Thickness-dependence of the B2–B19 martensitic transformation in nanoscale shape memory alloy thin films: Zero-hysteresis in 75 nm thick Ti51Ni38Cu11 thin films

Abstract The influence of film thickness on the B2–B19 martensitic transformation properties of nanoscale Ti 51 Ni 38 Cu 11 thin films with thicknesses ranging from 750 to 50 nm is reported. For these films an unexpected behavior of the phase transformation temperatures was observed: A f and O s initially decrease with decreasing film thickness but increase sharply again for thicknesses A f ) and 14 to ∼0 K, respectively. For the first time we can show that substrate-attached Ti–Ni–Cu thin films as thin as 50 nm show reversible B2–B19 phase transformations. Furthermore, it is shown that with decreasing film thickness a change in the tetragonality of the B19 martensite phase occurs. This leads to fulfilling the so-called λ 2 criterion, causing a vanishing hysteresis for a film thickness of 75 nm.