Irradiation-induced phase transformation in undeformed and deformed NiTi shape memory thin films by high-energy ion beams

Abstract

Irradiation-induced transformations in sputter-deposited Ti-rich NiTi thin films have been investigated with special attention given to the effects of prior deformation on the response of martensite phase. Irradiation at low fluence was carried out at room temperature with Au ions at 350 MeV, such that the projected ion range greatly exceeded the film thickness and lattice damage was attributable mainly to electronic stopping effects (43 keV nm⁻¹). Ion-beam modified microstructures were investigated using transmission electron microscopy, X-ray diffraction, and differential scanning calorimetry. Amorphous tracks were observed in both Ti₂Ni precipitates and martensite. The track diameter and the amount of amorphization were larger in the pre-deformed sample as compared to the undeformed sample. In both cases the displacive transformation temperatures and transformation enthalpies were depressed by irradiation, but to a greater degree in the pre-deformed material. Similarly, the extent of austenitic and R-phase regions observed to surround individual ion tracks was substantially greater for the pre-deformed films. These observations are discussed in terms of differences between the deformed and undeformed cases with respect to microstructure.

Acknowledgements

The authors are grateful to the Swiss National Science Foundations for funding this project. We additionally thank the personnel at the Centre Interdepartmental Electron Microscopy (CIME) at EPFL, and Dr Klaumünzer and Dr Schumacher for their help in irradiation experiments at HMI Berlin.