Experimental assessment of low-temperature martensite transformations in Ni-rich polycrystalline Ni-Ti alloys

Abstract

Ultrasonic velocity and attenuation measurements of a commercially available Ni-rich polycrystalline NieTi alloy were simultaneously obtained upon cooling from room temperature (RT) down to 130 K. The anelastic spectra show multiple anomalies in both velocity and attenuation curves, which evidence a complex nature of structural rearrangements exhibited by NieTi alloy, associated with relaxations and phase transformations. In particular, some evident anomalies at 285 and 180 K, not previously exploited using ultrasonic measurements on Ni-rich polycrystalline NieTi alloy, were associated with austenite to pre-martensitic (B2 / R) and pre-martensitic to martensitic (R/ B19’) phase transitions, respectively. The peculiar temperature separation between these transformations was interpreted based on chemical composition and the NieTi alloy microstructure evolution. X-ray diffraction (XRD), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC) were also used to add complementary results about phase transformations and thermal events exhibited by NieTi alloy at low temperatures.