Improving Functional Fatigue in NiTi-based Shape Memory Alloys

Shape memory alloys, especially NiTi, are used in a large number of applications in the biomedical sector and start to expand towards other markets like the automotive sector. Still one of the major drawback of binary NiTi is the rather poor fatigue life. This limits the design flexibility and usage of SMAs in several other technology fields. The fatigue life of TiNi-based alloys e.g. TiNiCu, can be greatly improved if the materials possess a unique microstructure with coherent precipitates, which act as nucleation centres for the phase transformation and a grain size in the sub µm range. They show a nearly perfect compatibility of the austenite and martensite phase, which can be expressed by the cofactor conditions. If fulfilled an unstressed transition layer between the corresponding phases is created, leading to a phase transformation without slip, reduced hysteresis and an increased fatigue life. As a result, magnetron sputtered TiNiCu based shape memory show negligible fatigue for 10 million cycles. Within this presentation several NiTi-based shape memory alloys will be discussed in terms of microstructure, compatibility and fatigue life to determine critical parameters for the design of SMAs with low functional fatigue.

Acknowledgements: Funding by the DFG priority program SPP1599 ferroic cooling is gratefully acknowledged.