Functional shape memory alloys need to operate reversibly and repeatedly. Quantitative measures of reversibility include the relative volume change of the participating phases and compatibility matrices for twinning. But no similar argument is known for repeatability. This is especially crucial for many future applications, such as artificial heart valves or elastocaloric cooling, in which more than 10 million transformation cycles will be required. We report on the discovery of an ultralow-fatigue shape memory alloy film system based on TiNiCu that allows at least 10 million transformation cycles. We found that these films contain Ti2Cu precipitates embedded in the base alloy that serve as sentinels to ensure complete and reproducible transformation in the course of each memory cycle.

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Micropatterned freestanding superelastic TiNi films in the thickness range between 20 and 100 µm are attractive materials for medical in vivo applications. Micropatterning of these materials by UV lithography and etching is a challenging task, as wet etching has severe design limitations due to its isotropic nature while dry etching shows much to low etching rates. This study presents a method to fabricate freestanding TiNi films (with thicknesses higher than 20 µm) with a minimum feature size 5 µm based on UV lithography, sacrificial layer and wet etching technology. This method was successfully applied to the fabrication of TiNi films with 50 µm thickness and 25 µm feature size. These superelastic films showed remarkable tensile strengths up to 1100 MPa and elongations upon fracture of more than 40%.

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