Anomalous Temperature Changes of Positron Lifetime and Electrical Resistivity in B2-NiTi Alloys

Abstract

The pre-martensitic phenomena in Ni-rich NiTi alloys have been studied by means of positron lifetime spectroscopy and electrical resistivity measurement. We have found anomalous positron lifetime changes above the martensitic transformation temperatures of Ni₅₀Ti₅₀–Ni₅₁Ti₄₉ alloys. Positron lifetime increases anomalously with decreasing temperature even at temperatures 100 degrees higher than M_s of Ni₅₁Ti₄₉ alloys. Almost the same change in positron lifetime has been observed on the subsequent heating run with little hysteresis. We have also found the anomalous behavior of positron lifetime and negative temperature dependence of electrical resistivity even in NiTi alloys that do not exhibit martensitic transformation. The observed pre-martensitic changes of positron lifetime and electrical resistivity directly reflect the intrinsic electronic-structural changes of NiTi alloys that cause so-called pre-martensitic phenomena.