Microstructure and shape memory behavior of Ti51.2(Pd27.0Ni21.8) and Ti49.5(Pd28.5Ni22.0) thin films

The shape memory characteristics of Ti-rich Ti(Pd,Ni) thin films annealed from the amorphous state has been clarified based on the microstructural observations compared with those of the near-equiatmic ones. The films with the compositions of Ti51.2(Pd27.0Ni21.8) and Ti49.5(Pd28.5Ni22.0) were crystallized at 773, 873 and 973 K for 1 h. The low-temperature crystallizations at <973 K from the amorphous state are effective for grain refinement without an additional element or thermo-mechanical treatment. The Ti51.2(Pd27.0Ni21.8) film annealed at 773 K shows fine plate-like Ti2Pd-type precipitates with a diameter <100 nm inside the B2 grain. These precipitates have the habit plane parallel to a {100} plane of the B2 matrix and with the orientation relationship of {100}B2∣∣{001}Ti2Pd and 〈010〉B2∣∣〈001〉Ti2Pd. It was found that the shape memory characteristics could be improved by the precipitation hardening of the Ti2Pd-type precipitates, probably because of the internal strain field produced along the {001} planes of the precipitates. The film shows the Ms temperature of 485 K, the recoverable strain of 2.53% and the residual strain of 0.14% under the constant stress of 440 MPa.