In-situ XRD and Electrical Resistivity Study of the Phase transformations in Ni-Ti Shape Memory Alloys (SMA)

In situ XRD has been used by the authors to study the structural changes during (i) the crystallization of Ni-Ti thin films [1], (ii) the growth of Ni-Ti thin films by sputtering [2-11], and (ii) the transformation characteristics of bulk N-Ti subject to thermomechanical treatments [12-16]. The phase transformations of Ni- Ti SMA can also be investigated by measuring some physical properties such as electrical resistivity (ER) as a function of temperature. During cooling of Ni-Ti SMA from B2-phase, the resistivity value decreases linearly with the temperature down to Rs , where R-phase self-accommodated (by twinning) starts to be formed. Twinning in an alloy matrix results in electron scattering, which in turn leads to the increase of the ER [17]. Additional cooling below Rf promotes the continuous increase of rhombohedral distortion angle of the R-phase. It is assumed that this rhombohedral distortion is the reason for a further increase of ER (between Rf and Ms). Below Ms, this distortion is relaxed by the R-phase transforming to monoclinic B19' martensite, giving a gradual decrease of ER.