Cu-rich Ti_(52.8)Ni_(22.2)Cu_(22.5)Co_(2.5) shape memory alloy films with ultra-low fatigue for elastocaloric applications

摘要

Elastocaloric cooling demands for ultra-low functional and structural fatigue in combination with a high effect size and low energy input. Recent advances in fine-grained sputtered Ti-rich Ti_(54)Ni_(34)Cu_(12) and Ti_(54.7)Ni_(30.7)Cu_(12.3)Co_(2.3) alloys show that a high fatigue resistance can be achieved. Ti_(54)Ni_(34)Cu_(12) shows a good compatibility (λ_2～0.9905) with coherent Ti_2Cu precipitates, whereas Ti_(54.7)Ni_(30.7)Gu_(12.3)Co_(2.3) shows a near perfect compatibility (λ_2～1.00083) but no Ti_2Cu and lower transition temperatures. To differentiate whether the crystallographic compatibility or Ti_2Cu precipitates influence the functional properties more, a TiNiCuCo alloy with a large expected fraction of Ti_2Cu precipitates was chosen. In this work, freestanding Ti_(52.8)Ni_(22.2)Cu_(22.5)Co_(2.5) films are fabricated by a multilayer sputter deposition approach. They show stable superelasticity for more than 2 × 10~7 cycles with almost no degradation. Temperature-dependent x-ray diffraction and scanning transmission electron microscopy-high-angle annular dark-field imaging investigations identify that a perfect crystallographic compatibility (λ_2～0.994 instead of 1) is not needed for high cyclic stability when combined with a small grain size (～300 nm) and Ti_2Cu precipitates. In situ x-ray diffraction studies of the stress-induced transformation reveal the presence of non-transformed austenite well above the superelastic plateau and an eased transformation perpendicular to the loading direction. In agreement with XRD studies, the adiabatic temperature change shows an increase with increasing strain up to -12.2 K for the reverse transformation. The material shows a stable isothermal entropy change of -21.8 J kg~(-1) K~(-1) over a wide range of 40 K. The average COP_(mat) reaches a value of 11.2, which makes Ti_(52.8)Ni_(22.2)Cu_(22.5)Co_(2.5) highly suitable for elastocaloric cooling applications.