Phase transformation and crystal structures of Ti2Ni3 precipitates in Ti-Ni alloys

In Ni-rich Ti-Ni alloys, three kinds of precipitates such as TiNi3, Ti2Ni3 and Ti3Ni4 appear with aging heat-treatments. The shape memory effect and superelasticity of the alloys are greatly affected by the presence of these precipitates. It is interesting to note that the Ti2Ni3 precipitate itself exhibits a phase transformation. However, the characteristics of this transformation is not understood in detail as yet. In the present study, the characteristics of this transformation were studied from crystallographic point of view. The crystal structures of the low- and the high-temperature phases were determined by combining several techniques such as electron diffraction, convergent beam electron diffraction, high resolution electron microscopy and powder X-ray diffraction with the Rietveld analysis. The results of these analyses revealed that the space group of the low- and the high-temperature phases are Bbmm and I4/mmm, respectively. It was reported that the Ti-2(Ni, Cu)(3) alloy shows a similar transformation. The relevance of the transformation in this alloy to Ti2Ni3 precipitates is also discussed.