Abstract
Ni–Ti Shape Memory Alloys (SMA) are of great technological interest because they have the best shape memory behaviour of all SMA. Moreover, Ni–Ti thin films are considered to be one of the most promising solutions for the development of new microactuators. In order to use Ni–Ti in the thin film-state, it is therefore important to know if the properties of the martensitic phase transformation responsible for the shape memory effect are different in Ni–Ti thin films and in bulk materials. For that purpose, 61Ni Nuclear Magnetic Resonance (NMR) measurements have been performed at very high magnetic field (14 T) in Ni–Ti bulk alloys as well as in thin films. The process of nucleation and growth of the R-phase and of the martensite in the different samples have been studied by means of a careful analysis of the 61Ni NMR spectra recorded at different temperatures from T>Rs to T<Mf. The complex structure of the NMR spectra during phase transformation has been interpreted as a sum of contributions arising from the coexisting crystalline phases. The spectral deconvolution is very difficult to achieve because the NMR martensitic line of complex shape is partly superimposed on the austenitic line. However, additional NMR measurements in a 61Ni enriched Ni–Ti bulk alloy allowed us to identify the microscopic interactions responsible for the martensitic line shape. With the help of this analysis it was then possible to determine the volume fraction of the different phases present at each temperature in the Ni–Ti bulk alloys and thin films. The NMR results showed that in Ni–Ti bulk alloys, where the R-phase transformation does not take place or is only partial, the martensitic transformation is not complete even at temperatures well below Mf as determined by calorimetry. On the other hand, in thin films showing a well distinct two-step transformation via the R-phase no remaining austenite could be detected below the Rf temperature.
