Study of the martensitic transformation in shape-memory nitinol alloy by time-of-flight neutron diffraction techniques

Abstract

The present paper reports a study of thermally induced martensite (M) and its parent (P) structure in the shape-memory NiTi alloy (Nitinol) by neutron time-of-flight diffraction. The specimen was a 0.6-mm-dia wire in which the martensitic reaction occurred at 33 ± 7°C with zero applied stress. We made seven measurements in a closed cycle of temperature between − 18 to 90°C, using neutrons with wavelengths between 0.72 and 4.15 Å, sorted into 120 channels of 1.5% resolution. Neutron scattered at angles between 30 and 60° were detected by an area detector. The provided scattering data were used in these programs for obtaining the spectrum of the Bragg peaks for given angle θ and T, the lattice constants of the unit cells of both P and M phases, and the specimen texture at a given T. We found, as did previous authors, that the P phase has a CsCl-type cell with the lattice constant 3.032 ± 0.5%, and the M phase is monoclinic which can be formed with both P112 1/ m and P12/ c 1 unit cells. The lattice constants of these cells were found to deviate ±0.4 to ±1.4% from their average values. Although the martensite is predominantly long-range ordered with either Ni or Ti atoms at the cell corners, our data lead us to suggest that in our specimen there are some small fractions of this structure in which the cell corners are occupied by both Ni and Ti atoms in four different ordered arrangements. These domains, together with the atomic positions deduced from our results, eliminate the controversies among the models proposed by previous X-ray and TEM workers, and unifies their findings with our experimental results. The Bragg peak line spectra obtained with a given angle θ, reveal that during the P → M reaction, each peak of the P phase is replaced by two peaks, each of which is composed of a number of peaks of the M phase. We have phenomenologically formulated this so-called “Bp transition”, and identified some families of habit planes. The “transformation twins” are discussed, and it is found that the habit planes {100} are the sources of the two twin systems (1 10) [111] and (1 12) [111] which are formed on {111} planes. The branching angles of all twins are estimated to be within the range 57–62°. The cycle of M → P → M transformations revealed a hysteresis of about 15°C, but no anomaly or premartensitic effects in the Bragg peak spectra.