MARTENSITIC TRANSFORMATIONS IN A TiNi THIN FOIL

Abstract
The intermetallic compound TiNi has long been known to exhibit "anomalies" above the M_s temperature - the so-called pre-martensitic phenomena. In particular electron diffraction patterns show extra reflections and the Transmission Electron Microscopy (TEM) images some mottling, "tweed-like" structure... However, it is suspected that some of these effects are not premonitory for the martensitic transformation in the bulk material. Recently it was pointed out that extra reflections observed in Cu-Zn-Al electron diffraction patterns were due to a surface effect. To clarify all these pretransition phenomena in TEM specimens, we have carried out an investigation on phase transformations in a Ti-50.5 at % Ni thin foil. In order to know the influence of the initial state at Room Temperature (RT) on the martensitic formation in a thin foil during an "in situ" cooling process, different thermal treatments were performed in the microscope. TEM samples heated up to 400°C and cooled down to RT exhibit in their electron diffraction patterns strong 1/2 100 and 1/4 210 type extra reflections, and do not transform into martensite when they are cooled down to Liquid Nitrogen (LN) temperature. The same thermal treatment performed on the bulk does not give rise to these extra reflections and does not suppress the martensitic transformation when the specimen is cooled down to LN temperature. It is thought that this thin foil behavior is due to contamination during the "in situ" thermal treatment. Interstitial ordering of atoms (possibly oxygen), which would adequately account for the experimental observations, is suggested.