Cu-rich Ti52.8Ni22.2Cu22.5Co2.5 shape memory alloy films with ultra-low fatigue for elastocaloric applications

Abstract

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 Ti54Ni34Cu12 and Ti54.7Ni30.7Cu12.3Co2.3 alloys show that a high fatigue resistance can be achieved. Ti54Ni34Cu12 shows a good compatibility (λ2 ∼ 0.9905) with coherent Ti2Cu precipitates, whereas Ti54.7Ni30.7Cu12.3Co2.3 shows a near perfect compatibility (λ2 ∼ 1.00083) but no Ti2Cu and lower transition temperatures. To differentiate whether the crystallographic compatibility or Ti2Cu precipitates influence the functional properties more, a TiNiCuCo alloy with a large expected fraction of Ti2Cu precipitates was chosen. In this work, freestanding Ti52.8Ni22.2Cu22.5Co2.5 films are fabricated by a multilayer sputter deposition approach. They show stable superelasticity for more than 2 × 107 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 (λ2 ∼ 0.994 instead of 1) is not needed for high cyclic stability when combined with a small grain size (∼300 nm) and Ti2Cu 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 COPmat reaches a value of 11.2, which makes Ti52.8Ni22.2Cu22.5Co2.5 highly suitable for elastocaloric cooling applications.