Premartensitic states in the Ti50Ni50-x57Fex alloys studied by the annihilation of positrons

The spectra of the angular correlation of annihilation photons (ACAP) for the (Ti50Ni50-xFex)-Fe-57 alloys with x = 0, 1.0, and 2.5 at. % Fe-57 were studied as a function of Fe-57 concentration and temperature at the stage preceding the B2 --> R martensitic transformation. Changes in the positron behavior caused by introduction of impurity Fe-57 atoms into the TiNi matrix are analyzed. It is shown that the qualitative reversible changes in the ACAP spectra at T-0 similar to 413 K cannot be attributed to the changes in the concentration of vacancy-type defect, but rather reflect changes in the electron band structure near the impurity atoms of Fe-57. Comparison with the Mossbauer experimental data obtained on the same alloys is carried out.