High ultimate tensile stress in nano-grained superelastic NiTi thin films

NiTi-films were fabricated by dc magnetron sputtering from melt-cast disc targets. The freestanding films revealed superelastic properties in tensile tests. At 37 deg. C superelastic properties were achieved showing a closed-loop hysteresis and a plateau of more than 5% strain. The ultimate tensile strength exceeded 1180 MPa for the sputtered films at a maximum strain of 11.5%. This remarkable improvement in mechanical properties over those reported in previous studies correlates with a textured, fine grained (50-200 nm), single phase microstructure, confirmed by transmission electron microstructure. Moreover, these grains revealed a texture which was not found in earlier studies concerning sputtered films. Finally, the prepared specimens did not reveal any evidence of disc or lens shaped Ti3Ni4 precipitates but a relatively homogeneous chemistry