Study of the martensitic phase transformation in Ni-Ti by nuclear magnetic resonance

In order to study the microscopic mechanisms underlying the martensitic phase transformations (MPT), nuclear magnetic resonance (NMR) experiments have been performed on NiTi shape memory alloys displaying a two-step transformation using the 61Ni-nuclei as microscopic probes. Systematic measurements of the spectral linewidths and Knight shifts as a function of temperature for polycrystalline Ti-50.6 at.% Ni wires and Ti-rich Ni-Ti thin films have been carried out. The volume fraction of martensite and R-phase at each temperature can then be determined by spectral deconvolution. The growth curve of martensite as obtained by NMR provides quantitative information that cannot be detected by other experimental techniques. It has been shown in the Ni-Ti wire sample that 5% of the austenite remains untransformed even at a temperature well below Mf as determined by differential scanning calorimetry. In contrast to the austenite, all the R-phase transforms into martensite. In the Ni-Ti thin films an additional spectral contribution is detected. Its origin is tentatively attributed to the presence of a supplementary phase which has been identified as Ti2Ni precipitates by TEM observations. (orig.)