. We study Ni-rich NiTiCr wires (0.25 wt.-% Cr) provided by Memory-Metalle, Weil am Rhein. The microstructure of the material was characterized in the as-received condition and after a number of different heat treatments using scanning and transmission electron microscopy (SEM and TEM). A prominent feature of the microstructure of our wires are ultra fine grains of close to 40 nm grain size (UFGs). During cooling from the high temperature regime, two sequential martensitic transformations occur: B2 → R followed by R → B19’. The results were confirmed by X-ray and in situ electron diffraction. We perform mechanical pull pull cycling at temperatures ranging from 20°C to 80°C. Moreover, bending rotation fatigue tests are conducted in air and in oil. The influence of surface conditions on fatigue life is studied. Results are reported and compared with data from binary NiTi wires. The effects of system parameters like the type of loading, temperature, surface quality and the presence of inclusions are discussed. The results obtained in the present study contribute to a better understanding of the microstructure and of pseudoelastic fatigue of NiTiCr shape memory alloys.
Summary: Pseudoelastic NiTiCr shape-memory alloy (SMA) wires are studied with regard to microstructure in the as-received condition and after a number of different heat treatments. Results of mechanical experiments such as thermo-mechanical pull pull cycling at various temperatures and rotation bending fatigue are reported and compared with data from binary NiTi wires.
