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.

The positive influence of crystallographic compatibility on the thermal transformation stability has been already investigated extensively in the literature. However, its influence on the stability of the shape memory effect or superelasticity used in actual applications is still unresolved. In this investigation sputtered films of a highly compatible TiNiCuCo composition with a transformation matrix middle eigenvalue of 1±0.01 are exposed to thermal as well as to superelastic cycling. In agreement with previous results the thermal transformation of this alloy is with a temperature shift of less than 0.1 K for 40 cycles very stable; on the other hand, superelastic degradation behaviour was found to depend strongly on heat treatment parameters. To reveal the transformation dissimilarities between the differently heat-treated samples, the microstructure has been analysed by transmission electron microscopy, in situ stress polarization microscopy and synchrotron analysis. It is found that good crystallographic stability is not a sufficient criterion to avoid defect generation which guarantees high superelastic stability. For the investigated alloy, a small grain size was identified as the determining factor which increases the yield strength of the composition and decreases the functional degradation during superelastic cycling. This article is part of the themed issue ‘Taking the temperature of phase transitions in cool materials’.

Titanium-rich TiNiCu shape memory thin films with ultralow fatigue have been analysed for their structural features by transmission electron microscopy. The stabilization of austenite (B2) and orthorhombic martensite (B19) variants epitaxially connected to Ti 2 Cu-type precipitates has been observed and found responsible for the supreme mechanical cycling capability of these compounds. Comprehensive ex situ and in situ cooling/heating experiments have demonstrated the presence of an austenitic nanoscale region in between B19 and Ti 2 Cu, in which the structure shows a gradual transition from B19 to B2 which is then coupled to the Ti 2 Cu precipitate. It is proposed that this residual and epitaxial austenite acts as a template for the temperature-induced B2$B19 phase transition and is also responsible for the high repeatability of the stressinduced transformation. This scenario poses an antithesis to residual martensite found in common high-fatigue shape memory alloys. research papers 1010 Torben Dankwort et al. Martensite adaption in TiNiCu shape memory alloys

Ultralow-Fatigue Shape Memory Alloy Films. -Ultralow-fatigue Ti 54Ni34Cu12 shape memory alloy films allow at least 10 million transformation cycles. These films contain Ti 2Cu precipitates embedded in the base alloy that serve as sentinels to ensure complete and reproducible transformation in the course of each memory cycle. Because TiNiAg shape memory alloys show transformation characteristics comparable with that of TiNiCu, they could be promising candidates for biocompatible ultralow fatigue shape memory films for applications such as artificial heart valves or elastocaloric cooling. -(CHLUBA, C.; GE, W.; LIMA DE MIRANDA, R.; STROBEL, J.; KIENLE, L.; QUANDT*, E.; WUTTIG, M.; Science (Washington, DC, U. S.) 348 (2015) 6238, 1004-1007, http://dx.