Search

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 0.1K 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. [ABSTRACT FROM AUTHOR]

The time-resolved strain and temperature profiles of magnetron-sputtered pseudoelastic TiNi films of 20 μm thickness are investigated during tensile stress cycling for different strain rates. Based on simultaneous infrared thermography and digital image correlation, a good correlation between the evolution of temperature profiles and the local formation and propagation of Lüders-like strain bands is observed. The evolution of temperature profiles can be quantitatively described by a phenomenological Tanaka-type model of the martensitic transformation combined with a heat transfer model taking into account the local interaction between transformed and untransformed regions in the transformation kinetics. The investigated film samples exhibit an undercooling of −16 K upon mechanical unloading under adiabatic conditions at a strain rate of 0.2 s −1 . A coefficient of performance of 7.7 is determined. Due to the high surface-to-volume ratio, the films show fast heat exchange in air of the order of 1.5 s. Based on these results, the prospects of SMA film-based elastocaloric cooling are discussed. [ABSTRACT FROM AUTHOR]