Evolution of temperature profiles in TiNi films for elastocaloric cooling

The time-resolved strain and temperature profiles of magnetron-sputtered pseudoelastic TiNi films of 20 mu m 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 Luders-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. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.