Crystal size induced reduction in thermal hysteresis of Ni-Ti-Nb shape memory thin films

Ni41.7Ti38.8Nb19.5 shape memory alloy films were sputter-deposited onto silicon substrates and annealed at various temperatures. A narrow thermal hysteresis was obtained in the Ni-Ti-Nb films with a grain size of less than 50 nm. The small grain size, or large amount of grain boundaries, facilitates the phase transformation, thus reduces the hysteresis. The corresponding less transformation friction and heat transfer during the shear process, as well as reduced spontaneous lattice distortion, are also responsible for this reduction of the thermal hysteresis.