Abstract
Sputtered Ti–rich TiNiCu alloys are known to show excellent cyclic stability. Reversibility is mostly influenced by grain size, crystallographic compatibility and precipitates. Isolating their impact on cyclic stability is difficult. Ti2Cu precipitates for instance are believed to enhance reversibility by showing a dual epitaxy with the B2 and B19 lattice. Their influence on the functional fatigue, if they partly lose the coherency is still unknown. In this study, sputtered Ti53.7Ni24.7Cu21.6 films have been annealed at different temperatures leading to a similar compatibility (λ2 ~ 0.99), grain size and thermal cyclic stability. Films annealed at 550 °C exhibit a superior superelastic fatigue resistance but with reduced transformation temperatures and enthalpies. TEM investigations suggest the formation of Guinier–Preston (GP) zone-like plate precipitates and point towards a coherency relation of the B2 phase and finely distributed Ti2Cu precipitates (~ 60 nm). Films annealed at 700 °C result in the growth of Ti2Cu precipitates (~ 280 nm) with an irregular distribution and a partial loss of their coherency. Thus, GP zones are assumed to cause the reduction of transformation temperatures and enthalpies due to increased internal stresses, whereas the coherency relation of both, Ti2Cu and GP zones, help to increase the superelastic stability, well beyond 107 cycles.
Highlights
Tailoring and understanding the fatigue mechanism in shape memory alloys (SMAs) is crucial for biomedical implants, actuation applications or possible future applications like elastocaloric cooling
Distinct intragranular precipitation is observed in the HT sample, which differentiate in shape and size: Larger roundish (Appendix Figs. 7 and 8) and rectangularly shaped precipitates which seem to exhibit an incoherent or semicoherent connection to the matrix and very small rectangularly shaped precipitates (Appendix Fig. 8), which potentially exhibit a coherent or semicoherent relationship to the matrix according to their orthogonal arrangement
Additional X-ray diffraction measurements of TiNiCu films annealed at higher temperatures (800 °C) suggest that only incoherent Ti2Ni precipitates remain at higher temperatures and are regarded as the stable phase in the investigated composition
Summary
Tailoring and understanding the fatigue mechanism in shape memory alloys (SMAs) is crucial for biomedical implants, actuation applications or possible future applications like elastocaloric cooling. In this study, sputtered Ti53.7Ni24.7Cu21.6 films have been annealed at different temperatures leading to a similar compatibility (k2 * 0.99), grain size and thermal cyclic stability.
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