The structure and morphology of the near-surface area in FeTi and NiTi compounds after activation treatment

The prehydrogenation activation of FeTi is a subject of current controversy. No agreement exists regarding the presence of metallic iron clusters and various suboxide compounds in the surface layers of activated FeTi. In order to gain more insight into these issues we carried out a transmission electron microscopy study of the near-surface area of FeTi and NiTi samples which had been activated by heat treatments at various partial oxygen pressures. These treatments of FeTi result in the formation of oxygen-stabilized Fe_3-xTi_3+xO_y precipitates and of titanium and iron oxides. No free iron precipitates were detected. We observed the presence of the corresponding nickel compounds in activated NiTi and, in addition, NiTiO₃ and free nickel precipitates. The results indicate a high sensitivity of the near-surface layers to the parameters involved in the activation treatment, e.g. partial oxygen pressure, temperature and duration.