Characterization of the martensitic transformation in Ni50−xTi50Cux alloys through pure thermal measurements

The addition of a third element to the Ni–Ti system often changes the product and the path of the martensitic transformation of the alloy, which is a direct B2–B19′ transformation for the NiTi alloy in the fully annealed state. In this study we investigate the martensitic transformation of fully annealed Ni50−xTi50Cux (x = 3–10 at%) shape memory alloy (SMA) samples using differential scanning calorimetry (DSC) and the four-probe electrical resistance (ER) measurements under stress-free conditions. DSC and ER data show that the ternary alloy goes through a direct B2–B19′ transformation for Cu content between 3 and 7 at% and through the two-stage B2–B19–B19′ transformation for Cu content between 8 and 10 at%. We find good agreement between the two techniques as regards the detection of the phase transformation temperatures. B19′ starting and finishing temperatures decreases with the increases of Cu content and show a significant reduction starting from 7 at%; the range of temperatures in which B19 is stable increases with increasing Cu content.

Research highlights▶ This work deals with DSC and electrical resistance (ER) measurements of Ni50−xTi50Cux alloys. The path of the transformation as well as the phase transformation temperatures are studied. ▶ We find good agreement between the two techniques: the alloys go through a B2–B19′ transformation when Cu content is below 8 at% and to the B2–B19–B19′ path when Cu belongs to the 8–10 at% range. ▶ A deep investigation on resistivity signal suggests that the B2–B19–B19′ transformation may originate starting from 7 at%; this is a starting point for future investigations.